FACILE ALKYLATIVE CYCLIZATION - REGIOSELECTIVITY IN THE SYNTHESIS OF 2-METHYL-4H-THIENO[2,3-B][1]BENZOTHIOPYRAN-4-ONES AND THIOPYRANO[2,3-B][1]BENZOTHIOPYRAN-5-(4H)-ONES

4-Hydroxydithiocoumarin (1) on treatment with propargyl bromide, 3-bromobut-1-yne and 3-chloro-3-methylbut-1-yne gives 2-alkyl-4H-thieno[2, 3-b][1]-benzothiopyran-4-ones (3a-c) under classical alkylation condition and yields thiopyrano[2, 3-b][1]benzothiopyran-5(4H)-one(4) and thio-pyrano[2,3-b][1]benzothiopyran-5(2H)-ones (5a and 5b) under phase-transfer catalysis condition in good yields. However, compound 1 reacts with allyl bromide under PTC condition to give 2-methyl-1, 2-dihydro-4H-thieno[2, 3-b][1]benzothiopyran-4-one (6) and 2-thioallylthiochromone (7a) under classical condition. All the other acetylenic and allylic halides used in this study furnish S-alkylated products (except 8, which is C, S-dialkylated) when reacted with compound 1 under both PTC and classical conditions.