Trichloro-1,2,4-triazine Diels-Alder reactions

An investigation concerning intramolecular aza Diels-Alder reactions of 3-(co-alkynyl)-l,2,4-triazines has been published by Taylor et al. [327] and trichloro-1,2,4-triazine has been introduced as novel triazine diene recently [328]. 1,2,4-Triazines are a useful alternative of 1,4-diaza-l,3-butadienes with regard to the aforementioned synthesis of pyrazines since Taylor s group has found them to undergo cycloadditions with nitriles followed by extrusion of nitrogen [329]. This reaction is noteworthy since it is a Diels-Alder reaction of both electron-deficient diene and dienophile. 

DIELS-ALDER REACTIONS 3-Acetyl-4-oxazoline-2-one. 1,3-Bis(trimethylsilyl-oxy)-l,3-butadiene. 1-Chloro-l-di-methylaminoisoprene. 1,3-Dihydroiso-thianaphthene-2,2-dioxide. 4,6-Di-methoxy-2-pyrone. Furane. trans -Methoxy-3-trimethylsilyloxy-l,3-butadiene. Trichloroethyl trans-1,3-butadiene-1-carbamate. Trichloro-1,2,4-triazine. 

Armstrong et al. (07JOC8019) investigated a synthetic approach to (+ )-epibatidine 128 from 3,5,6-trichloro-l,2,4-triazine 124 (Scheme 43). However, the approach was abandoned due to the poor selectivity in the flZfl-Diels-Alder reaction and the anticipated difficulty in the selective removal of the extra chlorine from 127. The authors did note the utility of this approach as a convenient means of varpng the heteroaromatic portion of epibatidine in analogue synthesis. 

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