Abstract: Stable .alpha.-substituted boronic esters, e.g. for use in peptide synthesis, are prepared by reacting a substituted alkene of the formula: ##STR1## with a disubstituted borane of the formula: ##STR2## wherein: (i) R.sup.1 and R.sup.2 are each selected from various groups which are preferably not leaving groups;(ii) X is halogen or other leaving group;(iii) Y is H or lower alkyl;(iv) Q.sup.1 and Q.sup.2 are each selected from various groups which are preferably such that the borane is non-hydrolyzable, and particularly Q.sup.1 and Q.sup.2 together may represent a residue of a diol such as catechol or pinanediol.