Anomalous Zemplén deacylation reactions of 2-O-acyl-3-O-alkyl or -3-O-glycosyl derivatives of d-galactose and d-glucose: synthesis of O-α-d-mannopyranosyl-(1→4)-O-α-l-rhamnopyranosyl-(1→3)-d-galactose and an intermediate for the preparation of 2-O-glycosyl-3-O-(α-d-mannopyranosyl)-d-glucoses