Enzymatic construction of highly strained carbocycles

Small carbocycles are structurally rigid and possess high intrinsic energy due to their ring strain. These features lead to broad applications but also create challenges for their construction. We report the engineering of hemeproteins that catalyze the formation of chiral bicyclobutanes, one of the most strained four-membered systems, via successive carbene addition to unsaturated carbon-carbon bonds. Enzymes that produce cyclopropenes, putative intermediates to the bicyclobutanes, were also identified. These genetically encoded proteins are readily optimized by directed evolution, function in Escherichia coli, and act on structurally diverse substrates with high efficiency and selectivity, providing an effective route to many chiral strained structures. This biotransformation is easily performed at preparative scale, and the resulting strained carbocycles can be derivatized, opening myriad potential applications. ; © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. This is an article distributed under the terms of the Science Journals Default License. Received for publication November 8, 2017. Accepted for publication February 6, 2018. We thank D. K. Romney, S. C. Hammer, and S.-Q. Zhang for helpful discussions and comments on the manuscript; C. K. Pier, O. F. Brandenberg, and A. M. Knight for sharing hemeprotein variants; K. Ding (D. J. Anderson Lab, Caltech) and J. Li (R. H. Grubbs Lab, Caltech) for generous donation of materials and reagents; S. C. Virgil and the Caltech Center for Catalysis and Chemical Synthesis, N. Torian and the Caltech Mass Spectrometry Laboratory, and M. K. Takase, L. M. Henling, and the Caltech X-ray Crystallography Facility for analytical support; and B. M. Stoltz for use of polarimeter and chiral gas chromatography equipment. Funding: Supported by NSF Division of Molecular and Cellular Biosciences grant MCB-1513007; Ruth L. Kirschstein NIH Postdoctoral Fellowship F32GM125231 (X.H.); and NSF Graduate Research Fellowship ...