Chemoselective Synthesis of N-Protected Alkoxyprolines under Specific Solvation Conditions

N-Protected hydroxyprolines (Hyp) were transformed chemoselectively into alkoxyproline derivatives by direct O-alkylation. The starting Hyp was transformed into the corresponding dianion in a mixture of dimethyl sulfoxide and tetrahydrofuran(1:16 v/v) as solvent. Under these conditions, the carboxy-anion showed reduced nucleophilicity because it was specifically solvated, and the more reactive oxy-anion was selectively alkylated. N-Protected trans-4-alkoxy-, cis-4-alkoxy- and trans-3-alkoxyprolines were thus obtained in a single step in very high overall yields and with complete stability of the stereogenic center configuration.