Diels-Alder approaches to pentacyclic triterpenes of the arborane and the fernane families
摘要:
The high pressure Diels-Alder reaction of diene 2 and ene-dione 3 does not lead to the arborane skeleton but to angular isomers 5. The AB + D synthesis of tetracyclic ene-dione 6, followed by construction of the E-ring by a double alkylation, yields the isoarborinol precursor 7; the fernane series is similarly accessed.
Diels-Alder condensation of the bicyclic diene 1 with 2,6-dimethylbenzoquinone 2 under thermal and Lewis acid-catalyzed conditions gives the diastereomers 3, 4, 5 and 8; their structures are demonstrated by NMR analysis, in full agreement with the conformations deduced from molecular mechanics.