Claisen rearrangements—VII

Direct Assembly of Prenylated Heteroarenes through a Cascade Minisci Reaction/Dehydration Sequence

The prenyl group is an important component in bioactive compounds. Herein, we report the assembly of prenylated heteroarenes through a cascade Minisci reaction and acid-promoted dehydration sequence. The use of potassium (3-hydroxy-3-methylbut-1-yl)trifluoroborate as a new coupling reagent allows the direct introduction of prenyl and 3-hydroxy-3-methylbutyl groups to a wide variety of electron-deficient heteroarenes. Synthetic application is also demonstrated.

Unified synthesis of caged Garcinia natural products based on a site-selective Claisen/Diels-Alder/Claisen rearrangement

A unified synthetic strategy toward caged Garcinia natural products has been designed and implemented. Central to the strategy is a tandem Claisen/Diels-Alder/Claisen rearrangement of a suitably substituted xanthone precursor to form forbesione (1a). Serving as a template, forbesione is then used to deliver representative members of this family, including desoxygaudichaudione A (4), desoxymorellin (5), and gambogin (10). Studies on the timing of this reaction cascade suggest that the C-ring Claisen/Diels-Alder rearrangement proceeds initially and is followed by the A-ring Claisen reaction. The electronic and steric effects that govern the outcome of this cascade are presented.

Minor constituents from Bupleurum fruticosum roots

A new triterpenoid saponin, 23-acetoxy,16 alpha-hydroxy-13,28-epoxyolean-11-en-3 beta-yl--beta-D-glucopyranosyl- (1-->2)---beta-D-glucopyranosil(1-->3)-beta-D-fucopyranoside, has been isolated from the roots of Bupleurum fruticosum, along with eight coumarins, scopoletin, scoparone, prenyletin, capensin, fraxetin, aesculetin,7-(3-methyl-2-butenyloxy-6-methoxycoumarin, 7-(2-hydroxy-3-methyl-3-butenyloxy)-6-methoxycoumarin and 5,7-dihydroxy-6-methoxy-8-(3-methyl-2-butenyl)coumarin. The last compound has not been previously encountered in nature. The structures were determined by analysis of NMR spectral data including two-dimensional techniques.