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Experimental study demonstrates pathways for generating tetrahydropyrimidoindoles and isotryptamines, highlighting versatile routes for indole ring functionalisation.
Fischer cyclisation of phenylhydrazones of 1,3-disubstituted 4-piperidones leads, in general, to both tetrahydropyrimido[3,4-a]indoles and 3-substituted 2-(2-alkylaminoethyl)indoles (isotryptamines). The latter can also be synthesised in low yield from 3-substituted 2-methylindoles by the Mannich reaction. They are cyclised to tetrahydropyrimido[3,4-a]indoles by treatment with aldehydes.
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Cattanach et al. (1971) studied this question.
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