An Efficient, Practical, and Enantioselective Method for Synthesis of Homoallenylamides Catalyzed by an Aminoalcohol-Derived, Boron-Based Catalyst

A practical catalytic
method for enantio­selective addition
of an allene unit to aldimines is disclosed. Transformations are promoted
by an in-situ-generated B-based catalyst that is derived from a simple,
robust, and readily accessible (in multigram quantities) chiral aminoalcohol.
A range of aryl-, heteroaryl-, and alkyl-substituted homoallenylamides
can be obtained in 66–91% yield and 84:16 to >99:1 enantiomeric
ratio through reactions performed at ambient temperature and in the
presence of 0.1–3.0 mol% of the chiral catalyst and a commercially
available allenylboron reagent. The catalytic protocol does not require
strict anhydrous conditions, can be performed on gram scale, and promotes
highly selective addition of an allenyl unit (vs a propargyl group).
The utility of the approach is demonstrated through development of
succinct approaches to syntheses of anisomycin and epi-cytoxazone.