Electrocyclic ring expansion

The thermal addition of dimethyl acetylenedicarboxylate to indoles, unlike the photocycloaddition (see Section 3.2.1.4.1.1.), proceeds via a polar stepwise process to yield, initially, 3,4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes which in some cases are isolable,13 141 but which, in general, ring open in situ to give the indolylacrylates 3 and/or undergo electrocyclic ring expansion to 1-benzazepines.21... 

Recently, further examples for dehydro Diels-Alder reactions were published, which are believed to proceed via intermediates of the type 205. To explain the formation of products besides those of the type 206, the authors [132] proposed a remarkable reaction cascade and supported it by quantum-chemical calculations. Accordingly, an isonaphthalene of the type 205 undergoes an electrocyclic ring expansion to give a l,2-dehydro[10]annulene derivative, in which a configurational isomerization occurs followed by an electrocyclic ring closure, yielding a further isonaphthalene of the type 205, and aromatization. 

A route to a 1,4-azaphosphinine is reported through a sequence involving cycloaddition and electrocyclic ring expansion (Scheme 4) <1996CEJ68>. 

Wender reported a clever combination of a [2-i-2]-photocycloaddition of unactivated diene 204 followed by a thermal electrocyclic ring-expansion to give 207 (Scheme 18.33) [137]. The reversible formation of the initial diradi-... 

All three levels of theory predict the ring expansion of singlet phenylcarbene ( A -la) to cycloheptatetraene (3a) to occur in two steps, via bicy-clo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The first step is addition of the carbene carbon to an adjacent 7t bond of the ring. The second step involves a six-electron, disrotatory, electrocyclic ring opening, which is allowed by orbital symmetry67 and thus proceeds by a highly delocalized transition state. Fig. 4... 

The ring expansion of arenes by electron-deficient singlet nitrenes is by far the most versatile synthetic route to 1H -azepines. The first l//-azepines were prepared independently in 1963 by Hafner, and by Lwowski, and their coworkers. They found that ethoxycarbonyl-nitrene (Scheme 26, path a R=C02Et), generated by photolysis of ethyl azidoformate, adds to benzene to give initially the unstable azanorcaradiene (227), electrocyclic ring... 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

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