On the Funicolides, Briaranes of the Pennatulacean Coral Funiculina quadrangularis from the Tuscan Archipelago: Conformational preferences in this class of diterpenes

Abstract

AbstractKinetic and equilibrium NMR studies of briarane diterpenes isolated from the pennatulacean coral Funiculina quadrangularis showed that funicolide A (1), funicolide D (5), and brianthein W (6), with R2 = Hβ, undergo slow flipping by rotation of the C(1)C(2)C(3)C(4) dihedral angle, giving rise to two observable conformers in a 4:96 population ratio, both having an axial AcOC(14) and C(16) pointing ‘downwards’, but differing for pseudoaxial or pseudoequatorial position of R1O, respectively. δ(C) for the minor conformers could be quickly assigned by an original emulation methodology. Similar studies revealed that funicolide B (2), 7‐epifunicolide A (4), funicolide E (7), and unnatural epibrianthein W (8) undergo similar motions, where, however, the nature of the α‐positioned substituent R2 determines which conformer predominates: axial R1O for R2 = Hβ (4 and 8) or equatorial R1O for R2 α‐OH, (2 and 7). In contrast, funicolide C (3) proved to undergo slow conformational motions that involve also the cyclohexene ring, resulting in two observable conformers characterized by either an equatorial AcOC(14) and trans‐diaxial C(2)/C(9) or an axial AcOC(14) and trans‐diequatorial C(2)/C(9) in a 9:1 population ratio, respectively. These observations, and molecular‐mechanics calculations for briaranes known to exhibit broad NMR signals, lead to general views on the conformational preferences of diterpenes of this class.