Thermal Isomerization of (+)-cis- and (−)-trans-Pinane Leading to (−)-β-Citronellene and (+)-Isocitronellene

Catalyzed and uncatalyzed rearrangement reactions of terpenoids play a major role in laboratory and industrial‐scale synthesis of fine chemicals. Herein, we present our results on the thermally induced isomerization of pinane (1). Investigation of the thermal behavior of (+)‐cis‐ (1 a) and (−)‐trans‐pinane (1 b) in a flow‐type reactor reveals significant differences in both reactivity and selectivity concerning the formation of (−)‐β‐citronellene (2) and (+)‐isocitronellene (3) as main products. Possible explanations for these results are discussed on the basis of reaction mechanism and ground‐state geometries for 1 a and 1 b. To identify side reactions caused from ene cyclizations of 2 and 3, additional pyrolysis experiments were conducted that enabled the identification of almost all compounds in the network of C10H18‐hydrocarbon products formed from 1. Rearrangements in the gaseous phase: Does the ratio of products formed from fragmentations of a cyclobutane ring depend on the stereochemistry of a carbon atom that is not part of the ring? A study investigating the stereochemical influence of such a carbon atom on the selectivity of products obtained from gas‐phase rearrangements (see scheme) of a rigid bridged cyclobutane ring is discussed.