Crystal structure of the product of a condensation reaction of (±)-gossypol with (R)-(+)-1-phenylethylamine. Why do diastereoisomeric diaminogossypols cocrystallize?

Abstract

The crystal structure of a compound obtained in a condensation reaction of (±)-gossypol with (R)-(+)-phenylethylamine has been determined by X-ray structure analysis. The crystals of C₄₆H₄₈O₆N₂·(C₄H₈O₂)₂ are monoclinic, space groupP2₁,a=21.243(3),b=8.666(1),c=28.651(4) Å, β=108.24(1)⁰,V=5009(3) β³,Z=4,D _x=1.195 g cm⁻³, μ(CuK _α)=0.66 mm⁻¹,T=292 K. The structure has been solved by direct methods and refined to the finalR value of 0.091 for 4290 observed reflections and 1049 parameters. There are two diastereoisomeric molecules of the host and four solvent molecules in the asymmetric unit. The host molecules are H-bonded into chains with diastereoisomeric molecules alternating along the chain. The fact that the host prefers the association mode in which molecules with a different configuration of the 2,2′-binaphthyl moiety are H-bonded explains why separation of diastereoisomeric diaminogossypols by fractional crystallization has been unsuccessful. The 1,4-dioxane molecules are accommodated in infinite channels but only every second guest molecule in a channel is H-bonded to the host.