Four-membered Rings as Neighbouring Groups

Acetolysis of the corresponding methylated exo-tosylates (533a) and (533b) also yields ring-enlarged acetates, but small amounts of unrearranged exo-acetates are also formed. The first-formed cation in the methylated derivatives is thus partially trapped under the same conditions. The difference in the rate of the ring expansion of the first-formed homoallylic cation has been explained in terms of steric and conformational effects. 

The effect of 2-methyl substitution on the solvolysis rates and products has been investigated. Solvolysis of the p-nitrobenzoates (534a) and (534b) in aqueous acetone gave an exojendo rate ratio of only 2.5 and the unrearranged alcohols as the products. 

The thermal stabilities of (549) and (550) differ quite markedly. In refluxing xylene (549) remains unchanged whereas (550) is decarbonylated smoothly. The isolated product of the reaction is the bicyclic diene (551), but the possibility that the first-formed product is the triene (552) can not be ruled out. The relative reactivity of (549) and (550) is consistent with assistance by the appropriately aligned cyclobutyl T-bond in the chelotropic loss of carbon monoxide from (550). Studies on the related pair of isomers (553) and (554) again demonstrated that the exo-isomer (553) was stable under conditions which gave loss of carbon monoxide from the endo-isomer (554). Furthermore, the initial product of decomposition of (554) was shown to be the diene (555). Its isomeric triene (556) was shown to be more stable, and its rate of formation in the thermal decomposition of (554) was slower than the rate of loss of carbon monoxide, which does not involve the breaking of the cyclobutyl cr-bond. 

Jorgensen has examined the importance of orbital interactions in determining the stabilities and reactivities of hydrocarbons containing bridged small rings, using 

Butler and Snow ° have prepared a series of 1,2-dimethylene-substituted polycyclic hydrocarbons, including (569), (570), and (571), and have prepared the corresponding adducts with dimethyl acetylenedicarboxylate. 

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