Synthesis of the phorboxazoles—potent, architecturally novel marine natural products

Abstract

The phorboxazoles have attracted the attention of synthetic chemists around the globe due to their potent biological activity and novel structure. A review of the recent synthetic efforts as well as new phorboxazole analogs is discussed.

Introduction

In 1995, Searle and Molinski reported the isolation of a pair of novel natural products from the Indian Ocean marine sponge Phorbas sp. collected off the Muiron Islands that demonstrated potent antifungal and cytostatic activity.¹ The subsequent structural elucidation of these two macrolides,^{2, 3, 4} phorboxazoles A (1, Figure 1) and B (2), has inspired considerable attention and synthetic creativity over the succeeding two decades that has resulted in 10 total syntheses as well as a number of additional synthetic approaches, the discovery of new members of the phorboxazole family, the creation of biological probes and the generation of analogs to uncover hints about their bioactivity. Some of the highlights of these efforts are reviewed here.

Figure 1

Structures of phorboxazoles A and B.

More recently, Capon et al.⁵ isolated both 1 and 2 from a Raspailia sp. sponge collected along the Australian Northern Rottnest Shelf while searching for nematocidal agents. This second isolation from such a taxonomically distinct source suggests a potential microbial biosynthetic origin or at least contribution that has yet to be uncovered, leaving synthesis as the primary source for further consideration of the natural products.

It is not surprising that the distinctive structural features of the phorboxazoles, which includes the presence of four unique pyran rings and two oxazoles, attracted the interest of a number of synthetic groups. In fact, the first synthetic approach to the molecule was reported before the final structure elucidation was published!⁶ Seitz and Hoffmann have previously examined some of the earliest syntheses of the phorboxazoles,^{7, 8} and therefore we will primarily focus on the efforts that were not previously highlighted.

With a target as structurally diverse as the phorboxazoles, there is an array of potential approaches to be considered. However, the need to form a macrolide left a limited number of options for the cyclization. The most commonly exploited approach was to close the ring via an olefination of the α,β-unsaturated macrolactone—a strategy that has proven to be quite effective in this regard. Notably, the Evans total synthesis⁹ and the Paterson synthetic approach¹⁰ utilized a Yamaguchi macrolactonization as an alternative. The other common disconnection was about the C16–C18 oxazole or the C19–C20 olefin to join the two macrolide fragments. As the two natural products differ only about the stereochemical disposition of the hydroxyl substituent at the C13 position, the phorboxazoles serve as an excellent vehicle for the study of the various approaches to solve the synthetic challenges posed. In this review, we will first evaluate the completed syntheses of 1 and 2, respectively, followed by the approaches of other groups in this regard and the efforts reported to date on the discovery of other members of the phorboxazole family, biological probes and additional analogs.

Discussion

Total syntheses

Earliest syntheses

As the earlier reviews covered the first generation syntheses of 1 by Forsyth and Smith as well as the syntheses of 1 by Pattenden and Williams and the synthesis of 2 by Evans, we will only point out a few of the salient highlights from these seminal works. Forsyth, who was the first to complete the synthesis of either natural product (Figure 2),¹¹ established the Michael cyclization to the pentasubstituted pyran and developed a biomimetic route to the oxazoles. He observed the same lack of diastereoselectivity in the formation of the C38 center observed by Molinski and Leahy,² ascertained the viability of the hetero-Diels–Alder approach to the pyrans and established the feasibility of the Still–Gennari macrocyclization. Before 2006, Evans had achieved the only synthesis of 2,⁹ which featured a macrolactonization strategy of an acetylene to enable a selective reduction to the Z C2–C3 olefin (Figure 3). In addition, Evans’ lithiation of the oxazole methyl, optimization of the C19–C20 Wittig and use of β-ketoesters enabled many of the efforts that followed. Smith’s pioneering use of the Petasis–Ferrier rearrangement serves as a hallmark of his synthesis of 1 (Figure 4),¹² and he implemented a Stille protocol for introduction of the exocyclic oxazole, allowing it to serve as a linchpin for the two complex fragments. Pattenden was the first to use a carbohydrate starting material (d-xylose) in his synthesis of 1,¹³ and his utilization of the Julia olefination for the introduction of the polyene side chain was adopted by several subsequent synthetic approaches (Figure 5). In addition to using a Horner–Wadsworth–Emmons approach to C19–C20 and C27–C28, Williams was able to take advantage of his in situ asymmetric transmetallation/allylation protocol in his synthesis of 1 (Figure 6).¹⁴

Figure 2

Retrosynthetic analysis of Craig Fosyth’s first-generation total synthesis of phorboxazole A.

Figure 3

Retrosynthetic analysis of Dave Evans’ total synthesis of phorboxazole B.

Figure 4

Retrosynthetic analysis of Amos Smith’s first-generation total synthesis of phorboxazole A.

Figure 5

Retrosynthetic analysis of Gerald Pattenden’s total synthesis of phorboxazole A.

Figure 6

Retrosynthetic analysis of David Williams’ total synthesis of phorboxazole A.

Recent syntheses

Second generation synthesis of 1 by Amos B Smith, III

In the years since those syntheses were completed, five additional total syntheses have been reported. The first of these was a second generation approach to 1 by Smith,¹⁵ who has shown an admirable commitment to developing scalable syntheses of scarce natural products in order to facilitate their biological investigation.^{16, 17} His retrosynthesis, which is a bit more convergent than his first generation approach but uses many of the same disconnections, is shown in Figure 7. Asymmetric hetero-Diels–Alder cyclization of 14 with Danishefsky’s diene (15) gave 17 in 95% ee using Jacobsen’s catalyst (16) on a >70-g scale (Scheme 1). Michael addition of an acetate provided excellent trans selectivity, which allowed for the completion of the initial pyran 18. An acetate aldol set the stage for the formation of acetal 21, which then underwent the Petasis–Ferrier rearrangement to bis-pyran 22. Once the ketone was reduced to the axial alcohol, the C3–C19 fragment (9) was completed as expected. Smith also used 14 as his entry point to the pentasubstituted pyran. Evans aldol addition followed by acetalization with 10 led to dioxanone 25, which underwent the two-step Tebbe olefination/Petasis–Ferrier rearrangement without incident (Scheme 2). α-Methylation and elaboration to aldehyde 8 yielded the Wittig coupling partner. The synthesis of Smith’s final pyran was achieved using an intermediate from his first generation synthesis (which could also be prepared from aldehyde 14, meaning all of the fragments emanate from the same starting material), and features an asymmetric Sharpless dihydroxylation to set the C37 and C38 stereocenters, a Stille coupling of 6 and 7 to install the diene and the introduction of the linchpin oxazole (Scheme 3). With all of the intermediates in hand, Wittig coupling of 8 and 9 proceeded with outstanding selectivity (Scheme 4). Macrocyclization was achieved using the Still–Gennari conditions to give the Z-olefin with 2.5:1 selectivity. The two fragments were united via a Stille coupling, and the peripheral vinyl bromide was installed followed by global deprotection to give 1.

Figure 7

Retrosynthetic analysis of Amos Smith’s second generation total synthesis of phorboxazole A.

Total synthesis of 1 by James White

In their synthetic approach to 1, the White group installed the elaborated polyene before introducing the bis-pyran portion of the macrolide (Figure 8).^{18, 19} Asymmetric allylation of tartrate-derived 52 leads to 53, which was oxidized and acetalized (Scheme 5). Protection of the secondary alcohol allows for the Wittig homologation to 58, which facilitated the generation of lactone 60. Asymmetric crotylation of 62 installed the C25 and C26 stereocenters with 92% ee (Scheme 6), which was further elaborated via a second asymmetric crotylation with good diastereocontrol. A palladium-mediated alkoxycarbonylation then created the pentasubstituted pyran 66, which was converted to 47 in anticipation of coupling to 46, which was achieved via deprotonation of the oxazole methyl (Scheme 7). Formation of enal 68 and Julia olefination provided the C20–C46 fragment. It should be noted that the vinyl bromide had to be installed after the introduction of the oxazole, as it promoted elimination to the undesired terminal alkyne.

Figure 8

Retrosynthetic analysis of James White’s total synthesis of phorboxazole A.

White prepared the bis pyran from the asymmetric allylation of 14 (Scheme 8). Seyferth–Gilbert homologation to the alkyne preceded bromoboration and extension to allylsilane 49. Asymmetric allylation of 74 proceeded with outstanding enantiocontrol (Scheme 9), which was extended to 77 via a second asymmetric allylboration. The cis pyran was again formed via a carbonylative process, which was then coupled with 49 to complete the C3–C19 fragments. The undesired diastereomer at C9 was able to be converted to 45 using an oxidation/reduction protocol. The final assembly of 1 was achieved via the Wittig union of the fragments, which selectively gave the trans product (Scheme 10). The trans-pyran was then closed and the macrolide formed via intramolecular Horner–Wadsworth–Emmons olefination with 4:1 selectivity. Global deprotection yielded 1.

