Cycloelimination cationic

Strategies based on very particular and specific reactions, such as Diels-Alder addition, either inter- or intramolecular (see also cycloeliminations), cationic cyclisations (important in the "biomimetic synthesis" of steroids), Pauson-... 

In a similar context Amdtsen developed a new pyrrole synthesis from alkynes, acid chlorides either imines or isoquinolines, based on the reactivity of isocyanides (Scheme 35a) [197]. Although all atoms from the isocyanide are excluded from the final structure, its role in the reaction mechanism is crucial. The process takes place through the activation of the imine (isoquinoline) by the acid chloride to generate the reactive M-acyliminium salt, which is then attacked by the isocyanide to furnish a nitrilium ion. This cationic intermediate coordinates with the neighboring carbonyl group to form a miinchnone derivative, which undergoes a [3+2] cycloaddition followed by subsequent cycloelimination of the isocyanate unit, to afford the pentasubstituted pyrrole adducts 243 and 244 (Scheme 35a, b). 

A most unusual result was encountered when the irradiation of 243 was carried out in pentane in the presence of both methanol and methyl trifluoro-acetate (excess).Under these conditions, the only cycloadducts observed consisted of a mixture of the two stereoisomers of 3-oxazoline, 260. This result suggests that 2-phenylazirinylidene (244) reacts with methanol to give mainly azirine 258, which is subsequently converted to nitrile ylide 259 (and thus cycloadduct 260) on further irradiation. The formation of 245 has been explained in terms of a photoinduced methoxy migration of 258 which competes with C-C bond cleavage of the azirine ring. Ciabattoni and Cabell have previously reported that 2-chloro-2/f-azirines undergo ready isomerization at room temperature via a 27r-electron azacyclopropenyl cation. A similar mechanism would rationalize the apparent photoconversion of azirines 258 and 245. The results obtained with this spiro azirine system indicate that cycloelimination of ethylene from 243 is much more efficient than C-C bond scis-... 

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