Organocuprate induced allylic versus 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 Acanthoscelides obtectus

The sulfinates (E)‐R1‐C≡C‐C(R2, CH=CHR3)‐OS(O)Me (1) have been treated with [R4CuBr]MgX·LiBr (4) in tetrahydrofuran. The ratio allylic versus propargylic 1,3‐substitution in 1, leading to R1‐C≡C‐CR2=CH‐CHR3R4 (2, Z/E ≥ 86/14) and (E)‐R1R4C=C=CR2‐CH=CHR3 (3) respectively, depends on the size of the substituents R1 and R3. As could be concluded from the reaction of (E)‐1 (R1=R2=H, R3= ‐CH(OEt)2) and of the corresponding methanesulfonate (E)‐5 with the heterocuprate [n‐C8H17CuBr]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‐C8H17)2CuMgBr·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).