Heterocyclic compounds with bridgehead nitrogen atoms. Part V. Pyrido[2,1,6-de]quinolizines (cycl[3.3.3]azines)

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Abstract
Alkyl quinolizin-4-ylideneacetates, obtainable in two stages from 4-chloroquinolizinylium perchlorate, react with alkyl propiolates in boiling nitrobenzene to give dialkyl cycl[3.3.3]azine-1,3-dicarboxylates. When t-butyl esters are used, the products can be thermolysed to give the parent cycl[3.3.3]azine or its 1-alkoxycarbonyl derivatives. The cyclazine diesters are readily hydrogenated to tetrahydro-derivatives, form Diels–Alder adducts with dimethyl acetylenedicarboxylate, and undergo electrophilic substitution in the 4- and 6-positions. The parent cyclazine is air-sensitive and is oxidised by halogens to a stable radical cation and to a dication; no substitution or addition products were obtained. Oxidation of diethyl cycl[3.3.3]azine-1,3-dicarboxylate gives an unstable radical cation which is readily converted into a derivative of 12c,14b-diazoniadibenzo[cd,lm]perylene. The 1H n.m.r. spectra are discussed and it is concluded that cycl[3.3.3]azine is an antiaromatic system.

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