Organocuprate induced allylic <i>versus</i> propargylic 1,3‐substitution in sulfinates and sulfonates derived from 3‐hydroxy‐1‐penten‐4‐ynes. Improved synthesis of a precursor for the presumed sex attractant of <i>Acanthoscelides obtectus</i>

Abstract

Abstract The sulfinates ( E )‐R 1 ‐C≡C‐C( R 2 , CH=CHR 3 )‐OS(O)Me ( 1 ) have been treated with [R 4 CuBr]MgX·LiBr ( 4 ) in tetrahydrofuran. The ratio allylic versus propargylic 1,3‐substitution in 1 , leading to R 1 ‐C≡C‐CR 2 =CH‐CHR 3 R 4 ( 2 , Z/E ≥ 86/14) and ( E )‐R 1 R 4 C=C=CR 2 ‐CH=CHR 3 ( 3 ) respectively, depends on the size of the substituents R 1 and R 3 . As could be concluded from the reaction of ( E )‐ 1 (R 1 =R 2 =H, R 3 = ‐CH(OEt) 2 ) and of the corresponding methanesulfonate ( E )‐ 5 with the heterocuprate [ n ‐C 8 H 17 CuBr]MgBr·LiBr, the ratio 2/3 is also influenced by the nature of the leaving group. A very high regioselectivity has been found if ( E )‐ 5 was reacted with the homocuprate ( n ‐C 8 H 17 ) 2 CuMgBr·LiBr instead of the heterocuprate. The latter reaction is part of an attractive route to the presumed sex attractant of the Male Dried Bean Beetle (Acanthoscelides obtectus).