Polyketide synthesis using the boron-mediated, anti-aldol reactions of lactate-derived ketones:: Total synthesis of (-)-ACRL, toxin IIIB

The boron-mediated, anti-selective, aldol reactions of ketone 2 (and related derivatives) proceed with high levels of asymmetric induction, diastereoselectivities of up to 200:1 in favour of the aldol adducts 4 are obtained with achiral aldehydes and reagent control operates with chiral aldehydes. These lactate-derived ketones provide a versatile chiral auxiliary for the synthesis of beta-hydroxy carbonyl compounds. Oxidative removal of the auxiliary provides enantiomerically pure aldehydes 5, while reductive deoxygenation gives the corresponding ethyl ketones 6. This practical asymmetric methodology for generating anti-aldols is illustrated by an efficient total synthesis of (-)-ACRL toxin IIIB (7), which proceeds in 15 steps from 2 with 21% overall yield and 88% diastereoselectivity.