The rearrangement of (-)-/3-caryophyllene (1) with sulfuric acid in ether has been reinvestigated. In the early stage, a multitude of products is formed. Of these, fourteen hydrocarbons (3, 4, 6-17) and four alcohols (5, 18-20) were obtained pure. From the very beginning, the tricycloundecenes 15-17 accumulate quickly. This indicates that a previously unknown rearrangement of 1 by protonation of the exocyclic double bond followed by transannular ring closure is a main reaction, and that the conformation ß is involved. Three hydrocarbons (3, 4, 8) and three alcohols (5,18,19) proved to be stable. They are the main components in the late stage. A complete rearrangement scheme followed from a force field analysis using MMP2. First, all conformations up to 4 kcal above the global minimum of each product and each carbenium ion necessary for its formation were detected automatically using the search program HUNTER. Second, the conformation favoring a specific transformation was searched for. For transannular cyclizations, the conformation with the shortest distance between the atoms to be bound and, for ring openings and 1,2-shifts, the conformation with the smallest dihedral angle between the empty p-orbital and the bond to be broken were selected. In all cases, the selection criteria proved valid: The stereochemistry of the conformations selected matched the stereochemistry of the products observed. On the basis of the results presented, a fully automated search program for favorable rearrangement paths may be developed.
(−)‐β‐Caryophyllene (1) adopts three conformations in solution: αα(48%), βα(28%), and ββ(24%). The conformations were identified by an analysis of the 13C‐ and 1H‐NMR spectra at −87.2 and −153.8° in connection with APT, HETCOR, and COSY spectra, and subsequent NOESY experiments. The activation parameters of the conversion αα → βα were determined from a bandshape analysis of exchange‐broadened 13C‐NMR spectra of 8‐[methylene‐13C]‐1 to give ΔH≠ = 5.9 ± 0.3 kcal/mol, ΔS = −8.1 ± 1.8 cal/mol. · K. and ΔG 298≠ = −8.3 ± 0.8 kcal/mol. The observed population ratio of the different conformers is best described by MM3.
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