Synthesis of the 4-methyl-1,2-oxazetidine-4-carboxylic acid moiety of the originally proposed halipeptin A and B structures

O-Alkylation of N-hydroxycarbamate 6 with iodo ester 5 affords 15, which was elaborated to mesylate 4. Intramolecular N-alkylation affords methyl N-Boc-4-methyl-1,2-oxazetidine-4-carboxylate (3). The geminal coupling constant of the methylene protons is 8.5 Hz, which is much smaller than the 12.0 Hz reported for halipeptins A and B. This confirms that the halipeptins do not contain an oxazetidinecarboxylic acid as originally proposed in structure 1, but a thiazoline as in the revised structure 2. The unusual O-alkylation of 5 probably proceeds by an electron transfer mechanism.