Ring expansions of cyclic conjugated systems

Examples treated in this book are summarized in Table 9. One frequently found type consists of ring expansions of cyclic conjugated systems with an exocyclic ylide function, such as pyridine /V-oxides or /V-imides incorporation of the exocyclic half of the ylide function expands the ring by one member. 

Cyclic addition reactions of 16a with diynes or cyanamide are interpreted as being initiated by an electrophilic attack of the cyanamide or the terminal carbon atom of a C=C bond. Both result in ring expansion and the formation of 2,4-As,As-diphosphapyridine 39 <1998ZNB443> or l,3-A5,A5-diphosphabenzenes 40, respectively (Scheme 10). The diyne reaction is not limited to conjugated systems. 1,5-Hexadiyne and 1,7-octadiyne can be used as well <1997JOM(529)223>. 

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