Enantioselective Allylations of Selected alpha, beta, gamma, delta-Unsaturated Aldehydes by Axially Chiral N,N'-dioxides. Synthesis of the Left-hand Part of Papulacandin D
Abstract
Background: Catalytic asymmetric allylation of aldehydes is a synthetically useful method for preparation of chiral homoallylic alcohols that may serve as convenient synthetic building blocks. The catalysis can be brought about with Lewis acid or Lewis bases, and Bronsted acids.
Objective: The main aim was to evaluate enantioselective allylation of variously substituted alpha, beta, gamma, delta-aldehydes with allyltrichlorosilane catalyzed by axially chiral Lewis bases with bis(tetrahybis(tetrahydroisoquinoline) N, N'-dioxide scaffold. Method: Allylations of several structurally different alpha, beta, gamma, delta-aldehydes were carried out in various solvents in the presence of a catalytic amount of an N, N'-dioxide to access conditions leading to the highest symmetric induction.
Results: Out of screening of several reaction conditions the highest asymmetric induction (up to 98% ee) was achieved in THF. A higher asymmetric induction was observed with substrates bearing additional substituents on alpha-carbon atom.
Allylation of (S)-(2E, 4E)-2,8-dimethyldecadienal gave rise to the corresponding homoallylic alcohol (an intermediate for synthesis of papulacandin D) with a high asymmetric induction of 92% ee. Conclusion: Enantioselective allylation of alpha, beta, gamma, delta-aldehydes with allyltrichlorosilane catalyzed by an axially chiral Lewis base gave rise to the corresponding homoallylic alcohols with a good enantioselection.
This method could be used for synthesis of valuable building block for organic synthesis as it was demonstrated by a synthesis of a papulacandin D intermediate.