Cheletropic disconnections

Cycloreversions pericyclic and cheletropic disconnections. The Woodward-Hojfmann rules... 

Besides the bond-pair cheletropic disconnection of oxiranes and aziridines to an alkene and "atomic oxygen" (from a carboxylic peracid) or a nitrene, respectively, and the hetero-Diels-Alder cycloreversion, of special interest are the 1,3-dipolar cycloeliminations of five-membered rings [-(34-2)] leading to 1,3-dipoles and an unsaturated acceptor or dipolarophile. So large is the number of different five-membered heterocyclic systems resulting from 1,3-dipolar... 

In the present chapter, however, because the problem is considered from a retrosynthetic point of view, we will distinguish only between heterolytic and homolytic disconnections -to which we will refer to as "retro-annulations"- and concerted or "pericyclic (or cheletropic) cycloreversions". In the same way that Woodward-Hoffmann rules [2] apply to pericyclic reactions, the Baldwin rules [3] may be said to apply to heterolytic as well as to homolytic "monotopic" annulations (see Table 6.1). Although in the preceding Chapter (see 5.5) we have already described some radical "monotopic" annulations, later on in this Chapter (see 6.1.3) and mainly in Chapter 7 we will refer to some new methods, syntheses and strategies which have been developed recently. 

In addition to being treated in a similar manner to acyclic systems, cyclic systems can also be disconnected according to concerted pericyclic or cheletropic cycloreversions. In this context, the Woodward-Hoffmann rules [2] are of... 

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