Organocatalyzed C-C Ring Construction: The Ito Synthesis of Guignardone H

M. Kevin Brown of Indiana University used an N-protonated oxazaborolidene organocatalyst to mediate the addition of 2 to 1, to give the cyclobutane 3 (Tetrahedron 2019, 75, 3265. DOI: 10.1016/j.tet.2019.04.028). Yu-Fei Ao of the Institute of Chemistry, Chinese Academy of Sciences used whole cells of Rhodococcus erythropolis AJ270 to effect the enantioselective hydrolysis of 4, leading after benzylation to 5 (Org. Chem. Front. 2019, 6, 808. DOI: 10.1039/C9QO00069K). Benjamin List of the Max-Planck-Institut für Kohlenforschung used a BINOL-derived imidodiphosphorimidate for the Nazarov reaction of 6 to 7 (J. Am. Chem. Soc. 2019, 141, 3414. DOI: 10.1021/jacs.8b13899). Song Ye, also of the Institute of Chemistry, Chinese Academy of Sciences, used an NHC catalyst to mediate the construction of 10 by the addition of 8 to 9 (Angew. Chem. Int. Ed. 2019, 58, 1183. DOI: 10.1002/anie.201813047).

Professor List also found that a chiral phosphoric acid mediated the enantioselective hydroxylation of 11 to 12 (Synlett 2019, 30, 49. DOI: 10.1055/s-0037-1611084). Jeffrey S. Johnson of the University of North Carolina used a quaternary ammonium phase transfer catalyst to cyclize 13 to 14 (J. Am. Chem. Soc. 2019, 141, 2645. DOI: 10.1021/jacs.8b13006). Tobias A. M. Gulder of the Technical University of Munich showed that modifying the ester of 15 made it an effective substrate for conversion to 16 by the flavin-dependent monooxygenase SorbC (Org. Lett. 2019, 21, 4520. DOI: 10.1021/acs.orglett.9b01398). John C.-G. Zhao of the University of Texas at San Antonio devised a catalyst self-assembled from Cinchona alkaloids and amino acid derivatives to direct the addition of 17 to 18, leading to 19 (Adv. Synth. Catal. 2019, 361, 208. DOI: 10.1002/adsc.201800987).

Mathias Christmann of the Freie Universität Berlin established that the proline-derived Hayashi-Jorgensen catalyst mediated the ring expansion of 20 to 21 with near-full retention of enantiomeric excess (Angew. Chem. Int. Ed. 2019, 58, 5075. DOI: 10.1002/anie.201813880). Eiji Yamamoto and Makoto Tokunaga of Kyushu University using a phosphonium phase transfer catalyst to effect the enantioselective protonation of the enolate derived from 22 with 23, to deliver the ketone 24 in high ee (Org. Lett. 2019, 21, 4030. DOI: 10.1021/acs.orglett.9b01216).

Efraim Reyes and Jose L. Vicario of the University of the Basque Country used an enantiomerically-pure phosphine to direct the transannular Morita-Baylis-Hillman cyclization of 25 to 26 (J. Am. Chem. Soc. 2019, 141, 9495. DOI: 10.1021/jacs.9b03679). Francesca Leonelli of the Università degli Studi di Roma "La Sapienza" used stoichiometric tyrosine to effect the intramolecular cyclization of the prochiral 27 to 28 (Eur. J. Org. Chem. 2019, 1594. DOI: 10.1002/ejoc.201801771).

Guignardone H (31) was isolated from the flowering plant Scyphiphora hydrophyllacea. Hisanaka Ito of the Tokyo University of Pharmacy and Life Sciences prepared the enantiomerically-enriched cyclohexenone core 30 via Shi epoxidation of the Birch reduction product 29. In the course of the synthesis, the authors corrected the structure of 31 to that shown (Org. Lett. 2019, 21, 3008. DOI: 10.1021/acs.orglett.9b00486).