Isolation and Structures of Sixteen New Asbestinin Diterpenes from the Caribbean Gorgonian Briareum asbestinum

Abstract

Sixteen new diterpenoids, representative of the asbestinane skeletal class, have been isolated from shallow water colonies of the Caribbean gorgonian octocoral Briareum asbestinum. The structures of these secondary metabolites, named asbestinin-11 [1], asbestinin-12 [3], asbestinin-13 [4], asbestinin-14 [5], asbestinin-15 [7], asbestinin-16 [8], asbestinin-17 [9], asbestinin-18 [10], asbestinin-19 [11], asbestinin-20 [12], asbestinin-21 [13], asbestinin-22 [14], asbestinin-23 [15], 11-acetoxy-4-deoxyasbestinin E [16], 11-acetoxy-4-deoxyasbestinin F [17] and 4-deoxyasbestinin G [18], were defined by chemical and spectroscopic methods. These specimens of B. asbestinum, collected in Puerto Rico, yielded almost exclusively diterpenoids possessing the asbestinane carbon skeleton thus suggesting minor biosynthetic variations for this gorgonian. In this paper, we also revise the structures of the known asbestinin-6 and asbestinin-7 to asbestinanes 2 and 6, respectively.

Peroxy natural products

“…The structure originally assigned to 11-acetoxy-4-deacetoxyasbestinin F 94 has been revised to 175. 95 Spectroscopic discrepancies observed for the enantioselectively synthesised structure originally proposed for alcyonin 96 have led to the proposal that the correct structure of the natural product is the allylic peroxide 176.…”

Total Syntheses of 11‐Acetoxy‐4‐deoxyasbestinin D, 4‐Deoxyasbestinin C, Asbestinin‐10, ‐20, ‐21 and ‐23

“…[8] Dess-Martin oxidation of asbestinin-20( 6)p roduced asbestinin-10 (14)a nd the structure of this natural product was confirmed. Enone reductionu nder Luche conditions afforded the allylic alcohols in a3 .2:1 ratio favouring asbestinin-20 (6), ar esult consistent with that reported by Ospina and Rodríguez when they reduced natural asbestinin-10 (14)u nder similarconditions. [8] The next target to be synthesizedw as asbestinin-21, which had been originally identified ast he ketone 15 (Figure 2), but was later reassigned to be the a-hydroxy ketone 16 (Scheme 7).…”

“…[8] The next target to be synthesizedw as asbestinin-21, which had been originally identified ast he ketone 15 (Figure 2), but was later reassigned to be the a-hydroxy ketone 16 (Scheme 7). [6,8] Dihydroxylation of 11-acetoxy-4-deoxyasbestinin D( 1)u nder standard Upjohn dihydroxylation conditions afforded ad iastereomericm ixture of 1,2-diols 41 a and 41 b in 56 %y ield, in which the crystalline diol 41 a predominated (5:1 ratio). [20] The diol 41 a was separated from the minor isomer 41 b and oxidized to give a-hydroxy ketone 16 by the use of the Dess-Martin protocol.…”

“…The isolation and characterization of the first members of the family—asbestinins‐1–5—were reported by Faulkner, Clardy and co‐workers in 1980 . The 39 asbestinins that have been isolated and characterized to date partition into two distinct structural groups: members that lack a C‐4 substituent, such as 11‐acetoxy‐deoxyasbestinin D ( 1 ) and 4‐deoxyasbestinin C ( 2 ), and those that are oxidised at C‐4, such as asbestinin‐12 ( 3 ) and asbestinin‐2 ( 4 ) (Figure ) . A biosynthetic route to the asbestinins from related briarellin natural products has been proposed in which Wagner–Meerwein rearrangement transfers a methyl group from C‐11 to C‐12 .…”

“…[1] The 39 asbestinins that have been isolated and characterized to date partitioni nto two distinct structural groups: members that lack aC -4 substituent, such as 11-acetoxy-deoxyasbestinin D( 1)a nd 4-deoxyasbestinin C ( 2), and those that are oxidiseda tC -4, such as asbestinin-12( 3)a nd asbestinin-2 (4) ( Figure 1). [2][3][4][5][6][7][8] Ab iosynthetic route to the asbestininsf rom related briarellin naturalp roductsh as been proposed in which Wagner-Meerwein rearrangement transfers am ethyl group from C-11t oC -12. [1,9] Theb riarellin naturalp roducts are, in turn, thought to be derived from the simpler cladiellins (eunicellins).…”