Second generation syntheses of 1 by Craig Forsyth

Forsyth has also recently disclosed an extensive study that culminated in a second-generation synthesis of 1, expanding on his de novo oxazole approach (Figure 9).^{20, 21} Hetero–Diels–Alder cyclization with Garner’s aldehyde (84) gave an excellent yield of exclusively 86, which is initially the trans-pyran (Scheme 11). Manta has also reported the synthesis of 86 using this methodology. Upon elaboration to aldehyde 88, this could be equilibrated to the desired cis product (89). A second hetero-Diels–Alder cyclization with diene 85 then provided bis-pyran 90. β-Ketoimide aldol addition to 91 and directed reduction gave the stereotetrad of the pentasubstituted pyran, which was extended to α,β-unsaturated ester 96 and closed via the Michael protocol (Scheme 12) to render primarily the trans-pyran. Equilibration via a retro-Michael/Michael process then afforded the desired stereochemistry, and the ester was extended and saponified to 82.

Figure 9

Retrosynthetic analysis of Craig Forsyth’s second-generation total synthesis of phorboxazole A.

Forsyth’s approach to the exoxyclic pyran ultimately emanated from d-xylose derived 100 (Scheme 13). Silylketene acetal addition gave >4:1 selectivity for 102, which was converted to the aldehyde and extended to 106. Julia addition of the terminus gave 108, which could be ketalized and advanced to vinyl bromide 81.

Forsyth explored an impressive array of approaches to optimize the final formation of 1, including a ring closing metathesis that dramatically improved the C2–C3 Z:E selectivity and a tandem de novo oxazole formation of both rings. The most direct synthesis is shown in Scheme 14. Ultimately, his second-generation synthesis achieves 1 in an overall yield of 6% with a longest linear sequence of 19 steps from 84.

Total synthesis of 2 by Guo-Qiang Lin and Wei-Shan Zhou

Two total syntheses of 2 have also been achieved in the last decade. The first of these came from the labs of Lin and Zhou (Figure 10).²² Asymmetric allylation and Wacker oxidation of the protected product gave an 87% ee of ketone 126 (Scheme 15). Aldol addition and oxidation led to diketone 128, which was closed to pyranone 129. Hydrogenation simultaneously set the C13 and C15 stereocenters, leading to aldehyde 122. Aldol addition of 123 (derived as the opposite antipode to 126, Scheme 16) gave a nearly 8:1 diastereomeric ratio of 132. Nysted olefination and cyclization resulted in bis-pyran 135, which was then prepped for the Wittig coupling. Sequential asymmetric crotylation of glyceraldehyde 136 using Roush’s tartrate-derived methodology produced 140 (Scheme 17), which was closed to the cis pyran using an intramolecular oxymercuration cyclization with good stereocontrol. Once the resultant iodide was transformed into a protected silyl ether, removal of the acetonide and periodate cleavage yielded ketone 144, which was adorned with the oxazole to complete the C20–C32 piece.

Figure 10

Retrosynthetic analysis of Gua-Lin Qiang and Wei-Shan Zhou’s total synthesis of phorboxazole B.

The exocyclic pyran was generated from the Sharpless asymmetric dihydroxylation of enoate 147 (Scheme 18). Upon extension to the allylic acetate, the primary TBS ether was converted to the corresponding aldehyde and subjected to a titanium silylketene acetal addition with 4:1 selectivity at C35. Deprotection allowed selective δ-lactone cyclization, which led to 154. Methyl deprotonation and ketalization was used to prepare 155 (Scheme 19), which underwent Julia olefination with 3 to C20–C46 fragment 118. Although its use in the synthesis of the phorboxazoles was well established by Pattenden, Lin and Zhou prepared 3 via dihydroxylation of bis-allyl ether 157 and Wipf’s hydrozirconation protocol (Scheme 20). The synthesis of 2 was completed via Wittig coupling of the C19-C20 olefin and Horner–Wadsworth–Emmons macrocyclization (Scheme 21).

Total synthesis of 2 by Steven Burke

The Burke synthesis of 2 (Figure 11) is a beautiful illustration of the use of two-directional synthesis.²³ Readily available diol 171 was ozonolyzed and converted to achiral tetraol 170 (Scheme 22). Palladium-catalyzed desymmetrization with Trost’s DPPBA ligand gave the bis-pyran 174 in 98% ee. Selective asymmetric dihydroxylation of the equatorial vinyl group completed the desymmetrization, which was able to be oxidized to carboxylate 177 and cyclized under Mitsunobu conditions to lactone 178, thus differentiating the two secondary alcohols. Upon reductive opening of the lactone, the exocyclic alkene was installed using the Tebbe reagent to generate 180. Removal of the acetonide and selective primary protection allowed for the introduction of nitrogen with diphenylphosphoryl azide. Burke was the first to utilize an asymmetric hetero-Diels–Alder approach to the pentasubstituted pyran, installing four of the centers in a single transformation in good yield and excellent enantioselectivity (Scheme 23). Reduction of ketone 184 gave a solid that could be recrystallized to even higher enantiopurity, and cyanation of the corresponding acetal afforded the axial nitrile, which could be methylated to ketone 187. Epimerization restored the desired C26 equatorial orientation, and installation of the oxazole resulted in the formation of the desired C20–C32 fragment.

Figure 11

Retrosynthetic analysis of Steve Burke’s total synthesis of phorboxazole B.

The exocyclic pyran was also prepared using a hetero-Diels–Alder cyclization which, under europium catalysis, returned 191 as a single diastereomer (Scheme 24). Hydrogenation set the C35 stereocenter and liberated the diol, which could be manipulated to selectively protected 169. Lithiation of 121 and addition of 169 formed the C32–C33 bond (Scheme 25), and the peripheral triene moiety was realized via the established Wittig/Julia methodology. Selective deprotection of the primary alcohol allowed the Wittig extension to α,β-unsaturated acid 199, which was coupled to amine 168 to provide the oxazole precursor. Wipf cyclodehydration achieved the oxazole formation, and the macrolide was closed via the Horner–Wadsworth–Emmons olefination (Scheme 26). Desilylation completed the synthesis of 2.

Additional synthetic approaches to the phorboxazoles

Phorboxazole macrolide

Among the reported synthetic approaches to the phorboxazoles, only the Paterson group has published a completed macrolide.¹⁰ The pentasubstituted pyran was assembled using the anti-aldol addition of chiral ketone 204 to aldehyde 62, intramolecular Tishchenko reduction of the ketone and eventually Michael cyclization to the pyran (Scheme 27). The C5–C9 pyran was prepared via the Michael allylation of enone 17 to set the requisite trans-pyran with excellent diastereoselectivity (94% de) using allyl tributylstannane (Scheme 28), and the subsequent aldehyde was converted into the corresponding acetylenic ester 211. Finally, the C9 side chain was deprotected and converted into siloxydiene 213 to set the stage for a second hetero-Diels–Alder cyclization. Following the Wittig coupling of 214 to 209, this was achieved to provide a nearly 1:1 mixture of diastereomeric pyrans 217 and 217a that proved resistant to improved diastereocontrol (Scheme 29). Reduction of ketone 218 followed by generation of the seco acid allowed for the Yamaguchi macrolactonization developed by Evans and synthesis of 220.

The Hoffmann group recognized that the 8-oxabicyclo[3.2.1]octane ring system mapped well onto the phorboxazole pyrans, and exploited that in their approach.²⁴ Reduction of meso ketone 221 with l-Selectride and protection set the stage for the asymmetric hydroboration of the olefin with diisopinocampheylborane to give 223 in 96% ee (Scheme 30).²⁵ Oxidation to the ketone and subsequent Baeyer–Villiger ring expansion provided 225, which underwent methanolysis to provide pyran acetal 226. Allylation proceeded with excellent trans selectivity (>99% de), which set the stage for the elaboration to aldehyde 230. The C11 stereocenter was introduced via asymmetric allylation, which allowed for the remaining elaboration of C3–C13 fragment 233.

Hoffmann’s approach to the pentasubstituted pyran started from the racemic dimethylated bicycle 234 (Scheme 31).²⁶ DIBAL reduction of the ketone and benzylation gave 235, which underwent asymmetric hydroboration to a mixture of 236 and 237. Both components of this mixture proved useful, as 236 was transformed into the C1–C7 portion of discodermolide while 237 was carried on toward the phorboxazoles.²⁷ Utilizing their PCC/Baeyer–Villiger protocol, Hoffmann synthesized 238, which was elaborated to acrylonitrile 241 and subjected to rhodium catalyzed oxazole cyclization. Finally, LAH reduction simultaneously reduced the oxazole β-methoxy substituent and the ester to provide the C15–C26 fragment.

