Stereochemistry of cyclic organophosphorus compounds. Part 13. Stereoisomerism in bis-(4-methyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl) oxide. Synthesis of diastereoisomers and their solution and solid state conformations

Abstract

Bis-(4-methyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl) oxide (4) has been found to exist in six diastereoisomeric forms [cis-trans-, trans-trans-, and cis-cis-(4a) and -(4b)] differing both in configuration at the phosphorus atoms and in the relative positions of the methyl groups on C(4) on the two 1,3,2-dioxaphosphorinan rings A and B. All diastereoisomers of (4) have been characterised by ¹H n.m.r., ³¹P n.m.r., and i.r. spectroscopy. Single-crystal X-ray analysis of the trans-trans-(4a) isomer revealed that one 1,3,2-dioxaphosphorinan ring adopts a strongly flattened chair conformation with an axial methyl group on C(4); an X-ray analysis of the cis-cis-(4a) isomer shows it to have a conformation with both sulphur atoms and both methyl groups in equatorial positions relative to the 1,3,2-dioxaphosphorinan rings. The conformation of all diastereoisomers of (4) in solution is discussed and the stereochemistry of bis-(4-methyl-1,3,2-dioxaphosphorinan-2-yl) oxide (5) is briefly considered.