We describe a tandem Mitsunobu/3,3-sigmatropic rearrangement of allylic azides on a chiral auxiliary system that favors one regioisomer thanks to its exceptional steric bias. The sequence may be completed by the oxidative cleavage of the auxiliary or by a ring-closing metathesis reaction that produces a carboor heterocycle directly and a recyclable form of the chiral auxiliary. Applications of the methodology to the total synthesis of (+)-coniine, (+)-lentiginosin, and (+)-pumiliotoxin C are reported.
Vinylalanes, prepared from the zirconium-catalyzedcarboalumination of alkynes, were added directly to aldehydes bearing an alpha chiral carbon to give good yields of the corresponding alcohols. The addition was more stereoselective than the addition of the corresponding vinyllithium or vinylmagnesium bromide.
[GRAPHICS]AlMe3 dramatically increases the diastereoselectivity of addition of vinyllithiums to alpha-chiral aldehydes but decreases that of methyllithium. Our results are explained in terms of an addition of the free vinyllithium on the Me3Al-aldehyde complex.