Highly Diastereo‐ and Enantioselective Synthesis of Nitrile‐Substituted Cyclopropanes by Myoglobin‐Mediated Carbene Transfer Catalysis

Practical Enzymatic Production of Carbocycles

The Pd-catalyzed carbon-carbon bond formation pioneered by Heck in 1969 has dominated medicinal chemistry development for the ensuing fifty years. As the demand for more complex three-dimensional active pharmaceuticals continues to increase, preparative enzyme-mediated assembly, by virtue of its exquisite selectivity and sustainable nature, is poised to provide a practical and affordable alternative for accessing such compounds. In this minireview, we summarize recent state-of-the-art developments in practical enzyme-mediated assembly of carbocycles. When appropriate, background information on the enzymatic transformation is provided and challenges and/or limitations are also highlighted.

Direct Cyclopropanation of α-Cyano β-Aryl Alkanes by Light-Mediated Single Electron Transfer Between Donor-Acceptor Pairs

Cyclopropanes are traditionally prepared by the formal [2+1] addition of carbene or radical based C1 units to alkenes. In contrast, the one-pot intermolecular cyclopropanation of alkanes by redox active C1 units has remained unrealised. Herein, we achieved this process simply by exposing β-aryl propionitriles and C1 radical precursors (N-oxy esters) to base and blue light. The overall process is redox-neutral and a photocatalyst, whether metal- or organic-based, is not required. Our findings support that single electron transfer (SET) from the α-cyano carbanion of the propionitrile to the N-oxy ester is facilitated by blue-light via their electron donor-acceptor (EDA) complex. The α-cyano carbon radical thus formed can then lose a β-proton to form a π-resonance stabilised radical anion that preferentially couples at the benzylic β-position with a decarboxylated C1 radical unit. This new transition metal-free chemistry tolerates both electron rich and electron deficient (hetero)aryl systems, even sulfide or alkene functionality, to afford a range of cis-aryl/cyano cyclopropanes bearing congested tetrasubstituted quaternary carbons.

Biocatalytic Strategy for the Highly Stereoselective Synthesis of CHF2 -Containing Trisubstituted Cyclopropanes

The difluoromethyl (CHF2 ) group has attracted significant attention in drug discovery and development efforts, owing to its ability to serve as fluorinated bioisostere of methyl, hydroxyl, and thiol groups. Herein, we report an efficient biocatalytic method for the highly diastereo- and enantioselective synthesis of CHF2 -containing trisubstituted cyclopropanes. Using engineered myoglobin catalysts, a broad range of α-difluoromethyl alkenes are cyclopropanated in the presence of ethyl diazoacetate to give CHF2 -containing cyclopropanes in high yield (up to >99 %, up to 3000 TON) and with excellent stereoselectivity (up to >99 % de and ee). Enantiodivergent selectivity and extension of the method to the stereoselective cyclopropanation of mono- and trifluoromethylated olefins was also achieved. This methodology represents a powerful strategy for the stereoselective synthesis of high-value fluorinated building blocks for medicinal chemistry, as exemplified by the formal total synthesis of a CHF2 isostere of a TRPV1 inhibitor.

An Enzymatic Platform for the Highly Enantioselective and Stereodivergent Construction of Cyclopropyl-δ-lactones

Abiological enzymes offers new opportunities for sustainable chemistry. Herein, we report the development of biological catalysts derived from sperm whale myoglobin that exploit a carbene transfer mechanism for the asymmetric synthesis of cyclopropane-fused-δ-lactones, which are key structural motifs found in many biologically active natural products. While hemin, wild-type myoglobin, and other hemoproteins are unable to catalyze this reaction, the myoglobin scaffold could be remodeled by protein engineering to permit the intramolecular cyclopropanation of a broad spectrum of homoallylic diazoacetate substrates in high yields and with up to 99 % enantiomeric excess. Via an alternate evolutionary trajectory, a stereodivergent biocatalyst was also obtained for affording mirror-image forms of the desired bicyclic products. In combination with whole-cell transformations, the myoglobin-based biocatalyst was used for the asymmetric construction of a cyclopropyl-δ-lactone scaffold at a gram scale, which could be further elaborated to furnish a variety of enantiopure trisubstituted cyclopropanes.

Diazoacetonitrile (N2 CHCN): A Long Forgotten but Valuable Reagent for Organic Synthesis

Diazoacetonitrile (N₂CHCN) is a small reactive diazoalkane. It has been synthesized for the first time already in 1898 by Theodor Curtius, however, did not gain much recognition in organic synthesis until recently. Only in 2015, after introduction of in situ and flow protocols for the safe generation of diazoacetonitrile, it started gaining popularity. In this minireview, the synthetic properties and applications of this valuable reagent are discussed.

Biocatalytic Strategy for Highly Diastereo- and Enantioselective Synthesis of 2,3-Dihydrobenzofuran-Based Tricyclic Scaffolds

2,3-Dihydrobenzofurans are key pharmacophores in many natural and synthetic bioactive molecules. A biocatalytic strategy is reported here for the highly diastereo- and enantioselective construction of stereochemically rich 2,3-dihydrobenzofurans in high enantiopurity (>99.9% de and ee), high yields, and on a preparative scale via benzofuran cyclopropanation with engineered myoglobins. Computational and structure-reactivity studies provide insights into the mechanism of this reaction, enabling the elaboration of a stereochemical model that can rationalize the high stereoselectivity of the biocatalyst. This information was leveraged to implement a highly stereoselective route to a drug molecule and a tricyclic scaffold featuring five stereogenic centers via a single-enzyme transformation. This work expands the biocatalytic toolbox for asymmetric C-C bond transformations and should prove useful for further development of metalloprotein catalysts for abiotic carbene transfer reactions.

Hemoprotein-Catalyzed Cyclopropanation En Route to the Chiral Cyclopropanol Fragment of Grazoprevir

Reactions that were once the exclusive province of synthetic catalysts can increasingly be addressed using biocatalysis. Through discovery of unnatural enzyme reactions, biochemists have significantly expanded the reach of enzymatic catalysis to include carbene transfer chemistries including olefin cyclopropanation. Here we describe hemoprotein cyclopropanation catalysts derived from thermophilic bacterial globins that react with diazoacetone and an unactivated olefin substrate to furnish a cyclopropyl ketone, a previously unreported reaction for enzyme catalysts. We further demonstrate that the resulting cyclopropyl ketone can be converted to a key cyclopropanol intermediate that occurs en route to the anti-hepatitis C drug grazoprevir.

Tryptamine Synthesis by Iron Porphyrin Catalyzed C-H Functionalization of Indoles with Diazoacetonitrile

The functionalization of C-H bonds with non-precious metal catalysts is an important research area for the development of efficient and sustainable processes. Herein, we describe the development of iron porphyrin catalyzed reactions of diazoacetonitrile with N-heterocycles yielding important precursors of tryptamines, along with experimental mechanistic studies and proof-of-concept studies of an enzymatic process with YfeX enzyme. By using readily available FeTPPCl, we achieved the highly efficient C-H functionalization of indole and indazole heterocycles. These transformations feature mild reaction conditions, excellent yields with broad functional group tolerance, can be conducted on gram scale, and thus provide a unique streamlined access to tryptamines.