New Cyclization Mechanism for Squalene: a Ring-expansion Step for the Five-membered C-ring Intermediate in Hopene Biosynthesis

New cyclization mechanism for squalene: a ring-expansion step for the five-membered C-ring intermediate in hopene biosynthesis.

Three triterpenes having the 6/6/5-fused tri- and 6/6/6/5-fused tetracyclic skeletons were isolated from an incubation mixture of the mutated F601A enzyme, these products being in accordance with a Markovnikov closure. Successful trapping of the tricyclic cationic intermediate by using the squalene analog having a highly nucleophilic hydroxyl group leads us to propose that the ring expansion pr...

Cross sections for electron collision with five-membered ring heterocycles

The absolute total cross-section (TCS) for electron scattering from isoxasole, (CH)3NO, molecule has been measured using the linear transmission technique for impact energies ranging from 1 to 400 eV. The measured TCS energy dependence appears typical for highly polar targets; over whole energy range applied, the magnitude of TCS generally decreases as the energy increases. Some narrow features...

Mechanism of squalene cyclization. The biosynthesis of fusidic acid.

It is generally accepted that the C-20 cation or its stabilized equivalent is an intermediate in the biosynthesis of certain triterpenes and sterols. To evaluate this hypothesis the biosynthesis of the protosterol fusidic acid was investigated. Fusidic acid was biosynthesized by incubating the fungus Fusidium coccineum on a medium containing (3RS,4R)(2-14C,4-3H)-meva10nic acid (MVA). The obtain...

RhI-catalyzed C-C bond activation: seven-membered ring synthesis by a [6+1] carbonylative ring-expansion reaction of allenylcyclobutanes.

Five-membered ring annelation in [2.2]paracyclophanes by aldol condensation

Under basic conditions 4,5,12,13-tetraacetyl[2.2]paracyclophane (9) cyclizes by a double aldol condensation to provide the two aldols 12 and 15 in a 3:7 ratio. The structures of these compounds were obtained from X-ray structural analysis, spectroscopic data, and mechanistic considerations. On acid treatment 12 is dehydrated to a mixture of the condensed five-membered [2.2]paracyclophane deriva...