Treatment of radical precursor 15a having a vinyl sulfide moiety with Bu3SnH in the presence of AIBN in boiling benzene afforded exclusively the 6-exo cyclization product 16a, whereas similar treatment of the exo-methylene compound 15b gave a mixture of the 6-exo cyclization product 16b and the endo-olefin product 17 formed by a 1,5-hydrogen shift. Based on these findings, the synthesis of (-)-aphanorphine was achieved using a sulfur-directed 6-exo-selective aryl radical cyclization of 22. (C) 2003 Elsevier Ltd. All rights reserved.
Synthesis of (−)-Aphanorphine Using Aryl Radical Cyclization
[structure: see text] The synthesis of (-)-aphanorphine was achieved by using Bu(3)SnH-mediated aryl radicalcyclization of 1-benzyloxycarbonyl-2-(2-bromo-4-methoxyphenylmethyl)-2-methoxycarbonyl-4-(phenylthiomethylene)pyrrolidine, leading to exclusive formation of the 6-exo cyclization product.