Nitrile ylides electrocyclization reactions

Ab initio and density functional calculations have been carried out on the mechanism of the 1,5-electrocyclization reactions of conjugated nitrile ylides (190). The results indicate that vinyl-conjugated systems (306) (X = CH2) cyclize via the classical electrocyclization pathway—a pericyclic, monorotatary process with a relatively early transition state in which there is substantial torsion of the vinyl group as well as pyramidalization at C(5). In contrast, systems with a heteroatom at the cyclization site 306 (X=NH, O) react via a pseudo-pericyclic process that is characterized by the in-plane attack of the lone pair of the heteroatom on the nitrile ylide. Such reactions have a lower activation energy. 

Some interesting new chemistry has been produced on the well-known 1,5-electrocyclization reaction of alkene-conjugated nitrile ylides but the greatest... 

A similar 1,5-electrocyclization involving an aromatic ring was observed for the nitrile ylide 310 (R =Ph, R =Me) (66). This reaction gave 311 as the initial product, which then rearranged via a 1,3-hydrogen shift to give the isoindole 312. 

Extensive work has been carried out on the 1,7-electrocyclization of diene-conjugated nitrile ylides (324) leading to fused heterocyclic systems containing the azepine ring (325). Reactions of this type for all 1,3-dipoles have been reviewed (197,198). 

The nitrile ylides were generated from amides via the imidoyl chloride-base method and hence the reaction is, overall, the electrocyclic equivalent of a Bischler-Napieralski type of process. However, it has the advantage that it is effective for cyclization on to both electron-rich and electron-poor aromatic rings, unlike the Bischler-Napieralski reaction itself, which is an electrophihc process and only works well for electron-rich rings. 

Nitrile ylides may also undergo electrocyclization reactions with neighboring re-systems, as exemplified by the conversion of 22 into 24 as shown in Scheme 5 (93JCS(P1)675). 

This reaction is likely to proceed via intermediate formation of nitrile ylides 15 and their electrocyclic ring closure as 1,5-dipoles (cf p 144) yielding the oxazole system [69a]. 

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