Preference for occupancy of axial positions by substituents bonded to the heterocyclic ring in penta-O-acetyl-(+)-catechin in the crystalline state

Abstract

The structure of penta-O-acetyl-(+)-catechin has been determined in the crystalline state. Crystals are monoclinic, space group C2, a= 2 320.0(7), b= 980.1(2), c= 1 108.0(3) pm, β= 100.64(2)°, Z= 4, D_c= 1.342 g cm^–3, R= 0.058 for 1 121 observations. One of the acetyl groups is disordered. Axial positions are occupied by the substituents bonded to heterocyclic ring atoms C(2) and C(3). The substituent bonded to C(2) occupies an equatorial position in previously reported crystal structures for related compounds. The root-mean-square dipole moment is 3.67 ± 0.26 D in dioxane at 25 °C. Conformational analysis demonstrates that the measured dipole moment in dioxane can be expected if either the same ring geometry or a diequatorial form occurs in solution. ¹H N.m.r. coupling constants J_2,3 6.5, J_3,4 5.0, and J_3,4 7.0 Hz suggest a heterocyclic ring conformation between these two extremes or a rapid flexing of the heterocyclic ring in solution at ambient temperature. Spectra recorded at –90 °C do not show a preferred conformation. Instead they show a slow exchange between different conformations of the heterocyclic ring.