Synthesis and conformation of substituted isothiochromans and derivatives. X-Ray analysis of cis-1,4-dimethylisothiochroman 2,2-dioxide, an S-heterocycle with preferred boat conformation

Abstract

Stereoselective syntheses for cis-1,4- and cis-1,3-dimethylisothiochromans and their 2,2-dioxides are described. In the 1,4- and 1,3-dimethyl sulphones, the cis- and trans-forms, respectively, are shown to be the more thermodynamically stable stereoisomers. A preferred boat conformation is inferred for these compounds, and is demonstrated for the cis-1,4-dimethyl sulphone (16) by X-ray analysis; crystals are monoclinic, space group P2₁/c, with Z= 4 in a unit cell a= 8·44 ± 0·01, b= 15·90 ± 0·02, c= 8·22 ± 0·01 Å, β= 114·37 ± 0·2°. The structure was solved by Patterson and Fourier methods and refined by least squares to R 0·081 for 1320 independent reflections measured on a diffractometer. 1-Methyl and 1,3-dimethylisothiochroman-4-ones intermediate in the synthesis are unstable to light, rearranging on irradiation to thiochroman-3-ones. Some heterocycles containing the 5,6-dihydro-2H-thiopyran ring are reported.