( Z ,2 R ,3 R ,4a R ,7 R ,12a S )-2,3,7,8,12,12a-Hexahydro-2,3-dimethoxy-2,3,7-trimethyl-4a H -[1,4]dioxino[2,3-c]oxecin-5(11 H )-one: a commensurate occupationally modulated structure revealing a condition for diffraction symmetry enhancement for non-parent reflections

Abstract

(Z,2R,3R,4aR,7R,12aS)-2,3,7,8,12,12a-Hexahydro-2,3-dimethoxy-2,3, 7-trimethyl-4aH-[1,4]dioxino[2,3-c]oxecin-5(11H)-one (dengue) crystallizes in the space group P31 and approximates the conditions necessary for diffraction symmetry enhancement without twinning for the h - k ≠ 3N reflections. The structure may be described as an occupancy modulation of a 1:1 disordered P3121 parent structure with Z = 3 that would only contribute to the h - k = 3N reflections. The crystal studied was a 0.717 (2):0.283 twin, but also had a stacking fault that on average caused the (1 - pj):pj population ratio for the alternative orientations of ordered columns along the three non-equivalent screw axes (j = 1, 2 or 3) of P31 to be describable by p1 = 0.068 (3), p2 = p3 = 0.960 (3). The effect of these stacking faults could be simulated using global parameters that modify an ordered prototype structure. The structure reveals that the ten-membered lactone ring incorporates a Z-configured double bond and that the methoxy-substituted stereogenic centers created during a trans-diol protection step each possess the R-configuration. Show more