Conformational studies of D-glycals by 1H nuclear magnetic resonance spectroscopy

Abstract

The preferred half-chair conformations of the fully O-acetylated D-glycals (and related compounds), as indicated by ¹H n.m.r. spectroscopy, show that the coupling constant ⁴J_2,4 is the most sensitive to conformational change. All six fully O-acetylated-D-glycals favour the ⁴H₅(D) conformation in solution, except di-O-acetyl-D-xylal which adopts the alternative ⁵H₄(D) conformation. These conformational preferences can be rationalised by considering the interactions between the substituents at C(3), C(4), and C(5) and the ‘allylic effect’. With the introduction of a substituent at C(2) in glucals and galactals, ring-flattening or inversion to the ⁵H₄ conformation can take place to relieve strain.