Synthesis of functionalised silanes for use in the asymmetric allylation reaction

摘要

Asymmetric allylation of aldehydes with allyltrichlorosilane reagents, in recent years, has become a powerful synthetic tool towards the enantioselective construction of homoallylic alcohols. In general, the reaction displays good diastereocontrol. Thus, when the allylation is carried out in the presence of a Lewis base, the homoallylic alcohols anti-3 and syn-4 are stereospecifically obtained from the (E)-2 and (Z)-2 silanes, respectively, indicating that the reaction is likely to proceed via a cyclic, chair-like transistion state. Herein, the synthesis of isomerically pure allylsilanes 2, functionalised in the γ-position is reported. This has enabled the range of valuable synthetic intermediates available via the asymmetric allylation reaction of various aromatic aldehydes to be extended. The resultant homoallylic alcohols have two new stereogenic centres. These molecules can now undergo an intramolecular SN2 reaction to afford the corresponding vinyl epoxides 5 and 6 with retention of the relative stereochemistry. A variety of chiral Lewis bases, including pyridine N-oxides and phosphine oxides, were synthesised and screened for asymmetric induction. The most notable result was achieved using chiral phosphine oxide BINAPO, which produced the syn-homoallylic alcohol 4 in 50 % ee.