578006-81-2Relevant articles and documents
Thermal isomerization of isoborneols and dehydroisoborneols to new chiral building blocks in terpenoid synthesis
Rueedi, Georg,Hansen, Hans-Juergen
, p. 1968 - 1989 (2007/10/03)
The substituted isoborneols 1a-1g and 5,6-dehydroisoborneols 6a-6c, readily prepared in excellent yields from (+)-camphor and (+)-5,6-dehydrocamphor (2) by aryl, vinyl, or alkyl Grignard addition in the presence of stoichiometric amounts of CeCl3, were thermally isomerized in a flow reactor system under DGPTI (dynamic gas-phase thermo-isomerization) conditions at temperatures between 480 and 630° to give the enantiomerically pure monocyclic carbonyl compounds 7a-7d, 19a,b, 23, and 24. In all cases, product formation proceeded highly regio- as well as stereoselectively. The absolute configurations of the new stereogenic centers were determined by 1H-NOE measurements. DGPTI of the aryl substrates 1a-1d is proposed to effect initial cleavage of the weakest single bond in the molecule under formation of a diradical intermediate state followed by intramolecular H-abstraction to afford the acetophenone derivatives 7a-7d. This reaction path was further supported by a 2H-labeling study showing the OH group to be the exclusive H-source. In contrast, DGPTI of the vinyl substrates 1e and 6b allowed concerted retro-ene and oxy-Cope rearrangements. In the case of 5,6-dehydro-2-phenylisoborneol (6a), concomitant diradical and retro-Diels-Alder reaction pathways could be observed. In addition, a new route to (+)-rrans-a-campholanic acid (9) and (+)-trans-a-dihydrocampholytic acid (14) is presented by regioselective Baeyer-Villiger oxidation and subsequent hydrolysis of 7c and 7d, respectively.
