Hemiacetals and their esters as side-products in lipase-catalysed transesterifications of vinyl esters with sterically hindered alcohols

Lipase-catalysed transesterifications of vinyl esters with various sterically hindered secondary alcohols sometimes give hemiacetals and hemiacetal esters as major side-products along with the expected esters, especially in the presence of aldehydes. The substrates, reaction conditions, and the lipases required for the formation of such hemiacetal products have been studied. Hemiacetals and their esters are very easily hydrolysed. Therefore, when conventional work-up procedures are used, the formation of such products in lipase-catalysed transesterification reactions may easily escape notice, leading to, e.g. an unexpectedly low enantiomeric purity of the isolated remaining substrate in a resolution reaction.