Exclusive alpha-coupling in the aldol reaction of unsaturated trimethylsilyl esters: an efficient and practical direct synthesis of unsaturated beta-hydroxy acids.

The lithium enolates of trimethylsilyl but-3-enoate and 3-methylbut-3-enoate reacted with aldehydes and saturated or aromatic ketones at -70 °C to give exclusively the α-condensation products in excellent yields. The unsaturated β-hydroxy acids thus obtained were directly identified, and the usual conversion into their methyl esters with diazomethane was not necessary. Unsaturated ketones underwent Michael reaction through α-addition leading to the unsaturated 5-oxo acids.