Optically pure N-hydroxy-O-triisopropylsilyl-alpha-L-amino acid methyl esters from AlCl3-assisted ring opening of chiral oxaziridines by nitrogen containing nucleophiles

This article reports a straightforward and unprecedented process of AlCl3-assisted oxaziridine ring opening by nitrogen containing nucleophiles, in a totally anhydrous milieu. Under these conditions, nucleophiles exclusively attack the carbon atom of the three-membered heterocycles, obtained from methyl esters of natural alpha-amino acids, generating N-hydroxy-alpha-(L)-amino acid methyl esters. No nitrones, amides, or other side products, either from unwanted rearrangements or due to the attack of the nucleophile on the N atom of the oxaziridine systems, are formed. The hydroxylamine compounds are recovered in excellent yields, after their site-specific conversion into the corresponding P-triisopropylsilyl derivatives, by exposure to triisopropylsilyl triflate in the presence of 1H-imidazole. Derivatization, performed immediately after the recovery of the N-hydroxylated precursors, allows the chiral integrity of the asymmetric (x-carbon atoms in the amino acid methyl esters to be retained. It also protects the obtained compounds from frame degradation by disproportionation. N-Hydroxy-O-triisopropylsilyl-alpha-(L)-amino acid methyl esters are important intermediates in the study of natural alpha-(L)-amino acid metabolic pathways and are ideal candidates as starting materials in the synthesis of biologically, pharmacologically, and nutritionally important N-hydroxy peptides.