With bulky silyl groups, synthesis

An intermediate in the synthesis of laulimalide by Davidson8 illustrates the differential protection of alcohols. The starting materials 56 and 57 already have an alcohol protected as a TBDMS silyl ether and a diol protected as an acetal. The alcohol in 58 is protected as a p-methoxybenzyl ether and the acetal hydrolysed by acetal exchange to give the free diol 60. Selective protection of the primary alcohol by a bulky acyl group (pivaloyl, i-BuCO ) 61 allows silylation of the secondary alcohol with a TIPS group 62. Finally the pivaloyl group is selectively removed by DIBAL reduction to release just one free alcohol 63. 

The asymmetric total syntheses of mtamycin B and oligomycin C was accomplished by J.S. Panek et al. In the synthesis of the C3-C17 subunit, they utilized a Mukaiyama aldol reaction to establish the C12-C13 stereocenters. During their studies, they surveyed a variety of Lewis acids and examined different trialkyl silyl groups in the silyl enol ether component. They found that the use of BFs OEta and the sterically bulky TBS group was ideal with respect to the level of diastereoselectivity. The stereochemical outcome was rationalized by the open transition state model, where the orientation of the reacting species was anti to each other, and the absolute stereochemistry was determined by the chiral aldehyde leading to the anti diastereomeric Felkin aldol product. 

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