Studies on the Minor Constituents of the Caribbean Gorgonian Octocoral Briareum asbestinum PALLAS. Isolation and Structure Determination of the Eunicellin-Based Diterpenoids Briarellins E-I.

Abstract

Five new eunicellin-type diterpenoids, briarellins E--I, along with several known diterpenoids of the asbestinane, briarane and eunicellane classes, were isolated from the Caribbean gorgonian octocoral Briareum asbestinum collected in Puerto Rico. The structures of these compounds were established on the basis of spectroscopic evidence.

A Unified Strategy for Enantioselective Total Synthesis of Cladiellin and Briarellin Diterpenes: Total Synthesis of Briarellins E and F and the Putative Structure of Alcyonin and Revision of Its Structure Assignment

“…Finally, Dess–Martin oxidation of 12 afforded briarellin F ( 13 ) in good yield. Synthetic briarellin E ( 12 ) was identical in all respects with a natural sample;81 spectral and optical rotation data for briarellin F ( 13 ) also compared well with those reported for the natural isolate 28…”

“…Having established that we could efficiently access cladiellins containing a hydroxyl substituent at C4, we desired to extend our approach to the total synthesis of the more complex briarellin diterpenes 28. Like most briarellin diterpenes,45 briarellins E ( 12 ) and F ( 13 ) were isolated by Rodríguez and co-workers from Caribbean gorgonian octococorals belonging to the genus Briareum (Figure 2).…”

“…Like most briarellin diterpenes,45 briarellins E ( 12 ) and F ( 13 ) were isolated by Rodríguez and co-workers from Caribbean gorgonian octococorals belonging to the genus Briareum (Figure 2). 10,28 The constitution and relative configuration of these coral metabolites were established on the basis of NMR studies and chemical correlations.…”

“…To address the unmet synthesis challenges posed by this C4 functionalization, and the additional oxepane ring found in the briarellins and asbestinins, we undertook the total synthesis of alcyonin (originally proposed structure 10 )27 and briarellins E ( 12 ) and F ( 13 ) 28. A full description of these studies, which led to a revised structure assignment for alcyonin ( 11 )16e and the first total syntheses of briarellin diterpenes ( 12 and 13 ), is the subject of this report 16f.…”

Strategic Use of Pinacol-Terminated Prins Cyclizations in Target-Oriented Total Synthesis

“…52 A wide variety of oxacyclic diterpenes has been isolated from soft corals and gorgonian octocorals. 29 The cladiellins (e.g., 62-64), 53 briarellins (e.g., 65), 54 and asbestinins (e.g., 68) 55 belong to the C2,C11-cyclized cembranoid diterpene family and have in common a rare oxatricyclic ring system composed of hexahydroisobenzofuran and oxacyclononane units, as well as six stereogenic carbons (C1-3, C9, C10, and C14; Figure 5). 56 Although the proposed SCHEME 6.…”