Synthesis of an Enantiomerically Pure Serine-Derived Thiazole

Previously reported methods for preparing enantiomerically pure thiazoles are inadequate for the synthesis of inherently labile O-alkyl serine-derived thiazoles. The intermediateN-Boc-(O-methylseryl) thiazolines are very susceptible to tautomerization, even under neutral conditions (Scheme 5). Herein, it is demonstrated that the choice of N-protecting group is critical to the preservation of enantiomeric purity. Thus, using an N-trityl protecting group, O-methyl serine was converted to the corresponding N-Boc-(O-methylseryl) thiazole 3 with no appreciable epimerization as indicated by (19)F and (1)H NMR of the corresponding Mosher's amide.