Lactones of methyl 3-O

Lactones of methyl 3-O-[(R)- and (S)-1-carboxyethyl]-alpha-D-gluco-, galacto- and manno-pyranoside were prepared by treatment of the sugar derivatives in acetic acid. The lactones were formed between the 1-carboxyethyl substituent and 2-OH or 4-OH in different proportions depending on the stereochemistry of the parent compounds. Relative formation rates in acetic acid-d4 and hydrolysis rates in buffered D2O solutions at pD 2.4, 4.6 and 7.4 were estimated. Hydrolysis of the formed lactones is relatively slow in D2O at pD 4.6, which permitted characterization of the lactones by 1H and 13C NMR spectroscopy in buffered D2O solutions. Hydrolysis of the lactones in 1 M aqueous NaOH at 80 degrees C gave no detectable isomerization of the alpha-carbon. The set of lactones formed from the 1-carboxyethyl substituted methyl glycosides used in this study showed large similarities in the NMR shifts (delta delta values). Deviations from the observed shift pattern were found for two lactones. Our findings strongly suggest that those two lactones differ from the rest by adopting a boat-like conformation, whereas the others adopt pseudo-chair conformations.