Abstract
This chapter provides an overview of the main types of reactions used in synthesis of fused ring systems containing one or more heteroatoms (N or S) at a ring junction. For nitrogen, such heterocyclic compounds are possible with the retention of a three-coordinate neutral, or four-coordinate positively charged, nitrogen atom. The chapter successively considers the formation of small (three- or four-membered) rings with one N at a ring junction, five-membered and then six-membered rings with one ring junction N atom, and finally rings containing two ring junction N atoms. A very large number of these systems with ring junction heteroatoms exist, and this number is constantly increasing. The synthetic methods for the formation of this type of heterocycle can be classified as follows: (a) various cyclocondensations between the corresponding heterocyclic derivatives and bifunctional units, (b) intramolecular cyclizations of electrophilic, nucleophilic, or (relatively rare) radical type, (c) cycloadditions, (d) intramolecular oxidative coupling, (e) intramolecular insertions, (f) cyclization of open-chain predecessors, and (g) various reactions (quite often unusual) which are specific for each type of system. Furthermore, the chapter discusses the synthesis methods for fused ring systems containing one or more S atoms at a ring junction. Preparations of bicyclic sulfonium ions utilize intramolecular alkylation of the tosylate precursor, while benzo-fused bicyclic sulfonium salts can be prepared via [4 + 2] cycloaddition reactions of 2-thianaphthylium ions with various butadienes.
Published Version
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