Yadav initiated his synthetic approach to the macrolide portion from vinyl furan 244, readily available from d-glucose (Scheme 32),²⁸ which provided the C13 and C15 stereocenters of phorboxazole A.²⁹ It is notable that d-galactose would correspond to the phorboxazole B stereochemistry. Upon opening the furan, the olefin was extended via a hydroboration/oxidation/Wittig sequence, and Michael addition of the secondary alcohol of 249 formed cis-pyran 250. Asymmetric allylation followed by acylation with acryloyl chloride gave 252, which was closed with first-generation Grubbs catalyst to lactone 253. Reduction to the ethyl acetal and Lewis acid mediated allylation yielded expected trans-pyran 255, which represents the C2–C16 portion of phorboxazole A.

Yadav also prepared the pentasubstituted pyran from the 8-oxabicyclo[3.2.1]octane ring system, starting from the asymmetric hydroboration of achiral dimethylated derivative 256 (Scheme 33).³⁰ Ring opening of Baeyer–Villiger lactone 258 revealed pyran 259 which, upon protection of the C20 Wittig precursor as a benzyl ether, was oxidized to lactone 262. This allowed for the epimerization of the C25 methyl using DBU with excellent selectivity and conversion. The C27 side chain was then introduced via acetylenation, and hemiketal reduction gave the elaborated pyran as a single isomer. Oxymercuration then gave ketone 265. The oxazole was introduced via a modified Julia olefination of 265 with sulfone 266 (Scheme 34), giving C20–C32 fragment 267 as a 9:1 mixture of olefin isomers.

The synthesis of the bis pyran was also achieved by the Rychnovsky group using their expertise with the Prins reaction.³¹ Epoxide 268 was opened with the lithium anion of 269 to give diol 270, which was smoothly closed to the pyran and extended to trans thioester 273 (Scheme 35). The C11 stereocenter was formed via reduction to the aldehyde and asymmetric allylation. Acylation of the C11 alcohol with oxazole 275 and conversion to the α-acetoxyether generated the Prins precursor, which proceeded as expected to the cis-pyran, representing the C3–C19 portion of phorboxazole B. Rychnovsky showed that the Prins cyclization could also be achieved to install the C11 stereocenter, but the yields were diminished presumably due to the proximity of the oxazole nitrogen.

The Prins reaction was also used by Rychnovsky to make the pentasubstituted pyran as well.³² Asymmetric crotylation of aldehyde 14 with 278 gave 279 in 98% ee (Scheme 36), setting the C22 stereocenter. Acylation with protected lactic acid yielded 281, which was transformed into Prins precursor 282 in standard fashion. This cyclization proceeded as expected with catalytic BF₃ etherate to set the remaining stereocenters of the pyran. Elaboration to ketone 284 was followed by Horner Wadsworth Emmons olefination to afford the C20–C29 fragment as a 3.9:1 mixture of olefin isomers.

The Clarke group also used the macrolide pyrans as a showcase for their work on the asymmetric Maitland–Japp reaction. Toward this end, diketene was able to be coupled to aldehyde 287 (setting the C11 stereocenter) and oxazole aldehyde 61 in a one-pot transformation using Ti(IV) and chiral ligand 295 to prepare pyranone 289 as a mixture of C15 epimers in 73% ee (Scheme 37).³³ The trans isomer could be converted to the desired cis isomer with Yb(OTf)₃. Decarboxylation, reduction and protection thus gave pyran 290, which was converted to the aldehyde and subjected to a second Maitland–Japp reaction using Chan’s diene and Evans’ pybox ligand, which established the trans pyran ring with excellent stereocontrol and complete enantiomeric enrichment. Once the exocyclic olefin at C7 was introduced, the primary alcohol was liberated and activated for displacement with an acetylenic anion. Radical bromination of the oxazole methyl finally gave C1–C19 fragment 294 functionalized for further elaboration.

Clarke’s initial efforts to utilize the Maitland–Japp reaction to generate the pentasubstituted pyran resulted in a C23-epimeric product, so he ultimately initiated his synthesis of the remainder of the macrolide fragment using the anti-aldol addition of 296 to 62 (Scheme 38).³⁴ Reductive removal of the chiral auxiliary followed silylation of the C26 alcohol, which allowed for outstanding Felkin–Ahn control of the subsequent crotyl addition. Acrylate 300 was added via a metathesis reaction, which set the stage for the Michael cyclization (13:1 selectivity) to the C20–C32 fragment.

Taylor was able to demonstrate that his electrophile-induced ether transfer methodology allowed access to the bis-pyran portion of 1 (Scheme 39).³⁵ Ether transfer of the BOM-protected analog of 28 with ICl conferred the C5 stereocenter with exquisite control (20:1), and the subsequent sulfone (303) could be cyclized and allylated to the trans pyran nearly quantitatively. Following elaboration to aldehyde 305, allylation gave a 1:1 mixture of C11 alcohols that could be separated and completely converted into the desired 306 via Mitsunobu inversion. Ether transfer again selectively introduces the C13 stereocenter of phorboxazole A, and cyclization leads to the C3–C17 fragment 308.

Donaldson also explored the use of a hetero-Diels–Alder approach to the phorboxazoles (Scheme 40).³⁶ Allylation of acetal 309 provided the trans pyran, and the resulting aldehyde was subjected to asymmetric allylation to create the separable diasteromers 314 and 315. The initial attempts to elaborate the second pyran via another hetero-Diels–Alder reaction proved possible but ultimately not as productive. Acryloylation of 314 followed by RCM gave the C3–C15 fragment.

The Panek group used their crotylsilane chemistry to approach the phorboxazoles.³⁷ Treatment of 317 (available in any isomeric form via Sharpless epoxidation) with aldehyde 318 gave cis-pyran 319 (Scheme 41) as a single diastereomer. Epoxidation and reduction of the ester set the stage for the selective methylation, which was achieved in excellent yield to give the fully substituted pyran 322 that was subsequently elaborated to C19–C28 fragment 326. Panek also demonstrated a unique way of installing the exocyclic oxazole to a related fragment via a Stille coupling to triflate 330 (Scheme 42).³⁸ It is clear that this strategy was also the plan for introducing the other oxazole based on the C19–C20 hydrostannylation.

Chakraborty prepared the pentasubstituted pyran from aldehyde 332 starting with an asymmetric aldol addition (Scheme 43).³⁹ Sharpless epoxidation installed the C22 stereocenter, but the radical-mediated ring opening gave a considerable amount of olefin 336 in addition to the anticipated 337. The mixture could be completely converted into the desired 337 via hydrogenation, which could be efficiently converted into alcohol 339. Upon oxidation to the aldehyde, acetylenation gave a mixture of diastereomeric propargyl alcohols, which could be closed to give 341 as well as the C26 epimer in a 2:1 ratio.

Nagaiah synthesized the pyran from the proline-mediated aldol addition of propionaldehyde to d-mannitol-derived glyceraldehyde 136 (Scheme 44).⁴⁰ Wittig elongation and reduction to the allylic alcohol provided the Sharpless epoxidation substrate, which was opened to diol 346. Selective primary alcohol oxidation and another Wittig elongation allowed for the Michael cyclization and generation of C20–C27 fragment 348.

Exocyclic fragments

In addition to completing the macrolide, Paterson also completed the entire exocyclic portion of the phorboxazoles.¹⁰ Mukaiyama aldol addition of 101 to 349 using Evans’ pybox catalyst installed the C37 stereocenter in >95% ee, and acid-catalyzed lactonization gave β-ketoester 351 (Scheme 45). Hydrogenation of the corresponding vinylogous carbonate simultaneously installed the C35 center and liberated the primary alcohol, which was oxidized to aldehyde 354. Meanwhile, the triene was formed from asymmetric acetate aldol addition to aldehyde 357 (Scheme 46) in a straightforward manner. The Nozaki–Kishi addition of 362 to 354 proceeded to give the fully elaborated C33–C46 fragment, though the reaction proceeded in disappointing yield and selectivity. The mixture could be oxidized for potential controlled reduction, but those results have not been published.

The Yadav group has also completed the entire exocyclic fragment.⁴¹ Acetylenic opening of 366 with 365 set the C35 center, which was converted into allyl chloride 368 (Scheme 47). Sharpless dihydroxylation was then used to install the C37 and C38 stereocenters, and the pyran was elaborated by formation of β-ketoester 372 and ketalization to 373. Upon formation of aldehyde 375, Julia olefination with 376 gave the full C31–C46 fragment as a 1:1 mixture of olefin isomers.

