Azirines fulvenes

The formation of 133 was rationalized by a stepwise mechanism. The initial addition of the fulvene to the activated 2H-azirine generates the zwitterionic intermediate 135 which then cyclizes to [2]p3U indine 133 via intermediate 136 (Scheme 36). 

Interestingly, the reaction of dimethyl fulvene (R = Me) with 2H-azirine 132 in an ultrasonic bath (neat) yielded the alkylation product 138. The structure of 138 was unambiguously assigned by single crystal X-ray structure. A possible mechanism to account for the formation of 138 involves an initial [4- -2]-cycloaddition followed by a subsequent... 

Inverse electron-demand cycloadditions of N-sulfonyl vinylimines and allenamides, exemplified by the formation of 180 from 181 and 182, have been examined. The synthetic limitations, mechanistic issues, and stereochemistry of the process have been addressed <04T7629>. Fulvenes have been used as dienophiles with N-sulfonyl vinylimines to synthesize the [IJpyrindine system <04OL3453> and in a formal [6+3] cycloaddition with an appropriate 2//-azirine to give the [2]pyrindine system <04TL1663>. 

Analogously, fulvene n-systems can act as a triene unit for enamines (Scheme 6.46a) [103], 2-oxyallyl cations (Scheme 6.46b) [104], chromium alkenyl carbene complexes (Scheme 6.46c) [105], 3-oxidopyrylium (Scheme 6.46d) [106], 3-oxidipyridinium betaines (Scheme 6.46e) [107], dienolates (Scheme 6.46f) [108], azirines (Scheme 6.46g) [109], and azomethine ylides (Scheme 6.46h) [110]. 

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