Conformational studies of alpha- and beta-pyrimidine 2'-deoxyribonucleosides in the syn and anti conformation

Proton magnetic resonance studies of a series of pyrimidine alpha- and beta-2'-deoxyribosides in 2H2O have been carried out at 250 MHz. The H-H coupling constants and chemical shifts are discussed in terms of the molecular conformation. Methyl substitution at the 6-position of the base leads to rotation from the anti to the syn conformation in both anomeric species. In both series, the 2'endo pucker is preferred when the base is anti. Rotation into the syn conformation leads to a shift towards the 3'endo pucker, the shift being larger in the alpha-series. In the alpha-series, a correlation between the ring pucker and the C(4')-C(5') conformer distribution is revealed. Changes in geminal coupling constant, J2'2", are noted in the alpha-series and related to the sugar pucker.