| Popis: |
Methyl 3,4-dichloro-4-deoxy-α- d - glycero -pent-2-enopyranoside and 3,4-dichloro-4-deoxy-β- d - glycero -pent-2-enopyranose have been synthesized fromn l -arabinose. The ring conformations of these unsaturated derivatives and those of the previously reported methyl β- d -glycoside and dimeric analogue have been determined by PMR spectroscopy, the conformational assignments being based upon both vicinal and long range coupling constants. Frequency swept spin decoupling experiments at 100 Mc/s showed that the bi-allylic and J 1,2 vicinal coupling constants in the methyl α- d -anomer are of the same sign, whereas its allylic and J 1,2 vicinal couplings have opposite signs. The α and β derivatves exist predominantly in different half-chair conformations, controlled in each case by the presence of the anomeric effect. The free energy difference of the anomeric methyl glycosides has been obtained by GLC analysis of equilibrated mixtures. The greater stability of the β-anomer is interpreted in terms of Van der Waals energies and dipole interactions. |
