Cyclopenta-1,2-dithioles, Cyclopenta-1,2-thiazines, and Methylenoindenes from New Molecular Rearrangements

The cyclobutanone oxime 6 reacts with
disulfur dichloride, N-chlorosuccinimide, and Hünig's
base
to give three unexpected 10 π pseudoazulenes in low yield:  the dark
blue cyclopenta-1,2-dithiole
7, its red isomer 10, and the orange
cyclopenta-1,2-thiazine 8. The benzo derivative
14 of oxime 6
gives an analogous benzo product 15 together with the
methylenoindene 16 in high yield. The
formation of all of these products can be explained by a unified
mechanism based on initial abnormal
Beckmann rearrangement of the oximes to cyanides followed by
cyclization and/or exhaustive
chlorination and dehydrochlorination. The 1,2-dithiole
7 is synthesized independently from
1-(cyanomethyl)cyclopentene 11 and the above reagents,
and 1-cyanomethylindene 17 is similarly
converted into methylenoindenes 18 and 19, which
are isomers of product 16. Structures 7 and
15
are confirmed by X-ray crystallography, and compounds 7,
8, 15, 16, 18, and
19 show birefringence
on heating in a hot stage polarizing microscope indicating, most
unexpectedly, liquid crystalline
behavior.