Nidulalin A (1) was isolated by Kawahara's group from the rice culture of an ascomycetous fungus, Emericella nidulans (Eidam) Vuill. var. lata (Thom and Raper) Subram. (anamorph: Aspergillus nidulellus, Samon and W Gams), strain IN-68.1 The structure of nidulalin A (1) was determined by X-ray crystallography and Mosher's method as well as by comparison of CD spectra with known xanthone derivatives.1 In the later studies, nidulalin A (1) was found to possess potent inhibitory activity against DNA topoisomerase II and immunomodulatory activity.2, 3 Although natural products having the dihydroxanthone skeleton have been seen to be widespread,4, 5, 6, 7, 8 only a few total syntheses have been achieved.9, 10 Interested in the structure and bioactivities of nidulalin A, we embarked on the synthetic studies of the natural product.

An overview of our total synthesis of nidulalin A (1) including the ORTEP drawing of X-ray crystallography of the key intermediates is disclosed in Scheme 1. (The spectrum data of compounds in Scheme 1 as well as 1H and 13C NMR spectra of synthetic (−)-nidulalin A (1) are provided as supplementary information.) The total synthesis started with dimethyl 5-hydroxy-3-methoxyphthalate (2).11 The hydroxy group of 2 was protected as methoxymethyl ether to provide 3, which was submitted to regioselective coupling to give ketone 5. Treatment of bis(methoxymethyl ether) 4 with t-BuLi in the presence of N,N′-tetramethylethylene diamine (TMEDA) at −78 °C produced regioselectively lithiated 4, which reacted with diester 3 regioselectively at the C-1 carbonyl group at −20 °C to provide ketone 5 in 90% yield. Acid hydrolysis of 5 gave trihydroxyketone 6 in quantitative yield. Oxidative cyclization of 6 was performed with lead (IV) tetraacetate (2 eq) in the presence of sodium acetate (23 eq) to afford tricyclic 7, the structure of which was confirmed by X-ray crystallography. (Crystallographic data (excluding structure factors) for the structures of 7, 8, 10, 13, 14, and 1 have been deposited with the Cambridge Crystallographic Data Center as supplementary publication numbers CCDC 728664 for 7, 728665 for 8, 728666 for 10, 728667 for 13, 728668 for 14, and 728669 for 1.)

Scheme 1
figure 1

Reagents and conditions: (a) MOMCl, NaH, DMF, 0 °C to rt, 40 min, quant.; (b) t-BuLi, TMEDA, Et2O-THF, −78 to −20 °C, 2 h, 90%; (c) TsOH•H2O, MeOH, reflux, 5 h, quant.; (d) Pb(OAc)4, AcONa, MeNO2, rt, 10 min, 39%; (e) NaBH(OAc)3, B(OAc)3, THF, rt, 30 min, 79%; (f) DIBAL, THF, −78 °C, 10 min then, 6 N HCl, rt, 14 h, quant.; (g) LiAlH4, THF, −60 °C, 2 h, 60% (recovery of SM, 30%); (h) TMSOTf, Et3N, CH2Cl2, 0 °C, 1 h; (i) SeO2, 1,4-dioxane, 50 °C, 20 h (60% from 11); (j) (−)-camphanic acid, WSCI-HCl, 4-DMAP, CH2Cl2, rt, 30 min, 75% (1:1); (k) K2CO3, MeOH, 0 °C, 5 h, 80% for (−)-nidulalin A (1), 76% for (+)-nidulalin A (ent-1).

Full size image

1,2-Reduction of 7 at the C-2 position with DIBAL (in CH2Cl2, −78 °C, 30 min) followed by hydrolysis of methyl vinyl ether (addition of THF, 6 N HCl, rt, 2 h) gave enone 8 (the ORTEP drawing is shown in Scheme 1). The hydride reduction of 8 gave 4-epi-nidulalin A exclusively.12 As the methoxycarbonyl group of 8 covered the α-face of C-4 carbon, hydride attacked the C-4 carbonyl group from the β-face to give the α alcohol (4-epi-nidulalin A). To invert the face selectivity, we set up cis-fused intermediate 10, which would be submitted to hydride addition from the α-face (the convex face) to give β-alcohol 11.

The cis-fused 9 was derived by 1,4-reduction of the oxidative cyclization product 7. The subsequent regio-selective reduction of C-2 ketone and hydrolysis gave enone 10 whose structure was determined by X-ray crystallography (the ORTEP drawing is shown in Scheme 1). The α-face of 10 was situated as the convex face as expected. Enone 10 was submitted to regio- and stereoselective reduction to afford β-alcohol 11. To avoid reduction at the C-9 position of ketone 10, the reaction mixture was quenched after stirring at −60 °C for 2 h. Dehydrogenation to construct Δ1,9a double bond of nidulalin A was performed in two steps. Treatment of ketone 11 with TMSOTf in the presence of Et3N provided silyl enol ether 12 concomitant with protection of alcohol. Allylic oxidation of 12 proceeded with SeO2 at 50 °C, accompanied by de-O-silylation at position C-4, to give (±)-nidulalin A [(±)−1]. Spectral properties of synthetic (±)−1 were identical with those of natural products including 1H NMR, 13C NMR, IR and MS.1

(±)-Nidulalin A in hand, we next examined resolving the enantiomers with a chiral auxiliary. Esterification of (±)-nidulalin A [(±)−1] with (−)-camphanic acid gave diastereomers separable by silica gel column chromatography. The absolute structures of both isomers were determined by X-ray crystallography (the ORTEP drawing of (4R, 4aS)-nidulalin A ester 13 is shown in Scheme 1). Saponification of camphanic ester of (4R, 4aS)-nidulalin A (13) gave orange crystals of 1, the solution of which showed the optical rotation [α]D24 –570° (c 0.28, CHCl3), levorotatory as the natural nidulalin A12 (orange needles, [α]D25 −463° (c 0.28, CHCl3)). (4S, 4aR)-nidulalin A (ent-1) was also obtained from the other diastereomer 14 by the same procedure as above. (4S, 4aR)-nidulalin A shows the optical rotation [α]D25 +569° (c 0.28, CHCl3), being (+)-nidulalin A. Therefore, (−)-nidulalin A was synthesized to confirm the structure of the natural product.

In conclusion, the first total synthesis of nidulalin A has been achieved. Construction of the stereogenic center at the C-4 position was accomplished through cis-fused tricyclic intermediate 10 to submit the stereospecific reduction. (−)- and (+)-nidulalin A was obtained from (±)-nidulalin A by derivation to (−)-camphanic esters.