n accordance with a preferred embodiment of the invention thecarboxylic acid may be of the formula: ##STR7## in which R.sup.1 is a hydrogen atom or acyl group, particularly a substituted acetyl group such as a phenylacetyl; alpha-aminophenylacetyl; alpha-aminoparahydroxyphenylacetyl; phenoxyacetyl;alpha-carboxyphenylacetyl or alpha-carboxy-3-thienylacetyl group or, when the carboxylic acid is of t