Acid-catalyzed cyclization of terpenes under homogeneous and heterogeneous conditions as probed through stereoisotopic studies: a concerted process with competing preorganized chair and boat transition states.
Based on stereoisotopic studies and beta-secondary isotope effects, we propose that the acid-catalyzed cyclization of geranyl acetate proceeds through a concerted mechanism. Under heterogeneous conditions (zeolite Y confinement), a preorganized chairlike transition state predominates, whereas under homogeneous conditions the boat- and chairlike transition states are almost isoenergetic. For the case of farnesyl acetate, we propose that under homogeneous conditions a concerted dicyclization occurs with a preorganized boat-chair transition state competing with the chair-chair transition state. Under zeolite confinement conditions, the chair-chairlike dicyclization transition state is highly favorable. The preference of chairlike transition states within the cavities of zeolite Y is attributed to a transition state shape selectivity effect.
Department of Chemistry, University of Crete, 71003 Voutes, Iraklion, Greece.
Christos
C
Raptis
Ioannis N
IN
Lykakis
Constantinos
C
Tsangarakis
Manolis
M
Stratakis
D002384:Catalysis; D003500:Cyclization; D005204:Farnesol / Q000031:analogs & derivatives / Q000737:chemistry; D011090:Polyenes / Q000737:chemistry; D013237:Stereoisomerism; D013729:Terpenes / Q000737:chemistry*; D017641:Zeolites / Q000737:chemistry
D011090:Polyenes; D013729:Terpenes; D017641:Zeolites; C037088:farnesylacetic acid; D005204:Farnesol