Metal-Mediated C-C Ring Construction: The Fox Synthesis of Piperarborenine B

Glenn C. Micalizio of Dartmouth College observed (Org. Lett. 2016, 18, 1250. DOI: 10.1021/acs.orglett.6b00072) that addition of the alkyne 2 to the cyclopropene 1 proceeded with high regio- and steroselectivity. The same reaction on the desilylated material delivered a product with the alkenyl substituent syn to the hydroxymethyl group (not illustrated). M. K. Brown of Indiana University found (Org. Biomol. Chem. 2016, 14, 5477. DOI: 10.1039/C5OB01837D) that Lewis acid promoted the heretofore unavailable 2+2 cycloaddition of the ketene derived from 5 to the alkene 4 to give 6.

Mariola Tortosa of the Universidad Autonoma de Madrid achieved (Angew. Chem. Int. Ed. 2016, 55, 6969. DOI: 10.1002/anie.201601976) remarkable enantioselectivity in the borylation of 7, leading to the lactone 8. In the course of a synthesis of hamigeran B, Jian-Hua Xie and Qi-Lin Zhou of Nankai University reduced the ketone 9 to the alcohol 10. Minoru Isobe, now at the Chulabhorn Research Institute, added (J. Org. Chem. 2016, 81, 1571. DOI: 10.1021/acs.joc.5b02735) 12 to 11 to give, via Brook rearrangement, the sulfonyl ketone 13. Antonio M. Echavarren of ICIQ effected (Angew. Chem. Int. Ed. 2016, 55, 7121. DOI: 10.1002/anie.201601834) the diastereoselective Au-mediated cyclization of 14 to 15.

Carlos Valdés of the Universidad de Oviedo coupled (Chem. Eur. J. 2016, 22, 6253. DOI: 10.1002/chem.201600837) the diazo alkane (J. Org. Chem. 2008, 73, 9479. DOI: 10.1021/jo8017704) derived from the tosylhydrazone 16 with the vinylboronic acid 17 to give the equatorial product 18. Jan Streuff of the Albert-Ludwigs-Universität Freiburg added (Chem. Eur. J. 2011, 17, 5507, DOI: 10.1002/chem.201100501; Org. Biomol. Chem. 2016, 14, 5673, DOI: 10.1039/C5OB02631H) acrylonitrile 20 to the enone 19 to give 21. Zhi-Xiang Yu of Peking University uncovered (Tetrahedron 2016, 72, 2752. DOI: 10.1016/j.tet.2016.01.046) a non-photochemical protocol for the Fe-mediated cyclocarbonylation of 22 to the cyclohexenone 23. Vy M. Dong of the University of California, Irvine cyclized (J. Am. Chem. Soc. 2016, 138, 3310. DOI: 10.1021/jacs.6b01445) 24 to 25 with high diastereo- and enantiocontrol.

Young Keun Chung of Seoul National University assembled (Synlett 2016, 27, 455. DOI: 10.1055/s-0035-1560806) the cyclooctadiene 27 by the direct cyclization of 26. Charles E. Jakobsche of Clark University found (Tetrahedron Lett. 2016, 57, 2782. DOI: 10.1016/j.tetlet.2016.05.039) that intramolecular alkylation converted 28 to the inside-outside macrocycle 29.

Pipararborenine B (32) shows significant and selective anti-cancer activity. Joseph M. Fox of the University of Delaware assembled (Angew. Chem. Int. Ed. 2016, 55, 4983. DOI: 10.1002/anie.201600766) the central cyclobutane core of 32 by cyclizing 30 to the bicyclobutane, that was opened to 31 by Cu-catalyzed Grignard addition.