Shibasaki synthesized an intermediate from Smith’s phorboxazole synthesis to demonstrate the utility of his catalytic asymmetric Michael additions to unsaturated acylpyrroles, installing the C35 stereocenter with >99% control (Scheme 48) and efficiently generating alkyne 33.⁴²

Nagaiah was able to demonstrate that d-mannitol, which is the established synthetic precursor to glyceraldehyde 136, could also be used for the generation of the exocyclic pyran.⁴⁰ Periodate cleavage of diol 385 allowed access to 386,⁴³ which could be selectively converted into aldehyde 388 (Scheme 49). An asymmetric acetate aldol was then used to install the C35 stereocenter, which was then extended to β-ketoester 391 and cyclized to the C31–C39 fragment.

Another approach to the exocyclic pyran from chiral sulfoxide 395 was reported from the Carreño and Solladié groups (Scheme 50).⁴⁴ Reduction of this diketone proceeded with excellent diastereocontrol to set C37, which was then used to set C35 via a directed reduction. The terminal ester was converted into the methyl ketone, which could then be ketalized to 399. The sulfoxide could then be converted into aldehyde 401 via the Pummerer reaction.

Hoffmann also completed a synthesis of a ring-opened exocyclic pyran that incorporated the exocyclic oxazole as well (Scheme 51).⁴⁵ Addition of serine to 402 and aromatization gave 403, which could be converted into aldehyde 405. Extended homologation via vinylogous Horner–Wadsworth–Emmons reaction and installation of the C37 and C38 stereocenters completed the C28–C41 fragment.

Additional phorboxazoles and analogs

In 2009, Molinski reported the isolation of hemi-phorboxazole A (407, Figure 12),⁴⁶ which contains the full macrolide but only a nitrile instead of the exocyclic oxazole and hinting at its biosynthetic origin. Smith was able to confirm the structure of 407 via its synthesis from an advanced synthetic intermediate.⁴⁷ This synthesis provided sufficient material to allow for the biological evaluation of 407, which proved to be essentially inactive compared with 1. Interestingly, synthetic analog 408 showed much greater activity than the natural product.

Figure 12

Examples of synthetic analogs of the phorboxazoles.

The extensive synthetic efforts detailed above have also allowed for the generation of phorboxazole analogs and tool compounds useful for understanding its mechanism of action. Both the Forsyth and Smith groups prepared and evaluated alkyne analog 409,^{48, 49} demonstrating that it was nearly as active as 1 against several cancerous cell lines. In fact, changes to the terminus are reasonably well tolerated and even could be used to make more active analogs such as vinyl chloride 410. Although modifications such as ketalization at the C33 position do not destroy activity, the loss of either the macrolide or a significant portion of the exocyclic terminus dramatically reduces activity. Some purely synthetic analogs such as acetal 411 are quite active, while the activity of others such as triazole 412 (formed through a copper catalyzed click 1,3-dipolar cycloaddition) remain unreported.⁵⁰

Forsyth has prepared some phorboxazole-based biological tool compounds such as biotinylated analog 413 and fluorescent analog 414.^{51, 52} The latter was shown to be taken up in HeLa cells and to induce extranuclear cytokeratin intermediate filaments to associate with cyclin-dependent kinase 4 (Cdk-4), a kinase that has been explored as a cancer target.⁵³ The full mechanism of action of the phorboxazoles remains undetermined at this stage.

Conclusions

The novel structure and biological properties of the phorboxazoles have clearly attracted the interest of the synthetic community. It has been through these efforts that we have begun to understand some of the structural requirements for their impressive biological activity and gained a glimpse of their biochemical target. Given the impressive strides that have been made in this regard, it is reasonable to think that there remain chapters of the phorboxazole story to be written.

(a) 16, 4 Å MS, Me₂O, RT; (b) TFA, 0 °C; (c) (1) Cp₂TiMe₂, ethyl pivalate, THF, 55 °C; (2) HSiEt₃, 10% Pd-C, CH₂Cl₂, RT; (d) (1) 20, Sn(OTf)₂, N-ethylpiperidine, −55 to −78 °C; (2) LiOH, H₂O₂, THF, 0 °C; (e) (1) HMDS, CH₂Cl₂, RT; (2) 12, TMSOTf, −78 to −30 °C; (f) (1) Cp₂TiMe₂, ethyl pivalate, THF, 53 °C; (2) Me₂AICl, Cs₂CO₃, CH₂Cl₂, RT; (g) (1) K-selectride, THF, −78 °C; (2) TBSOTf, 2-6-lutidine, CH₂Cl₂, −78 °C; (h) DDQ, H₂O, CH₂Cl₂, RT; (2) MsCl, DIPEA, CH₂Cl₂, 0 °C.

(a) (1) 24, Bu₂BOTf, DIPEA; (2) LiOH, H₂O₂, 0 °C; (b) (1) HMDS, CH₂Cl₂; (2) 10, TMSOTf, TfOH, 2-6-DTBMP; (c) (1) Cp₂TiMe₂, THF, 55 °C, (2) Me₂AICl, CH₂Cl₂, −78 °C; (d) (1) LiHMDS, Mel, HMPA, −78 °C; (2) NaBH₄, EtOH, −10 °C; (3) KH, DMBCl, 15-c-5; (e) (1) TBAF, THF, RT; (2) SO₃ pyr, Et₃N, DMSO, CH₂Cl₂, RT.

(a) (1) DTBMP, MeOTf, 4 d; (2) O₃, PPh₃; (b) (1) 30, n-BuLi, THF; (2) K₂CO₃, MeOH; (c) (1) AD Mix β, 0 °C, 5 d; (2) Me₂C(OMe)₂, PPTS; (d) (1) t-BuLi, Mel; (2) TBAF; (e) (1) TEMPO, MeCN, pH 6.7 buffer, NaOCl, NaClO₂; (2) FeCl₃-6H₂O; (3) TIPSCl, imidazole; (f) (1) Bu₃SnH, PdCl₂(PPh₃)₂; (2) l₂; (g) (1) TMS-acetylene, t-BuLi, BF₃-Et₂O; (2) MeOTf, DTBMP; (3) HCl (cat.), MeOH; (h) (1) SO₃, pyr, DMSO; (2) CrCl₂, Bu₃SnCHBr₂, Lil, THF/DMF; (i) Pd₂(dba)₃—CHCl₃, DMF, RT, 4 h, Ph₂PO₂NBu₄; (j) (1) 38, iPrMgCl, −78 °C; (2) PTSA, MeOH; (k) Pd(PPh₃)₄, (Me₃Sn)₂, LiCl, Dioxane, 90 °C.

(a) (1) PBu₃, DMF, RT; (2) 8, DBU, DMF, RT; (b) (1) KOH, 18-c-6, THF, 4% H₂O; (2) Dess–Martin, NaHCO₃, CH₂Cl₂, 0 °C; (3) DDQ, pH 7 buffer, CH₂Cl₂; (c) (1) EDCl-Mel, HOBT, 42, CH₂Cl₂; (d) K₂CO₃, 18-c-6, toluene, RT; (e) Pd₂(dba)₃, CHCl₃, 4, AsPh₃, DIPEA, Ph₂PO₂NBu₄, DMF, RT; (f) (1) AgNO₃, NBS, acetone; (2) PdCl₂(PPh₃)₂, HSnBu₃, THF (6:1, ext:int); (3) NBS, MeCN, 0 °C; (4) 6% HCl, THF, RT.

(a) H₂C=CHCH₂MgBr, (−)-(lpc)₂BOMe, Et₂O, −100 °C; (2) NaH, Mel, THF, heat; (b) O₃, CH₂Cl₂, PPh₃, −78 °C; (c) MeOH, PPTS (cat.); (d) (1) TBSCl, imidazole, DMF, −78 °C; (2) TBDPSOTf, 2,6-lutidine, CH₂Cl₂; (3) MeOH, PPTS (cat.); (e) PhSSiMe₃, Znl₂, EDC; (f) (1) (COCl)₂, DMSO, Et₃N; (2) Ph₃P=C(CH₃)CO₂Me, toluene, heat; (g) (1) DIBAL-H, toluene, −78 °C; (2) TBSCl, DIPEA, CH₂Cl₂; (h) (1) AgNO₃, 2,6-lutidine; (2) TPAP, NMO, CH₂Cl₂.

(a) Ph₃P=CC(CH₃)CHO, benzene, heat, 20 h; (b) trans-CH₃CH=CHCH₃, t-BuOK, n-BuLi, THF, then (+)-(lpc)₂BOMe, THF, −70 °C, 6 h; H₂O₂, NaHCO₃, RT, 15 h; (2) NaH, THF, heat, 40 min, then PMBCl, n-Bu₄NI, heat, 6 h, 89%; (c) (1) OsO₄ (cat.), NMO, THF-H₂O, RT, 10 h, 84%; (2) NalO₄, H₂O-THF, RT, 30 min, 98%; (d) trans-CH₃CH=CHCH₃, t-BuOK, n-BuLi, THF, then (−)-(lpc)₂BOMe; HOCH₂CH₂NH₂, MeOH, RT, 3 h, 53%; (e) (1) TIPSOTf, 2,6-lutidine, CH₂Cl₂, RT, 2 h, 99%; (2) AICl₃, EtSH, CH₂Cl₂, −20 to −4 °C, 3.5 h, 78%; (f) Pd(OAc)₂ (3 eq.), CO, MeOH–MeCN, 70 h; (g) (1) LiAIH₄, Et₂O, 0 °C; (2) TBAF, THF, 0 °C; (3) TBSCl, DIPEA, DMAP, CH₂Cl₂, (4) NaH, PMBCl, TBAI, THF.

