Ylide compounds seven-membered rings

The formation of a minor side product 172 was observed which, if moisture was not rigorously excluded, could become a major product in this reaction (Scheme 36). The formation of compound 172 can be rationalized by protonation of the sulfur ylide 166, followed by deprotonation of one of the readily accessible methyl hydrogens to give the ylide 169, which then reacts at the carbonyl center to give the seven-membered ring intermediate 170. The methyl sulfide is then displaced by the alkoxide to give the oxirane 171, which upon addition of sodium azide, opens up to the azido alcohol 172 <1999T10659>. 

Relatively Uttle research effort has been reported with seven-membered ring carbonyl ylides which in turn will lead to oxabicyclo[4.2.1]nonan-2-ones [125]. In one example, the decomposition of compounds having a tethered diazocarbonyl functionality on the cycloalkanone ring systems 135 in the presence of Rh2(OAc)4 provided the seven-membered carbonyl yUdes. This was demonstrated by trapping experiments using a dipolarophile like NPM to provide the cyclooctanoid systems 136 (Scheme 41). 

Desmotropy of some five-, six-, and seven-membered heterocyclic rings and of some important pharmaceutical compounds and tautomeric equilibria of heterocyclic carbenes with mesoionic compounds and ylides are described. 

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