Abstract
Asymmetric crotylation has firmly earned a place among the set of valuable synthetic
tools for stereoselective construction of carbon skeletons. For a long time the field
was heavily dominated by reagents bearing stoichiometric chiral auxiliaries, but now
catalytic methods are gradually taking center stage, and the area continues to develop
rapidly. This account focuses primarily on preformed organometallic reagents based
on silicon and, to some extent, boron. It narrates our endeavors to design new and
efficient chiral Lewis base catalysts for the asymmetric addition of crotyl(trichloro)silanes
to aldehydes. It also covers the development of a novel protocol for kinetic resolution
of racemic secondary allylboronates to give enantio- and diastereomerically enriched
linear homoallylic alcohols. As a separate topic, cross-crotylation of aldehydes by
using enantiopure branched homoallylic alcohols as a source of crotyl groups is discussed.
Finally, the synthetic credentials of the developed methodology are illustrated by
total syntheses of marine natural products, in which crotylation plays a key role
in setting up stereogenic centers.
1 Introduction
2 Pyridine N-Oxides as Lewis Base Catalysts
3 Bipyridine N,N′-Dioxides as Lewis Base Catalysts
4 Chiral Allylating Reagents
5 Synthetic Applications
6 Concluding Remarks
Key words
allylation - asymmetric catalysis - crotylation - enantioselectivity - stereoselectivity
- Lewis bases
