"tert-Amino effect" in heterocyclic synthesis. The effect of a p-quinone moiety on the [1,6] H-transfer and 1,5-electrocyclization reactions

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Abstract

(Dialkylamino)benzoquinone 15 and (dialkylamino)naphthoquinones 32 and 34-37 undergo a thermal cyclization to the corresponding pyrrolo[1,2-a]indoles 41, 43, and 45-47 and to the pyrido[1,2-a]indole 44, respectively. A corresponding hydroquinone, viz. (E/Z)-2,5-dimethoxy-α-(phenylmethylene)-3,6-di(1-pyrrolidinyl)benzene- acetonitrile (18), cyclizes only slowly to pyrrolo[1,2-a]indole 42. The naphthohydroquinones 38-40 do not undergo a thermal rearrangement. The results demonstrate the accelerating effect of the quinone function on the rate of the reaction, as a result of stabilization of the “negative end” of the intermediate 1,5-dipole. The presence of an electron-donating group at the β-carbon atom of the vinyl moiety lowers the rate of the reaction. Moreover, this influence is demonstrated by oxidation of one of the sulfur atoms in 35 to the ketene dithioacetal S-monooxide 36, which undergoes a fast thermal isomerization to 47. Cyclization of the pyrrolidinylnaphthoquinones 32, 34, and 35 yielded exclusively products in which H-11a and CN have a trans relationship, while in the case of piperidinylnaphthoquinone 37 predominantly trans-1H-benzo[f]pyrido[1,2-a]indole 44a was formed. The trans stereochemistry of 43 was determined by single-crystal X-ray analysis. Heating of (dialkylamino)naphthoquinone 33 afforded the indoline 50 in low yield.

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