Bicyclo nitrogen heterocycles

The thermally induced intramolecular 2 + 2-cycloaddition of A(-(buta-2,3-dienyl)-2-cyclopropylidenemethyl)anilines (12) yielded functionalized bicyclo[4.2.0]nitrogen heterocycles (13) in high yield. DFT calculations indicate that the mechanism involves a concerted process in which a strained small ring is essential (Scheme 4). ... 

Nitro-3-phenylisoxazole-5-carboxylates (290) act as heterocyclic 1-azadiene components in [4 + 2] cycloaddition reaction with some enamines affording bicyclo derivatives 295 probably via a stepwise ionic cycloaddition involving the Michael adducts 292 as primary intermediates. Further intramolecular attack by the isoxazole nitrogen on the immonium carbon followed by elimination of the amine and aromatization of dihydropyridine 293 afforded 294, which was transformed into pyridine 295 by simple reduction164 (equation 63). 

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