SCOPE AND MECHANISM OF DEPROTECTION OF CARBOXYLIC ESTERS BY BIS(TRIBUTYLTIN) OXIDE

Methyl and ethyl esters of aliphatic and aromatic carboxylic acids as well as benzyl carboxylates, thiol esters and double esters such as (pivaloyloxy)methyl carboxylates have been successfully cleaved with bis(tributyltin) oxide to give the free carboxylic acids in good yields. The reaction is carried out in aprotic solvents under essentially neutral conditions and thus this method can serve as an ideal procedure for the cleavages of esters with other functional groups and/or protecting groups acid and/or base sensitive. We demonstrated that the reaction displays a high level of chemoselectivity between methyl and ethyl esters versus tert-butyl esters and gamma-lactones. Bis(tributyltin) oxide is also a highly efficient reagent for the cleavage of acetates of primary and secondary alcohols and phenols. The limitations we found in the use of this reagent include the lack. of cleavage of esters sterically hindered around the carboxyl carbon and the carbinol group (i.e., esters of tertiary alcohols) and in carboxylic esters that contain a fluoroalkyl substituent. A resonable mechanistic explanation is discussed to account for the reaction pathway of the acyloxygen cleavage of(-)-(1R)-menthyl acetate.