Synthesis and stereochemistry of Δ2,2′-bi-(2H-1,4-benzothiazine) derivatives. Crystal structures of cis- and trans-Δ2,2′-bi-(3-p-bromophenyl-2H-1,4-benzothiazine)

Abstract

Δ^2,2′-Bi-(2H-1,4-benzothiazine) derivatives (II) have been prepared by oxidation of the corresponding 3-substituted 2H-1,4-benzothiazines (I) by use of chloranil as dehydrogenating agent. The reaction afforded both cis- and trans-isomers of (II) which in solution were found to be easily interconvertible even at room temperature. Comparison of the physicochemical properties of the isomers showed that, despite the presence of bulky substituents at the 3- and 3′-positions, the major isomer at room temperature had the cis-configuration. The relative stability of the geometrical isomers has been explained in terms of the molecular structures of the cis- and trans- isomers of the 3-p-bromophenyl derivative (IIa) which were determined by three-dimensional X-ray analysis. The cis-isomer crystallizes in space group P, with Z= 2, in a unit cell of dimensions a= 10·18, b= 12·90, c= 10·65 Å, α= 113° 56′, β= 95° 57′, γ= 103° 5′. Crystals of the trans-isomer are monoclinic, space group P2₁/c with Z= 2, in a unit cell of dimensions a= 7·057, b= 12·70, c= 14·04 Å, β= 105° 35′. The two structures were determined by the heavy-atom method, and refined to R 0·055 and 0·069 for 3779 and 1125 independent reflexions.