.beta.-aminothiol ester and process for preparing thereof

A process for preparing a .beta.-hydroxythiol ester having the formula (I):

• ##STR27## wherein R.sup.1 is an alkyl group having from 1 to 4 carbon atoms or phenyl, R.sup.2 is an amino-protecting group selected from the group consisting of benzyl, methoxyphenyl, and trialkylsilyl wherein each alkyl group has from 1 to 4 carbon atoms, and R.sup.3 is a hydroxy-protecting group selected from the group consisting of benzyl, tetrahydropyranyl, and trialkylsilyl wherein each alkyl group has from 1 to 4 carbon atoms, which comprises reacting a .beta.-hydroxythiol ester having the formula (III):
• ##STR28## where R.sup.1 is as above, with a boron compound having the formula (IV):
• ##STR29## wherein each of R and R' is an alkyl group or a cycloalkyl group, or R and R' taken together form a ring including the boron atom, in the presence of a tertiary amine, the obtained reaction mixture being reacted with an imine having the formula (V):
• ##STR30## wherein R.sup.2 and R.sup.3 are as above, followed by treating the resultant product with hydrogen peroxide.