The chemoenzymatic potential for the construction of complex chiral molecules has not been fully explored. Now, Candida antarctica lipase B has been used to synthesize complex functionalized planar chiral macrocycles, providing a platform for the efficient and sustainable preparation of molecules that are of particular interest in drug discovery.
The creation of new molecules is one of the main goals of chemists. The extremely high complexity of natural products with several stereocentres and planar chirality — which is critical for biological activity — have made the continuous development of innovative approaches necessary for the synthesis of new molecules for drug discovery.
In this regard, transition metal catalysts have been shown to be excellent for many reactions, such as cycloaddition, C–C bonding, oxidation, reduction, and so on1, which facilitated the total synthesis of natural products and a new generation of pharmaceuticals.
Selectivity is a critical challenge in the preparation of these kinds of complex molecules. In this respect, biological enzymes display high selectivity and activity at mild conditions for a manifold of chemical reactions. However, most have evolved a high specificity for their natural cognate substrates, which limits their applicability in synthetic chemistry. Although enzyme engineering can be used to modify the substrate scope, the latter usually remains restricted to a few molecules for single protein mutants.
Lipases are a very impressive example of natural enzymes that present remarkable versatility by recognizing an array of non-natural substrates and generally displaying a very broad substrate scope2. In nature these enzymes catalyse fat hydrolysis, and the reaction is performed at an oil–aqueous interface, where the lipase works well. To achieve this, nature evolved most lipases with a particular catalytic action mechanism. One structural motif among different lipases is the presence of an oligopeptide chain — called a lid — which keeps the active site secluded from the solvent when the lipase is surrounded by water (the so-called closed conformation). In the presence of hydrophobic interfaces, the lid moves away from the active site. This generates the so-called open conformation, uncovering a very highly hydrophobic area for fixing the enzyme on the hydrophobic interface and performing catalysis3. The other important characteristic structural motifs are the active site (which contains the catalytic triad residues and the oxyanion hole) and the well-structured hydrophobic active site binding pocket to accommodate the long lipid chains3.
In particular, a lipase from Candida antarctica fraction B (CALB) is probably the most used enzyme in catalytic applications. Its potential and versatility has been demonstrated in terms of activity, enantio- and regioselectivity in many different processes (transesterification, hydrolysis, Michael addition, and so on)4.
Now, writing in Science, Shawn K. Collins and colleagues from the University of Montreal in Canada, report the enantio- and atroposelective enzymatic synthesis of planar chiral macrocycles by means of CALB. They found that this lipase can promote an enantioselective macrocyclization from common and simple building blocks, whereas similar transformations usually require less available or pre-functionalized substrates5.
The chemoenzymatic chiral macrocyclization procedure involved a one-pot tandem process including sequential lipase acylation of a hydroxyl group of aromatic diols — using diesters as aliphatic linkers — followed by racemization of the monoesterified intermediate molecule via a free rotation of the aromatic ring, and then an enantioselective ring-closing process promoted by a second lipase acylation (Fig. 1).
Using the commercial immobilized form of CALB (Novozym 435), the authors obtained high yields of different enantioenriched planar chiral macrocycles (>99% in almost all cases)5.
A very important issue is how this lipase is able to accommodate the prochiral aromatic plane in the active site environment, discriminating between different conformations of the cyclophane substrate to achieve this extremely high enantioselectivity. Computational docking was employed by the authors to answer this question. The assembly and location of the aromatic diol in the oxyanion channel demonstrated the stabilization of the structure in the cavity with the alcohol extending toward the catalytic serine (Ser 105), which is crucial for the first acylation process6. Within the active site the monoesterified intermediate orients itself with the carboxylic group directed toward Ser 105 and the aromatic substituent in a conformation that mirrors the final synthesized cyclophane. At this point (the serine-catalysed reaction in the second acylation process) it seems to be geometrically challenging to obtain extremely high selectivity for planar chirality. The produced enantiomer is oriented in a way that locates its carboxylic groups close to the Ser 105, with one substituent pointing to the exterior of the active site. However, the docking showed how the opposite enantiomer (the non-produced one) is transported away from the active site (>2.5 Å) because of a clash between a substituent group and leucine 140. This prevents binding to the hydrophobic site delineated by leucine 140, alanine 141 and leucine 144 (lid amino acid)6.
This chemoenzymatic process constitutes a dynamic kinetic resolution transformation that differs from the classical strategy, where excess acylation agent must be used and an additional catalyst — often a transition metal — is mandatory for the racemization step. In this approach, developed by Collins and colleagues, lipase is the sole catalyst employed and no excess of diester linker is used.
The researchers also demonstrated the broad applicability of the method; aromatic rings with different substituents — and even more complex structures, for example biaryl molecules — were successfully transformed by the lipase when used as building blocks5.
Therefore, the concept of using this lipase to accommodate and generate these macrocycles with exquisite selectivity, where the enzyme’s active site can be considered a structural chiral platform, opens research avenues, such as expanding the use of CALB for synthesis of other macrocycles; evaluating its molecular complexity; combining CALB with metal catalysts for sustainable synthesis of highly complex molecules by cascade processes starting from small building blocks; or the possibility of exploring whether other lipases also show CALB’s synthetic capacity.
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Palomo, J.M. Synthetic complexity created by lipases. Nat Catal 3, 335–336 (2020). https://doi.org/10.1038/s41929-020-0453-x
Current Opinion in Green and Sustainable Chemistry (2021)
Polymer International (2020)