Medium Size Seven- to Nine-Membered Rings

Cycles of Medium Size (Seven- to Nine-Membered Rings) 

Among azepines and their benzo derivatives, antiaromatic l//-azepine 14a is known as a very unstable (even at -78°C in CDC13 solution) red oil which in the presence of acid or base rearranges to the marginally 

Prinzbach and Limbach have studied the valence isomerism between N-substituted azepines 14b and benzeneimines 14c (76CB3505) although 14b is much more stable (actually it is the only form detected by 1H NMR), the compound could react, depending on R, as 14c with diazomethane. Later, Prinzbach et al. reported the study of the equilibrium 14b (90%)/14c (10%) in the case of R = p-tosyl [the compound has the following C-substituents 3,6-dichloro-4,5-di(methoxycarbonyl)l in the solid state (X-ray) only 14b is present [86CB616]. 

Of the four dibenzazepines, 577-dibenz[b,d]azepine (19), 577-dibenz [b,e]azcpinc (20), 577-dibenz[b,/]azepine (21), and 677-dibenz[c,e]azepine (22), only 21 is known as such (74CRV101 84JHC197), while 22 exists as the nonaromatic 577-tautomer (81LA240). 

3-Benzodiazepines (24b) exist as such and not as the antiaromatic 3//-tautomers 24a [73JCS(P1)2543 83H2173], A different tautomer (5H) was observed for Tofizopam (25) using H and 13C NMR (74CB3894) and X-ray structure analysis [86JST(147)143], The same behavior is observed 

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Overman and Blumenkopf prepared various seven- to nine-membered rings starting from vinylsilane 238 which was condensed with various acetals (Scheme 13.86) [56, 98, 99]. The desired medium-sized rings were obtained in moderate to good yields. 

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