(a) Et₂NH, n-BuLi, THF, −78 °C then 60; (b) (1) MeOH, PTSA; (2) MnO₂, CH₂Cl₂; (3) TBSCl, imidazole, CH₂Cl₂; (c) 3, NaHMDS, THF, −78 °C; (d) (1) MeOH, PTSA; (2) Dess–Martin.

(a) (1) H₂C=CHCH₂MgBr, (+)-(lpc)₂BOMe, Et₂O, −100 °C; (2) TESOTf, 2,6-lutidine, CH₂Cl₂; (3) O₃, CH₂Cl₂-MeOH, PPh₃, −78 °C; (b) 71, NaOMe, MeOH-THF; (c) (1) PPTS, MeOH; (2) B-Br-9-BBN, CH₂Cl₂; (3) TESOTf, 2,6-lutidine, CH₂Cl₂; (d) Me₃SiCH₂MgCl, NiCl₂(dppp).

(a) (+)-(lpc)₂BOMe, CH₂=CHCH₂MgBr, Et₂O, −100 °C; (b) (1) TBSOTf, 2,6-lutidine, CH₂Cl₂, RT; (2) OsO₄(cat.), NalO₄, H₂O-THF; (c) (1) (+)-(lpc)₂BOMe, CH₂=CHCH₂MgBr, Et₂O, −100 °C; (2) TBDPSOTf, 2,6-lutidine, CH₂Cl₂, RT; (3) HCl (3N), THF–H₂O; (d) (1) PdCl₂(MeCN)₂, (10 mol%), CO (1 atm), MeOH, p-benzoquinone; (2) DIBAL-H, CH₂Cl₂, −78 °C; (f) 49, SnCl₄, CH₂Cl₂, −78 °C (inverted unwanted diastereomer by [O]/[H] process).

(a) Bu₃P, DMF, DBU, then 44; (b) (1) MsCl, Et₃N; (2) MeOH, PPTS, CH₂Cl₂; (3) Et₃N, MeCN; (c) (1) DDQ, CH₂Cl₂;(2) (MeO)₂P(O)CH₂CO₂H, DCC, Ch₂Cl₂; (3) NH₄F (excess), MeOH, 50°C; (4) Dess–Martin, CH₂Cl₂; (d) K₂CO₃, 18-c-6, toluene, −78°C to RT; (2) TBAF, THF; HCl (6%), THF.

(a) ZnCl₂, CH₂Cl₂, 0 °C, then TBAF; (b) K-selectride, THF, −78 °C; (c) (1) TBDPSCl, imidazole, DMAP, CH₂Cl₂; (2) DDQ, pH 7 buffer, CH₂Cl₂, t-BuOH; (3) (COCl)₂, DMSO, iPr₂NEt, CH₂Cl₂; (d) DBU, benzene, reflux; (e) (1) 85, ZnCl₂, CH₂Cl₂, 0 °C; (2) TBAF; (3) Ph₃PCH₃Br, n-BuLi, THF, 0 °C; (f) AcCl, MeOH, 0 °C.

(a) (c-hexyl)₂BCl, Et₃N, Et₂O, 0 °C, then 9 1, −78 °C; (b) Me₄NBH(OAc)₃, HOAc, MeCN, −20 °C; (c) DBU, CH₂Cl₂, then DIBAL-H, −78 °C; (d) Ph₃PCHCO₂Et, benzene, reflux; (e) (1) KOtBu, THF, −50 °C; (2) TESCl. Et₃N, DMAP, CH₂Cl₂; (f) KH, THF, methyl pivolate, 0 °C to RT; (g) (1) DIBAL-H, CH₂Cl₂, −78 °C; (2) Ph₃PCHCO₂Me, MeCN, 65 °C; (h) LiOH, THF, H₂O (recycled deprotected alcohol: TESCl, Et₃N, DMAP, then aq NH₄Cl).

(a) 101, Ti(OiPr)₂Cl₂ (1.2 eq.), toluene; (b) Me₃O⁺BF₄⁻, proton sponge, CH₂Cl₂, 3 A sieves; (c) (1) DDQ, CH₂Cl₂; (2) SO₃-Pyr, DMSO, iPr₂NEt, CH₂Cl₂; (d) 105, toluene, 60 °C; (e) 107, NaHMDS, THF, −78 °C; (f) MeOH (16 eq.), toluene, reflux; (g) TsOH, MeOH; (h) (1) TBAF, THF, 0 °C; (2) TBSCl, imidazole, DMAP, CH₂Cl₂; (i) (1) Bu₃SnH, Et₃B, benzene; (2) NBS, MeCN; (2) Ba(OH)₂, MeOH, H₂O.

(a) (1) 83, EDCl-Mel, HOBt, CH₂Cl₂; (2) Dess–Martin, NaHCO₃, t-BuOH, CH₂Cl₂; (3) (CCl₂Br)₂, PPh₃, DIPEA, CH₂Cl₂, RT; (b) (1) TBAF, THF; (2) 42, EDCl-Mel, HOBt, CH₂Cl₂; (c) (1) DDQ, pH 7 buffer, t-BuOH, CH₂Cl₂; (2) Dess–Martin, NaHCO₃, t-BuOH, CH₂Cl₂; (d) (1) K₂CO₃, toluene, 18-c-6, RT, 3 h; (2) PTSA, MeOH; (e) TFA, CH₂Cl₂, then pH 7.4 buffer; (2) 81, PyAOP, i-Pr₂NEt, CH₂Cl₂; (f) (1) Dess–Martin, NaHCO₃, t-BuOH, CH₂Cl₂; (2) (CCl₂Br)₂, PPh₃, i-Pr₂NEt, CH₂Cl₂; (g) TBAF, EtOAc, THF; (2) aq HCl, THF.

(a) (1) (−)-lpc₂BAIIyl, Et₂O, −78 °C; (2) TBSCl, imidazole, DMF; (b) PdCl₂ (cat.), CuCl, O₂, DMF/H₂O, RT, 6 h; (c) (1) LDA, 127, THF, −78 °C; (2) Dess–Martin, CH₂Cl₂, RT; (d) (1) HF in CH₃CN, RT, 12 h; (2) TBSCl, imidazole, DMF, RT; (e) H₂, 10% Pd/C, EtOAc (saturated with 0.1N HCl), 8 h (f) (1) TBSCl, imidazole, DMF; (2) DDQ, CH₂Cl₂, H₂O; (3) Dess–Martin, CH₂Cl₂, RT.

(a) (1) (+)-lpc₂BAIIyl, Et₂O, −78 °C; (2) PMBOC(=NH)CCl₃, cyclohexane/CH₂Cl₂, BF₃OEt₂, 0 °C; (b) PdCl₂ (cat.), CuCl, O₂, DMF/H₂O, RT, 6 h; (c) (1) LiHMDS, TMSCl, THF, −78 °C; (2) 122, TiCl₄, −78 °C; (d) (1) BzCl, pyr, RT; (2) Nysted reagent, TiCl₄, RT, 30 min; (e) (1) DIBAL-H, CH₂Cl₂, −78 °C; (2) MsCl, Et₃N, CH₂Cl₂, 0 °C; (3) DDQ, CH₂Cl₂/H₂O; (f) Et₃N, MeCN, reflux; (g) (1) NH₄F, MeOH, 50 °C; (2) DIPEA, MsCl.

(a) (1) (−)-137, 4 Å MS, toluene, −78 °C, 7 h; (2) Ac₂O, Et₃N, DMAP (cat.), CH₂Cl₂, RT, overnight; (b) O₃, MeOH, −78 °C; then PPh₃, RT; (c) (+)-137, 4 Å MS, toluene, −78 °C, 6 h; (d) Hg(OAc)₂, toluene, 0 °C, l₂, 30 °C; (e) (1) PMBOC(=NH)CCl₃, BF₃OEt₂, 0 °C; (2) NaCN, DMSO, 70 °C; (f) (1) DIBAL-H, CH₂Cl₂, 0 °C, then 1N HCl; (2) NaBH₄, MeOH; (g) (1) TBDPSCl, Et₃N, CH₂Cl₂; (2) HIO₄, EtOAc; (3) MeLi, THF, −78 °C; (4) Dess–Martin, CH₂Cl₂, RT; (h) 145, LDA, THF, −78 °C.

