Aldol addition reaction of a lithium ester enolate in the solid state

Studies on the synthesis of bafilomycin A(1): stereochemical aspects of the fragment assembly aldol reaction for construction of the C(13)-C25) segment

Highly stereoselective syntheses of aldols 8a-c corresponding to the C(13)-C(25) segment of bafilomycin A(1) were developed by routes involving fragment assembly aldol reactions of chiral aldehyde 6a and the chiral methyl ketones 7. A remote chelation effect plays a critical role in determining the stereoselectivity of the key aldol coupling of 6a and the lithium enolate of 7b. The protecting group for C(23)-OH of the chiral aldehyde fragment also influences the selectivity of the lithium enolate aldol reaction. In contrast, the aldol reaction of 6a and the chlorotitanium enolates of 7a,c were much less sensitive to the nature of the C(15)-hydroxyl protecting group. Studies of the reactions of chiral aldehydes with Takai's (gamma-methoxyallyl)chromium reagent 40 are also described. The stereoselectivity of these reactions is also highly dependent on the protecting groups and stereochemistry of the chiral aldehyde substrates.

A solution to the cyclic aldol problem

[formula: see text] A protocol for achieving stereoselective aldol reactions with cyclic ketones is presented. In terms of yield, the process is particularly effective when a quaternary center at the alpha-carbon of the beta-hydroxy ketone product is created. The stereochemical outcome, anti or syn, is achieved by the Lewis acid-mediated ring expansion of stereochemically homogeneous epoxides in a reaction related to the pinacol rearrangement.