Methyl-branched octanoic acids as substrates for lipase-catalyzed reactions

Hydrolyses of racemic methyl-branched octanoic acid thiolesters are described using six commercial lipases as catalysts. Branching at positions 2, 4 and 5 greatly reduced activity; branching at the 3-position virtually eliminated activity. The reactivities of the racemic branched thiolesters relative to the unbranched ester were very similar for each lipase preparation examined. In reactions involving configurationally pure 2-methyloctanoic acids, the S-enantiomer reacted faster both in esterification of aliphatic alcohols and in hydrolyses of aliphatic alcohol esters with all of the lipases examined. Stereobiases in hydrolyses of the octanoic acid esters branched at other positions were low and variable. In sharp contrast to the hydrolyses of the thiolesters of 2-methyloctanoic acid, two aryl esters of 2-methyloctanoic acid catalyzed by R. miehei lipase hydrolyzed with a bias for the R-configuration. A view of the ester-enzyme complex is offered to explain the relative rates of reaction of the racemic esters.