(a) (1) NaH, TBSCl, THF, 0 °C; (2) PCC, CH₂Cl₂, RT; (3) Ph₃P=CHCO₂Et, benzene, reflux; (b) (1) AD-mix β, t-BuOH/H₂O, RT; (2) PMBOC(=NH)CCl₃, cyclohexane/CH₂Cl₂, BF₃OEt₂, 0 °C; (3) LiAIH₄, Et₂O, RT; (c) (1) (COCl)₂, DMSO, CH₂Cl₂, −78 °C, then Et₃N; (2) CH₃CH(PPh₃)CO₂Et, CH₂Cl₂, reflux; (d) (1) DIBAL-H, CH₂Cl₂, −78 °C; (2) Ac₂O, Et₃N, CH₂Cl₂, RT; (e) (1) Bu₄NF, THF, RT; (2) Dess–Martin, CH₂Cl₂; (f) (1) TiCl₂(OiPr)₂, toluene, −78 °C, then 152; (2) Me₃OBF₄, proton sponge, CH₂Cl₂, RT; (g) (1) K₂CO₃, EtOH; (2) 10% TFA, CH₂Cl₂, then Et₃N; (h) (1) TBSCl, Et₃N, CH₂Cl₂; (2) TIPSCl, AgNO₃, pyr.

(a) (1) LiNEt₂, THF, −78 °C, then 120; (2) PPTS, MeOH, 30 °C; (b) (1) Dess–Martin, CH₂Cl₂, pyr, RT; (2) 3, NaHMDS, THF, −78 °C; (c) (1) NH₄F, MeOH, 50 °C; (2) Dess–Martin, CH₂Cl₂, pyr, RT.

(a) AD-mix-α, t-BuOH/H₂O, RT, 87% ee, 66%; (b) (1) HBr, AcOH; (2) K₂CO₃, MeOH, 88%; (c) (1) TMSC₂H, BuLi, BF₃OEt₂, THF, −78 °C; (2) Me₃OBF₄, proton sponge, CH₂Cl₂, RT, 70% for two steps; (d) CAN, CH₃CN/H₂O, RT, 91%; (e) 161, PPh₃, DEAD, THF, 0 °C, 81%; (f) TBAF, THF, RT, 99%; (g) (1) Cp₂ZrHCl, THF; then NBS; (2) (NH₄)₆Mo₇O₂₄·4H₂O, 30% H₂O₂, EtOH, 56% for two steps.

(a) (1) Bu₃P, DMF, then 118, DBU, RT; (2) NH₄F, MeOH, 50 °C; (b) (1) Dess–Martin, pyr, CH₂Cl₂, RT; (2) DDQ, CH₂Cl₂, pH 7 buffer; (c) (1) (EtO)₂P(O)CH₂CO₂H, DCC, CH₂Cl₂; (2) K₂CO₃, 18-c-6, toluene, −20 °C; (d) TBAF, THF, RT; (2) 6% HCl, THF.

(a) (1) O₃, DMS, −78 °C to RT; (2) In, allyl bromide, H₂O, RT, 24 h; (b) 173, Grubbs' second-generation, CH₂Cl₂:DMF (20:1), RT, 30 min; (c) Pd₂dba-CHCl₃ (2 mol%), (R,R),DPPBA (6 mol%), Et₃N, CH₂Cl₂, 0 °C; (d) (1) Ac₂O, DMAP, pyr, RT, 1 h; (2) AD-mix β, 0 °C, 3 h, (96% BORSM); (e) (1) 2,2-DMP, PPTS (cat.), 1,2-DCE, 40 °C, 30 min; (2) disiamylborane, RT, 1.5 h, NaBO₃, H₂O, RT, 1.5 h; (f) RuCl₃ (3 mol%), NalO₄, 0 °C, 5 h; (g) (1) NaOH, MeOH, then Dowex CCR-3; (2) DIAD, PPh₃, THF, 1 h; (3) TBDPSCl, Et₃N, RT (h) (1) LiBH₄, THF, 0 °C; (2) TESCl, 2,6-lutidine, CH₂Cl₂; (i) (1) Dess–Martin, CH₂Cl₂, RT, 4 h; (2) Cp₂TiMe₂, THF/toluene, 80 °C; (j) 1) PTSA, MeOH, 40 °C; (2) TESCl, imidazole, −78 °C; (k) DEAD, PPh₃, N₃-PO(OMe)₂; (l) PPh₃, H₂O, THF, reflux.

(a) 16 (2 mol%), RT, 14 h, then HF/pyr/MeOH, −20 °C, 15 min; (b) (1) NaBH₄, EtOH, −10 °C, 15 min (98.5% ee after recrystallization); (2) PMB-Cl, KH, RT, 2 h; (c) 0.5 eq. THM-OTf, TMS-CN, MeCN, 0 °C 2 h; (d) AIMe₃ 5% Ni(acac)₂, 0 °C, 42 h; (e) KHMDS, LiCl, PhCH₃, 55 °C 2 h, 91%; (f) 145, LDA, THF, −78 °C to RT.

(a) [Eu(fod)₃], CH₂Cl₂, 0 °C; (b) Pd-C, H₂, EtOAc; (c) (1) MMTr-Cl, pyr, DMAP; (2) TBDPSCl, AgNO₃, pyr, 50 °C.

(a) (1) LiNEt₂, 169, THF, −78 °C; (2) PPTS, MeOH; (b) (1) Dess–Martin, CH₂Cl₂; (2) 194, toluene, 80 °C; (c) (1) DIBAL-H, CH₂Cl₂, −78 °C; (2) Dess–Martin, CH₂Cl₂, 2,6-lutidine; (d) 3, NaHMDS, THF, −78 °C to RT; (e) (1) PTSA, MeOH, MeOH; (2) Dess–Martin, CH₂Cl₂; (f) (1) Ph₃PCHCO₂Et, toluene, 80 °C; (2) LiOH, THF/MeOH/H₂O (4:1:1); (g) 168, EDC-Mel, HOBt, CH₂Cl₂.

(a) (1) Dess–Martin, CH₂Cl₂; (2) 2,6-DBMP, PPh₃, (CCl₂Br)₂, 0 °C, then DBU, MeCN, RT; (b) (1) DDQ, CH₂Cl₂, pH 7 buffer; (2) DIC, (MeO)₂P(O)CH₂CO₂H, CH₂Cl₂; (c) K₂CO₃, 18-c-6, toluene, −40 °C to RT; (d) (1) TBAF, THF; (2) 0.72 N HCl, THF RT, 60 h.

(a) (c-Hex)₂BCl, Et₃N, Et₂O, 0 °C, 1 h; 62, −78 to −20 °C, 18 h; H₂O₂, MeOH, pH 7 buffer; (b) Sml₂ (30 mol%), iPrCHO, THF, −20 °C; (c) (1) TBSCl, Imidazole, DMAP, DMF, 80 °C, 50 h; (2) DDQ, 20:1 CH₂Cl₂/H₂O, 20 °C; (d) (COCl)₂, DMSO, −78 °C, 10 min, −78°C, 1 h; Et₃N, −78 to −40 °C, 40 min; (e) LiCl, DBU, (MeO)₂P(O)CH₂CO₂Me, 4:1 CH₃CN/CH₂Cl₂, 20 °C, 2.5 h; (f) DIBAL, CH₂Cl₂, −78 °C, 5 h; (h) SO₃-pyr, Et₃N, 2:1 CH₂Cl₂/DMSO, 0 °C, 2 h.

(a) (1) H₂C=CHCH₂SnBu₃, TMSOTf, CH₂Cl₂, −78 to −50 °C; (2) TBAF (1 M THF). AcOH (10:1, v/v), THF, 0 °C; (b) (1) OsO₄ (4 mol%), NMO, t-BuOH-THF-H₂O (4:4:1); (2) Ph₃P=CH₂, THF, −40 to −20 °C; (3) Pb(OAc)₄, Na₂CO₃, CH₂Cl₂, 0 °C to RT; (c) (1) CBr₄, Ph₃P, CH₂Cl₂, −10 °C; (2) NaHMDS, THF, −98 °C; (d) (1) n-BuLi, THF, −78 °C, (2) MeOC(O)Cl, HMPA; (e) (1) HF. py, MeCN, 0 °C; (2) DMP, CH₂Cl₂; (f) MeC(O)CH₂P(O)(OMe)₂, Ba(OH)₂·xH₂O, THF; (g) TESOTf, Et₃N, Et₂O, 0 °C.

(a) LiHMDS, DMF, 0 °C, 30 min, then 209, DMF, 0 °C, 1 h; (b) DIBAL-H, CH₂Cl₂, −78 °C; (c) 213, HDA cat. (10 mol%), 4 A molecular sieves; (d) TBAF-AcOH (1:2, mol/mol), THF, 0 °C; (e) LiAI(OtBu)₃H, THF, −78 to −10 °C (2) BPSCl, imidazole, DMF; (f) conc. HCl (1%), MeOH; (2) LiOH. H₂O, THF, H₂O; (g) 2,4,6-trichlorobenzoyl chloride, Et₃N, THF, (2) DMAP, PhMe.

