Isomerization medium-sized rings

Caryophyllenes, as an example of two naturally occurring isomeric sesquiterpenes containing a medium-sized ring, in which the success of the total syntheses lies in the stereoselective control of a chiral centre, in a common synthetic key intermediate, which governs the configuration (JE or Z) of the double bonds present in each one of the two isomers. In this context, a brief reference to Cecropia Juvenile Hormone synthesis by the Syntex group, as well as to Johnson s cationic cyclisation of unsaturated polyolefins to fused polycyclic compounds, is made. 

The conversion of small rings to smaller ones, without loss, is not common. 3-Chloroazetidine isomerizes reversibly to 2-chloromethylaziridine (Section 5.09.2.2.5). Flash vacuum pyrolysis can convert isoxazoles to azirines (Section 5.04.4.3). More common is the isomerization of medium-sized, i.e. five- or six-membered rings, e.g. certain succinimides (Scheme 23) (81JOC27) to azetidinediones, or bicyclic 1,2-dioxetanes to bis-oxiranes (Section 5.05.4.3.2). 

The thermal hexa-1,4-diene l-methyl-2-vinylcyclopropane ( homo-ene ) equilibrium usually lies well to the left, presumably because of ring strain, but by the simple expedient of incorporating a hydroxy substituent in the hexadiene (16), cyclopropyl aldehydes (17) can be obtained in good yields. " In related work, Dauben et al. have shown that medium-sized cyclic trienes can be thermally isomerized to tricyclic compounds e.g. (18). ... 

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