Ring contractions of thiochroman-4-ones and thiochromen-4-ones
3-Bromothiochroman-4-one and its S-oxide undergo ring contraction on heating, especially in the presence of sodium acetate, to give mixtures which include thioindigo (9) and the ethanediylidenethioindigo (10). Brominated 2,2-dimethylthiochroman-4-one reacts on silica gel to form thioindigo, thioindirubin, and the bis(benzothieno)pyran (17).
Bromination of thiochromanone gives a number of products, including 2,3-dibromothiochromen-4-one S-oxide which, on heating with sodium acetate in acetic acid, is converted efficiently into thioindigo. The mechanism of this reaction is investigated.