(a) (1)²⁴; (2) NaH, PMBCl, (n-Bu)₄Nl, THF, reflux, 6 h; (b) (+)-lpc₂BH, THF, −10 °C, 1 week; (c) PCC, DCM, RT, 5 h; (d) m-CPBA DCM, RT, overnight; (e) MeOH, conc. H₂SO₄ (cat.), RT, overnight; (f) allyltrimethylsilane, TMSOTf, MeCN, −20 °C to RT, 1 h; (g) TIPSCl, imidazole, DMF, RT, 16 h; (h) (1) DIBAL-H, −20 °C, THF, 2 h; (2) TBSCl, imidazole, DMF, RT, 16 h; (i) O₃, DCM, −78 °C then PPh₃ (j) (−)B-allyl-β-diisopinocampheyl-borane, toluene, −78 °C, 5 h then H₂O₂, NaOH; (k) TBSOTf, 2,6-lutidine, DCM, 0 °C, 30 min; (l) O₃, DCM, −78 °C then PPh₃.

(a) (1) DIBAL-H, THF, −78 °C, 11 h; (2) NaH, THF, BnBr, reflux, 16 h; (b) (−)-(lpc)₂BH, THF, −10 °C, 14 d, then NaOH, H₂O₂, 3 h; (c) (1) PCC/SiO₂, DCM, RT, 15 h; (2) m-CPBA, NaHCO₃, DCM, RT, 15 h; (d) H₂SO₄ (cat.), MeOH, 16 h; (e) (1) DIBAL-H, THF, 0 °C, 4 h; (2) PCC, DCM, RT, 15 h; (f) Ph₃PCHCN, toluene, LiCl, RT, 20 h; (g) N₂C(CO₂Me)₂, Rh₂(OAc)₄, CHCl₃, 15 h, reflux; (h) LiAIH₄, THF, −78 °C, 3 h.

(a)²⁷; (b) (1) 50% AcOH, H₂SO₄ (cat.), 2 h; (2) NaBH₄, MeOH, 0 °C to RT, 1 h; (c) (1) PTSA (cat.), acetone, RT, 3 h; (2) NaH, BnBr, THF, reflux, 8 h; (d) BH₃-DMS, NaOH, H₂O₂, THF, 8 h; (e) (1) IBX, THF, DMSO, RT, 2 h; (2) Ph₃P=CHCO₂Et, benzene, RT, 12 h; (f) CSA, MeOH, RT, 2 h; (g) (1) NaH, THF, −78 °C, 4 h; (2) iPr₂NEt, MOMCl, CH₂Cl₂, 0 °C, 6 h. (h) (1) DIBAL-H, CH₂Cl₂, −78 °C, 1 h; (2) (+)-allyl diisopinocampheylborane, Et₃N, H₂O₂, 1 h; (i) iPr₂NEt, acryoyl chloride, CH₂Cl₂, 0 °C to RT, 4 h; (j) Grubb's 1st gen., Ti(iOPr)₄, CH₂Cl₂, 60 °C, 6 h; (k) DIBAL-H, CH₂Cl₂, −78 °C, 30 min then CSA, EtOH; (l) allyltrimethyl silane. TMSOTf, CH₂Cl₂, 0 °C, 1 h.

(a) (−)-lpc₂BH, THF, −23 °C, 24 h, NaOH (3N), 30% H₂O₂, RT, 6 h; (b) (1) PCC, CH₂Cl₂, RT, 3 h; (2) m-CPBA, NaHCO₃, CH₂Cl₂, RT, 10 h; (c) H₂SO₄-MeOH, 0 °C to RT, 12 h; (d) (1) LiAIH₄, THF, 0 °C to RT, 6 h; (2) BnBr, NaH, THF, RT, 12 h; (e) (1) AcOH-H₂O (3:2), 55 °C, 15 h; (2) PCC, NaOAc, celite, RT, 3 h; (f) DBU, THF, RT, 12 h; (g) n-BuLi, TMS-acetylene, THF, −78 °C, 1.5 h; (h) Et₃SiH, BF₃-Et₂O, MeCN/CH₂Cl₂ (1:1), −40 °C, 2 h; (i) HgO, H₂SO₄, acetone, 50 °C, 1 h.

(a) (1) 161, PPh₃, DEAD, THF, RT, 12 h; (2) oxone, THF-MeOH-H₂O (2:1:1), 12 h; (b) NaHMDS, THF, −78 °C to RT, 4 h; 265.

(a) (1) n-BuLi, BF₃OEt₂, THF, −94 °C; (2) TBAF, THF; (b) (1) IBX, DMSO; (2) Ac₂O, DMAP, pyridine; (c) TMSOTf, 272, CH₂Cl₂, −78 °C; (d) (1) DIBAL-H; (2) (−)-lpc₂B-allyl, Et₂O, −100 °C; (3) NaOH, H₂O₂; (e) (1) 275, DCC, DMAP; (2) DIBAL-H; (3) Ac₂O, DMAP, pyridine; (f) (1)TMSBr, CH₂Cl₂; (2) CsOAc, 18-c-6, benzene.

(a) 14, 0 °C 3 d; (b) DCC, CH₂Cl₂, 280, (c) (1) DIBAL-H, −78 °C, (2) Ac₂O, pyr, DMAP; (d) 0.1 eq. BF₃OEt₂, HOAc, hexanes, 140 min, 0 °C; (e) (1) H₂, Pd(OH)₂/C; (2) Swern Ox.; (f) KHMDS, (EtO)₂P(O)CH₂CO₂Et, THF, 25 °C.

(a) Ti(OiPr)₄, 295, MeOH, CH₂Cl₂, −78 °C, then 61, TiCl₄, −78 °C to RT; (b) (1) DMF, H₂O, mw (300 W), 160 °C, 10 min; (2) NaBH₄, MeOH, 0 °C, (3) TIPSCl, imidazole, DMF, 50 °C; (c) (1) 10% Pd-C, H₂, MeOH; (2) Dess–Martin, CH₂Cl₂; (d) ent-288, TiCl₄, CH₂Cl₂, −20 °C to RT; (e) DMF, H₂O, mw (300 W), 160 °C; (f) (1) 10% Pd-C, H₂, MeOH (2) Ph₃PCH₃⁺Br⁻, LDA, THF; (g) (1) Tf₂O, pyr; (2) PhHNOCCCH, LDA, THF; (h) (1) Boc₂O, pyr, DMAP, CH₂Cl₂; (2) LDA, NBS, THF, −100 to −78 °C.

(a) 296, Et₃N, (c-hex)₂BOTf, CH₂Cl₂, −78 °C, 3 h, then 62; (b) (1) TBSOTf, 2,6-lutidine, CH₂Cl₂, 1 h; (2) DIBAL-H, CH₂Cl₂, 0 °C, 15 min; (c) Dess–Martin, CH₂Cl₂, 0 °C to RT, 1 h; (2) E-crotylboronic pinacol ester, hexane, 0 °C to RT, 17 h; (d) 301, Grubb's second generation (30 mol%), Cul (20 mol%), Et₂O, reflux, 3 h; (e) TFA, CH₂Cl₂, H₂O, RT.

(a) (1) BOMCl, DIPEA; (2) lCl, PhSH, Et₃N; (3) TPAP, NMO; (b) (1) LiHMDS; (2) AICl₃, AllylTMS; (c) (1) Li/NH₃; (2) DDQ; (d) (3) TBSCl, imidazole; (4) O₃, PPh₃; (d) (1) allylMgBr; (49% of correct diastereomer) (2) DEAD, PPh₃, p-NBA, K₂CO₃, MeOH (recycled 46% of unwanted diastereomer); (e) (1) BOMCl, DIPEA; (2) ICl, PhSH, Et₃N; (3) (NH₄)₆Mo₇O₂₄-4H₂O, H₂O₂; (f) (1) LiHMDS; (2) NaHMDS, allyl iodide, MeOH; (3) TMSOTf, Et₃SiH.

(a) Hg(OAc)₂, MeOH, NaBH₃CN, THF; (b) LiAI(OtBu)₃, THF; (c) Ac₂O, NaOAc, (d) TMS-allyl, TMSOTf, CH₃CN; (e) O₃, CH₂Cl₂, −78 °C, PPh₃; (f) (−)-(lpc)₂B-allyl, toluene, −78 °C, salt-free then NaBO₃, toluene, H₂O; (g) H₂C=CHCOCl, Et₃N, DMAP; (h) Grubb's cat. (0.2 eq.), Ti(iPrO)₄.

