Z-a-Silyl vinyl sulfides

Z-a-Silyl vinyl sulfides are intriguing species since the silyl and the thioether functions exert an opposing polarization on the olefinic bond (Scheme 38). 

Under strictly anhydrous conditions which minimize competing protiodesilylation, (Z)-a-silyl vinyl sulfides with an co-carbonyl moiety attached to the S atom undergo a fluoride-promoted intramolecular cyclization which yields 2-alkenyltetrahydropyran-3-ols. The substrates are derived from (Z)-a-silyl enethiols by alkylation with co-halocarbo-nyl compounds (Scheme 157) <2000EJ02391>. 

Me2AlCl catalyzes the ene reactions of a variety of aldehydes with (Z)-3P-acetoxy-5,17(20)-pregnadiene at —78°C. The stereoselectivity with aliphatic aldehydes is >10 1 in favor of the 22oi-isomer, while aromatic aldehydes produce predominantly the 22p-isomer (eq 7). Me2AlCl is also the Lewis acid of choice for ene reactions of a-halo aldehydes (eq 8). Ene reactions of vinyl sulfides to produce enol silyl ethers are also catalyzed by Me2 AlCl (eq9).i  

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