Preparation and conformation of some 1,4-oxathianium salts

Abstract

Ethylation of 6-methyl-2-oxo-1,4-oxathian afforded approximately equal amounts of trans- and cis-4-ethyl-6-methyl-2-oxo-1,4-oxathianium salts (3) and (4). To determine their relative configurations the salts were converted into the ethylene 2-spiro-orthoesters (6) and (7), and into the 3-acetyl-3-ylides (8) and (9). A mixture of trans- and cis-4-ethyl-2-methyl-1,4-oxathianium salts (15) and (16) was synthesized for comparison purposes.

Evaluation of the cumulate ¹H and ¹³C n.m.r. data allows the assignment of trans- and cis-structures to (3), (6), (8), (15) and (4), (7), (9), (16), respectively. In addition, conformational properties of the same compounds in solution are discussed.

The isomer pairs (3)/(4), (6)/(7), and (15)/(16) were thermally equilibrated. An unexpectedly high proportion of (6) in the (6)/(7) equilibrium mixture was observed. Surprisingly, the ylide isomers (8) and (9) resisted equilibration at 100 °C.