Polyketide building blocks via diastereoselective nitrile oxide cycloadditions with homoallylic alcohols and monoprotected homoallylic diols.

A modular approach to Delta(2)-isoxazolines, latent aldol adducts and polyketide building blocks, is reported. The magnesium-mediated, hydroxyl-directed method allows for the diastereoselective access to a wide variety of masked beta-hydroxy ketones, starting from readily available aliphatic and aromatic oximes, homoallylic alcohols and monoprotected homoallylic diols. The utility of the prepared Delta(2)-isoxazolines as polyketide building blocks is demonstrated by their ready conversion into the corresponding beta-hydroxy ketones. The anti-diastereoselectivity of the reaction was established by derivatization, NOE studies and comparison of known compounds. A rationale for the observed diastereoselectivity is proposed.