Hetero-Diels-Alder-Reaktion mit 1,3-Thiazol-5(4H)-thionen |
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Authors: | Peter Tromm Heinz Heimgartner |
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Abstract: | Hetro-Diels-Alder Reaction with 1,3-Thiazol-5(4H)-thiones On heating in toluene to 180° and on treatment with BF3·Et2O in CH2Cl2 room temperature, 1,3-dienes react with the C?S group of 1,3-thiazol-5(4H)-thiones 1 in a reversible Diels-Alder reaction to give spiro4.5]-heterocycles of type 6. A 1:1 mixture of two regioisomeric cycloadducts is formed in the thermal reaction with 2-methylbuta-1,3-diene (isoprene, 5b ). In contrast, the formation of one regioisomer is strongly preferred in the BF3-catalyzed reaction. Frontier-orbital control as well as steric factors seem to be responsible for the observed regioselectivity. BF3-Catalyzed, cyclic 1,3-dienes and 1 also undergo a smooth Diels-Alder reaction. Whereas cyclohexa-1,3-diene ( 5c ) reacts with 1a and 1b to give a single isomer (presumably the ‘exo’-adduct), cyclopenta-1,3-diene ( 5d ) leads to a ca. 3:1 mixture of ‘exo’-and ‘endo’-isomer. |
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