THERMAL CIS-TRANS REARRANGEMENT OF SEMIRIGID POLYENES AS A MODEL FOR THE ANTICARCINOGEN BETA-CAROTENE - AN ALL-TRANS-PENTAENE AND AN ALL-TRANS-HEPTAENE

To assess the possible role of a 15,15' 90-degrees-twisted, singlet diradical in the anticarcinogenicity of beta-carotene, activation parameters for the thermal, cis-trans interconversion of all-trans-beta-carotene (1) and 15,15'-cis-beta-carotene (2), polyenes of order 11, are needed. Indirect achievement of this goal is initiated here by determining activation parameters for cis-trans rearrangement in a set of semirigid, all-trans-polyenes of order 3, 5, and 7 (9 to follow). These data also provide a test of theoretical calculations of stabilization energies of polyenyl radicals. For the coherent series, 3(1,1,1), 5(2,1,2), and 7(3,1,3), these activation parameters (DELTA-H double-ended-dagger (kcal/mol), DELTA-S double-ended-dagger (cal/mol.K)) are found: 38.9, -6.0; 32.1, -4.4; 27.5, -4.4. The trend is a diminishing return as the order is increased. ''Stabilization energies'', defined as extra stabilization associated with delocalization of the odd electron and derived with allyl as the reference point (SE1 = 13.5 kcal/mol), are pentadienyl, SE2 = 16.9 kcal/mol, and heptatrienyl, SE3 = 19.2 kcal/mol. Heptaene 7 rearranges in a range, 64-104-degrees-C, that is close enough to 37-degrees-C to forewarn of thermal lability of beta-carotene in cell membranes.