1-Azatrienes electrocyclic ring closure

Ring closure to a bicyclic product could also be expected in the irradiation of peril uoroazacyclohexadiene 25, but the product obtained, in 87% yield, is peril uoro[l-isopropyl-3-(methylimino)cyclobutene] (27), which is presumed to be formed via ring opening to azatriene 26, followed by electrocyclic ring closure to the cyclobutene and a 1,3-fluorine shift.20... 

The electrocyclic ring closure of 1-azatrienes to 1,2-dihydropyridines is a favorable process. The possibility of stereoisomerism at the nitrogen atom in certain derivatives (oximes and their benzoates) provides an important argument in favor of the steric and/or stereoelectronic factors discussed above for... 

A synthesis of the tunicate-derived natural product grossularine-2 rested on two key features the lithiation of a l-blocked-2-substituted imidazole at C-5, allowing introduction of tin for a subsequent coupling, and the electrocyclic ring closure of an isocyanate (a 2-azatriene in which one of the double bonds was the imidazole 2,3-double bond). 

The condensation of iV-triphenylphosphoraniliden-benzamidine with acyclic a,P-unsaturated aldehydes produces dihydropyrimidines in good to high yields. The reaction mechanism probably involves an aza-Wittig reaction followed by a 6e-n -electrocyclic ring closure of the azatriene intermediate to give dihydropyrimidines, which is oxidized to the corresponding pyrimidines. 

Mechanistically, as illustrated in Scheme 12.66, this intermolecular process (279 -I- 280 282) involves a tandem Knoevenagel condensation/6jt-electron electrocyclic ring-closure of 1-azatrienes 281 [126,127] to give fused heterocyclic system 282 [128,129]. The regiochemical control [5], head-to-head versus head-to-tail (i.e., the Hickmott-Stille s aza-cycloaddition) [130], is the challenge in this and other related cycloadditions, which can be unpredictable and leads to complex mixtures [4-6,131]. However, in our cycloadditions, the use of a, 3-unsaturated iminium salts has led to head-... 

Cyclisations where the benzene ring carries an electron-withdrawing group can be effected using the variant shown below - the substrate is simply heated strongly - the mechanism of ring closure probably does not involve electrophihc attack on the benzene ring but rather is better viewed as the electrocyclic cyclisation of a 1,3,5-3-azatriene. ... 

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