One version of the tert-amino effect operating in tertanilines possessing an ortho-vinyl substituent affords a fused tetrahydropyridine ring by an isomerization process with the formation of a new carbon-carbon bond between the vinyl and tert-amino moieties. Since its discovery in 1984, this type of cyclization has been efficiently used for the construction of fused-ring systems. However, it has been found that the electron-deficient heterocyclic analogues of tert-anilines, such as diazines, may undergo isomerization at considerably lower reaction rates and, as a consequence, their reactions may require harsh conditions. Recent synthetic studies have indicated that the rate and yield may be improved by incorporating the b-vinylic carbon atom into an electron-deficient ring and/or buttressing the tert-amino group, and/or performing the reaction by means of microwave irradiation.This review describes relatively recent synthetic transformations involving this type of tert-amino effect and devotes special attention to structure-reactivity relationships including regio-and stereochemical aspects.
Novel Extensions of the tert-Amino Effect: Formation of Phenanthridines and Diarene-Fused Azocines from ortho-ortho′-Functionalized Biaryls F o r m a t i o n o f P h e n a n t h r i d i n e s a n d D i a r e n e -F u s e d A z o c i n e s This work is kindly dedicated to Prof. László Tőke on the occasion of his 75 th birthday.Abstract: Phenanthridines and azocines fused to two benzene rings or one benzene and one pyridazinone ring, which are otherwise difficult to access, were prepared via two new extensions of the tertamino effect. The synthetic pathway includes three steps: i) Suzuki reaction of an ortho-functionalized phenylboronic acid with orthodisubstituted benzenes or pyridazinones; ii) the Knoevenagel condensation reaction of the biaryl aldehydes formed with active methylene compounds to obtain vinyl derivatives or, through their cyclization, phenanthridines via a tert-amino effect; and iii) thermal isomerization of vinyl or phenanthridinium compounds to fused azocines via another type of tert-amino effect.
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