Trimethylsilylcyanid als Umpolungsreagens, XX. Zur Regioselektivität der Addition von Carbonyl‐;Verbindungen an umgepolte α,β‐ungesättigte Aldehyde

Trimethylsilyl Cyanide — A Reagent for Umpolung, XX. — On the Regioselectivity of the Addition of Carbonyl Compounds to α,β‐Unsaturated Aldehydes after Umpolung By addition to t BuMe 2 SiCN to α,β‐unsaturated aldehydes 5 the silylated cyanohydrins 6 are produced. Deprotonation of 6 provides allylic anions which can be attached to aldehydes and ketones in α‐position (kinetic control) (→ 8 ) or γ‐position (thermodynamic control) (→ 9 ). In contrast to the Me 3 Si group which has been applied so far the t BuMe 2 Si group is stable under the reaction conditions. Therefore, the ratio of α/γ adducts can be tuned from >95:5 to <5:95 depending on the substitution pattern of 6 , the aldehydes and ketones, the reaction time and temperature, and even by some additives. The γ adducts 9 are easily transformed into γ‐butyrolactones 27 by acidic hydrolysis.