The N-(1-deoxy-D-fructos-1-yl) derivatives (Amadori compounds) of the endogenous opioid pentapeptide, leucineâ enkephalin (11), leucineâ enkephalin methyl ester (12) and of structurally related peptides (9,10) are synthesized. The equilibrium compositions of the prepared Amadori compounds 9â 12 in D2O and [²H6]DMSO are determined using ¹³C NMR spectroscopy. In water, the β-pyranose, α-furanose and β-furanose forms are detected, the β-pyranose tautomer being the most abundant at equilibrium (67â 75%). The α-pyranose form and open-chain keto form are not detected. In dimethyl sulfoxide, the equilibrium compositions of 9â 12 are markedly shifted towards a higher proportion of furanose forms, amounting to two-thirds of the mixture. In addition to the α- and β-furanoses and β-pyranose tautomers, DMSO solutions of compounds 9â 12 contain at equilibrium a relatively high proportion of the acyclic hydrate (gem-diol) form (ca. 10%).