| Abstract: | Independent Synthesis of 2H-Thiopyrane-2-ylidene-thioketones Diels-Alder reaction of 1-phenylisoprene (3) with thiophosgene (4) or methyl cyanodithioformate (7) leads to the 3,5-disubstituted thiopyranes 5 and 8 , rather than to the 4,6-disubstituted compounds. The α-pyrones 9 react with Lawesson reagent to give the pyrane-2-thiones 10. Rearrangement of 10 with NaSH yields the thiopyrane-2-ones 11 , which are once again sulfurised with Lawesson reagent to form 12. After alkylation of 12 with phenacyl bromide the pyrylium bromides 13 can be isolated. These lose sulfur on heating in pyridine and can be transformed to the 2H-thiopyrane-2-ylidene thioketones 2 with Lawesson reagent. Compound 2a is identical with the main isomer obtained by heating 3-methyl-5-phenyl-1,2-dithiolium perchlorate in pyridine/glacial acetic acid according to the literature. |
