It is well known that the sulfonium ion formed in situ from a sulfoxide, by the action of anhydrides such as trifluoroacetic anhydride, is a powerful electrophile reacting with nucleophilic carbon species (Pummerer reaction). Recently, we explored the intramolecular cyclization using the Pummerer reaction of simple alkyl sulfoxides and developed an efficient method for the preparation of N-heterocycles possessing isoquinoline, quinoline, and benzazepine ring systems. During the study, it was unexpectedly discovered by chance that boron trifluoride diethyl etherate, when used together with trifluoroacetic anhydride, markedly facilitated the cyclization. The high efficiency of this modified method was encountered particularly in the cyclization for compounds with a weak nucleophilic aromatic π-bond, which in the absence of the Lewis acid undergo no cyclization. The finding in this modified method has greatly extended the synthetic range of the N-heterocycles. Thus, this review demonstrates that the route leading to aromatic condensed N-heterocycles using Pummerer-type cyclization reaction as a key step, provides a convenient and useful method for the preparation of N- heterocycles with a variety of substitution in the ring system.
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