Synthesis of Pyrrolo [2,1-b] thiazolines Using N-(Trimethylsilylmethyl)-2- methylthiothiazolinium Trifluoromethanesulfonate

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• N-(Trimethylsilylmethyl)-2-methylthiothiazolinium trifluoromethanesulfonate(4), readily prepard from 2-methylthiothiazoline (2) and trimethylsilylmethyl trifluoromethenesulfonate (1), reacted with activated alkenes (5) and alkynes (6) in the presence of cesium fluoride in acetonitrile to give the corresponding 4, 5-dihydropyrrolo [2, 1-b] thiazoline and pyrrolo [2, 1-b] thiazoline derivatives. The 1, 3-dipolar cycloaddition reaction is one of the most important reaction to construct five-membered heterocycles. We have reported that tailor-made azomethine and thiocarbonyl ylids can be generated by the 1, 3-elimionation reaction of trimethylsilylmethyl- substituted ketene N,S- or S,S- acetals promoted by fluoride ion and react with dipolarophiles to give five-membered heterocycles. As an unpublished result, we have recognized that the reactuion of 3-trimethylsilylmethylamino-3-methylthio-2- cyanoacrylonitrile with dipolarophiles in the presence of a fluoride ion gives five-membered heterocycles but the reaction of 3-trimethylsilylmethylamino-2-cyanoacrylonitrile with dipolarophiles gives no cycloadducts. These facts have shown that the elimination of a methylthio group following cleavage of carbon-silicon bond is a useful tool for the generation of 1,3-dipolar. Here we descrive the reaction of N-(trimethylsilylmethyl)-2- methythiothiazolinium trifluoromethanesulfonate, which has a Me3SiCH2 -N=C-SMe system, with dipolarophiles gives 4,5-dihydropyrrolo [2,1 - b] thiazoline derivatives. Trimethysilylmethyl trifluoromethanesulfonate (1) is an interesting compound to the preparation of silicon-containing sulfonium, nitrogen and other ylids. Starting material 1 was prepared according to the literature. A solution of 2-methylthiothiazoline (2) in dichloromethane was treated with 1 and stirred at room temperature for over night. After the solvent was removed N- (trimethylsilymethyl)-2-methylthiothiazolinium trifluoromethanesulfonate (4) was afforded. The crude salt 4 was used in the next step without purification. A solution of 4, thus obtained, and dimethyl fumarate(6b) in the prsence of cesium fluoride in acetonitrile was stirred at room temperature for 24h. The usual work-up after treatment with aqueous ammonium chloride and separation by preparative t.l.c. gave dimethyl 4,5-dihydropyrrolo [2, 1-b] thiazoline -5, 6-dicarboxylate(7b), formal [3+2] cycloaddition product, in 53% yield. The cycloadditon behavior of an unsymmetrically substituted dipolarophile was studied to determine the regioselectivity of the reaction. The reaction of 4 and methyl acrylate (6a) under the similar conditions gave methyl 4,5-dihydop yrrolo [2,1-b] thiazoline-6-carboxylate (7a) in 57% yield, exclusively. When methyl cinnamate (6d) was used as a dipolarophile, cycloadduct 7d was obtained as a single pro duct. The representative results are listed in Table 1. N-(Trimethylsilylmethyl)-2- methylthiobenzothia-zolium trifluoromethenesulfonate (5) was also prepared and reacted with activated alkenes (6a-6d) to give the corresponding [3+2] cycloadducts (8a-8d) in a manner similar to that described for 7a. The reaction also shows complete regiospecificity in the cycloaddition with unsymmetrically substituted olefins. However the reactions of 4 and 5 with N-methylmaleimide was unsuccessful under the present reaction conditions. Salt 4 and 5 reacted with alkynes (9a and 9b) under the similar conditions to give [3+2] cycloaiddition products, pyrrolo [2, 1-b] thiazoline derivatives(10a and 10b) and pyrrolo [2, 1-b] benzothiazole derivatives (11a and 11b). (see Table 2) This work demonstrates a mild and simple procedure for the preparation of pyrrolo [2, 1-b] thiazoline and pyrrolo [2, 1-b] benzothiazole derivatives.

著者

• 高良 真也

  Faculty Of Liberal Arts Nagasaki University

• 上江田 一雄

  長崎大学教養部

• Kohra Shinya/ueda

  Faculty Of Liberal Arts Nagasaki University/faculty Of Liberal Arts Nagasaki University

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