| Summary: | The pyrolysis of a number of C2, C3 and C4 oxygenated pinanes has been studied. Substituents at C2 or C4 significantly affect the mode of cleavage of the pinnae cyclobutane ring. All pyrolysis reactions previously reported proceeded exclusively via initial 1,6-bond cleavage. For several compounds in this study an additional reaction path was identified, which involved the initial cleavage of the four membered ring by fission of the 5,6-bond.
[diagram here]
The relative effectiveness of substituents in inducing initial cleavage at the adjacent C-C bond of the cyclobutane ring (either 1,6- or 5,6-bonds) is in the order C=C 〉 C=O 〉 CH₃ 〉 OH 〉 H.
Substituents also influence the subsequent reactions of the 1,4-diradicals (e.g. (a) and (b) ) formed by cleavage of the 1,6- or 5.6-bonds. Isopropenylcyclohexane derivatives were observed as products only when the pinane substrate contained an sp²-hybridised centre at C2 or C4, immediately adjacent to the cyclobutane ring.
[diagram here]
For pinane derivatives with hydroxyl groups at C2 or C4, an acyclic dienol formed as a primary product may lead to the formation of carbonyl compounds via a 1-hydroxy-2-isopropenylcyclopentane derivative.
[diagram here]
In addition, cis-pinocarveol (97), on pyrolysis, undergoes a fragmentation reaction involving the C3 hydroxyl group and leading, after recombination of radicals, to a rearranged aldehyde (102) and a diene (91).
|
|---|