(a) TMSOTf (0.5 eq.), CH₂Cl₂, 20 °C; (b) m-CPBA, CCl₄, RT; (c) LiAIH₄, THF, 0 °C; (d) CH₃MgBr/Cul, THF, 0 °C; (e) Tf₂O, pyr, CH₂Cl₂, −15 °C; (f) (1) TMSOTf, 2,6-lutidine; (2) LDA, THF, TMS-acetylene, HMPA, −10 °C; (g) (1) Dess–Martin, CH₂Cl₂, RT; (2) LiAIH₄, THF, 0 °C; (h) (1) TMSOTf, 2,6-lutidine; (2) NBS, AgNO₃; (3)Bu₃SnH, PdCl₂(PPh₃)₂.

(a) (Me₃Sn)₂CuCNLi₂, THF, −78 °C; then Mel, DMPU, −78 °C to RT; (b) Tf₂O, Et₃N, THF, −78 °C; (c) Pd₂(dba)₃-CHCl₃ (6 mol%), P(t-Bu)₃ (12 mol%), LiCl, NMP, 60 °C.

(a) (1) 333, TiCl₄, DIPEA, CH₂Cl₂, −78 to 0 °C, 0.5 h then 332, −78 to 0 °C, 1 h; (2) NaBH₄, EtOH, 0 °C, 15 min; (3) TBDPSCl, Et₃N, DMAP, CH₂Cl₂, 0 °C to RT, 4 h; (b) Ti(Oi-Pr)₄, (−)-DIPT, TBHP, CH₂Cl₂, 4 Å MS, −20 °C, 3 h; (c) Cp₂TiCl₂, Zn, ZnCl₂, THF, −20 °C to RT, 12 h; (d) H₂, 10% Pd-C, CH₃COONH₄, MeOH, RT, 0.5 h; (e) (1) H₂, Pd-C, MeOH, RT, 24 h; (2) 2,2-dimethoxypropane, CSA, CH₂Cl₂, 0 °C, 1 h; (f) (1) TBAF, THF, 0 °C to RT, 4 h; (2) TBS-OTf, 2,6-lutidine, CH₂Cl₂, 0 °C, 0.5 h; (3) HF-pyr, THF, 0 °C to RT, 18 h; (g) SO₃-pyr, Et₃N, DMSO:CH₂Cl₂ (2:1.6), 0 °C, 0.5 h; (2) TMS-acetylene, n-BuLi, THF, −78 °C to RT, 1 h, then aldehyde, 0 °C, 15 min; (g) (1) MsCl, DMAP, pyridine, 0 °C, 0.5 h; (2) CSA, CH₂Cl₂, 0 °C to RT, 3 h.

(a) l-proline, EtCHO, DMF, 4 °C, 36 h; (b) Ph₃P=CHCOOEt, benzene, 80 °C, 12 h; (c) (1) TBSOTf, 2-6,lutidine, CH₂Cl₂, 0 °C, 1 h; (2) DIBAL-H, CH₂Cl₂, −78 °C to RT, 4 h; (d) (1) (−)-DIPT, Ti(iPrO)₄, TBHP, CH₂Cl₂, −20 °C, 10 h; (2) MeLi, Cul, Et₂O, −20 °C, 6 h; (e) (1) TEMPO, BAIB, CH₂Cl₂, RT, 1 h; (2) Ph₃P=CHCOOEt, benzene, 80 °C, 12 h; (f) TBAF, THF.

(a) 101, [Cu((R,R)-Ph-pybox)]-(SbF₆)₂ (8 mol%), CH₂Cl₂, −98 °C to −78 °C; (b) PPTs, MeOH; (c) K₂CO₃, MeOH; (d) (MeO)₂SO₂, K₂CO₃, acetone; (e) H₂, Pd-C, EtOAc; (f) Swern oxidation.

(a) MnO₂, CH₂Cl₂; (b) MeON(Me)C(O)CH₂,P(O)(OEt)₂, LiCl, DBU, MeCN-CH₂Cl₂ (3:1); (c) DIBAL-H, THF, −78 °C; (d) (1) 358, Sn(OTf), N-ethylpiperidine, CH₂Cl₂, −40 °C, (2) 357, −98 to −78 °C; (e) (1) MeNHOMe.HCl, ImH, CH₂Cl₂; (2) Mel, Ag₂O, Et₂O, reflux; (f) DIBAL-H, THF, −78 °C; (g) nBu₃SnCHI₂, CrCl₂, DMF; (h) NBS, MeCN, 0 °C; (i) NiCl₂-CrCl₂ (10:1), tBuC₅H₄N, THF; 354; (j) TPAP, 4 Å molecular sieves, CH₂Cl₂.

(a) (1) n-BuLi, BF₃Et₂O, −78 °C; (2) NaH, Mel, THF; (3) PTSA, MeOH; (b) (1) LiAIH₄, THF, reflux, 2 h; (2) PPh₃, CCl₄, reflux, 12 h; (c) AD-mix β. 0 °C, 48 h; (d) (1) 2,2-DMP, PTSA, acetone; (2) DDQ, CH₂Cl₂-H₂O (7:3); (e) (1) IBX; (2) LiHMDS; CH₃COOEt, −78 °C; (f) PDC; (g) PPTS, MeOH, 36 h; (h) (1) MOMCl, DIPEA; (2) HCOONa, Nal, DMF, 80 °C, 3 d; (3) NaBH₄, MeOH, 0 °C; (i) Dess–Martin Ox.; (j) 375, NaHMDS, THF, −78 °C.

(a) 379, toluene, 65 °C, 36 h, then H₈-BINOL (5 mol%), Sm(O-i-Pr)₃ (5 mol%), CMHP, THF/toluene 25 °C, 0.7 h; (b) PhSeSePh, NaBH₄, EtOH/AcOH, 25 °C, 15 min; (c) EtSLi, EtOH, 25 °C, 40 min; (d) (1) CH₃I, Ag₂O, MS 3 Å, toluene, 45 °C, 36 h; (2) LiAIH₄, EtO₂, 25 °C for 30 min then reflux for 40 min; (3) TBSCl imidazole, CH₂Cl₂, 25 °C, 50 min, (4) H₂ (5 atm), Pd(OH)₂, AcOEt/EtOH, NaHCO₃, 25 °C, 18 h, (e) (1) PCC, AcONa, MS 3 Å, CH₂Cl₂, 25 °C, 20 min then (CH₃O)₂P(O)C(N₂)COCH₃, K₂CO₃, CH₃OH, 25 °C, 80 min.

(a)⁴²; (b) (1) NalO₄, CH₂Cl₂, 0 °C, 1 h; (2) NaBH₄, MeOH; (3) NaH, THF, BnBr, 0 °C to RT, 12 h; (c) (1) 50% aq. AcOH, RT, overnight; (2) NalO₄, CH₂Cl₂, NaHCO₃, 0 °C to RT, 4 h; (3) Ph₃P⁺CH₂OMeCl⁻, t-BuOK, THF, −78 °C to RT, 4 h; (d) (AcO)₂Hg; NaBH₄, CO₂, 0 °C, THF/H₂O (1:4); (e) 389, TiCl₄, EtN(iPr)₂, CH₂Cl₂, 0 to −78 °C, 1 h; (f) (1) Proton Sponge, Me₃OBF₄, CH₂Cl₂, 0 °C, 24 h; (2) EtOC(O)CH₂COOK, MgCl₂, imidazole, THF, RT, 12 h; (g) PPTS, MeOH/CH₂Cl₂, 0 °C to RT, 3 h.

(a) NaH (2 eq.), t-BuLi (2 eq.), THF, 0 °C; (b) DIBAL-H, THF, −78 °C, 30 min; (c) (1) Me₄NHB(OAc)₃, HOAc, RT, 2 h; (2) HCl-N(OMe)Me, Me₃Al, CH₂Cl₂, heat, 2 h; (d) (1) (MeO)₂C(CH₃)₂, acetone, CSA, RT, 24 h; (2) MeMgBr, THF, 0 °C to RT, 1 h; (e) PPTS, MeOH, RT, 24 h; (f) NaH, Mel, THF, 0 °C, 10 h; (g) TFAA, 2,6-lutidine, CH₃CN, RT, 10 min.

(a) (1) IBCF, N-methylmorpholine, l-serine methyl ester, CH₂Cl₂; (2) Burgess' reagent, THF; (3) CuBr₂, HMTA, DBU, CH₂Cl₂; (b) (1) DIBAL-H, THF; (2) propane-1,3-dithiol, BF₃OEt₂; (c) (1) TBSOTf, 2,6-lutidine, CH₂Cl₂; (2) Hg(ClO₄)₂, CaCO₃, MeCN/H₂O; (d) (1) E-(EtO)₂P(O)CH₂CH=CHCO₂Et, NaH, CH₂Cl₂ 2) AD-Mix β, t-BuOH/H₂O.