Synthesis and diazomethane-catalysed 1→2 acyl migration of the l-arabinosyl esters of N-acylamino acids

The fully benzylated α- and β-l-arabino-pyranosyl (1 and 2) and -furanosyl esters (3 and 4) of N-acetyl-d-alanine and N-tert-butoxycarbonyl-l-phenylalanine have been synthesised. Catalytic hydrogenation of 3 and 4 gave both anomers of 1-O-(N-tert-butoxycarbonyl-l-phenylalanyl)-l-arabino-pyranose (5) and -furanose (6), which were characterised as the triacetates 7 and 8, respectively. Treatment of the cis-oriented β-anomers of 5 and 6 with 0.5 equiv. of diazomethane at 0° for 1 h led to the 1→2 acyl rearrangement, with pyranose—furanose interconversion and anomerisation, to give, upon acetylation, a mixture of 1,3,4- and 1,3,5-tri-O-acetyl-2-O-(N-tert-butoxycarbonyl-l-phenylalanyl)-α,β-l-arabino-pyranose and -furanose, the structures of which were determined by ¹H- and ¹³C-n.m.r. spectroscopy. The 1→2 acyl-migration step in the l-arabino series is immediately followed by isomerisation into the four possible forms.