New methods for probing the chirality of 18O-labelled phosphinic acids

Abstract

The chirality of singly ¹⁸O-labelled methyl(phenyl)phosphinic acid can be probed by conversion into the diastereoisomeric myrtenyl esters by the action of myrtenyl bromide on the potassium salt of the acid in Me₂SO in the presence of 18-crown-6. The Mitsunobu reaction applied to the esterification of singly labelled methyl(phenyl)phosphinic acid with myrtenol gives some doubly labelled ester. The minor retention of configuration at phosphorus that occurs in the alkaline hydrolysis of (S_P)-methyl methyl(phenyl)phosphinate is due to racemisation of the ester by methoxide ion formed in the hydrolysis. Alkaline hydrolysis of the corresponding (S_P)-menthyl ester occurs with > 98% inversion of configuration at phosphorus.