Agnatasterone A and B, Unusual Pregnane Steroids Isolated from the North‐East Atlantic Sponge Axinella agnata

Abstract

Two novel pregnatrienolones isolated in very small amounts from the North-East-Atlantic demosponge Axinella ugnuta (Tetractinomorpha, Axinellida) are unique in having C(2)=C(3) (or C(3)=C(4)), C(7)=C(8), and C(16)=C(17) bonds and a 12P-OH group which, being strongly H-bonded to a 20-keto group, resists acylation. 'Hand ' k -N M R spectroscopy of the steroids and of products of their selective epoxidation or reduction allow us to propose the structures (+)-12P-hydroxy-5cc-pregna-2,7,16-trien-ZO-one ( = agnatasterone A, (+)-1), and (+)-12g-hydroxy-5cc-pregna-3,7,16-trien-20-one ( = agnatasterone B, (+)-5), for the two steroids with minimal recourse to model compounds. 1.Introduction. -In the area of natural products marine sponges of the order Axinellida have aroused much interest for a wide variety of N-containing, unusual metabolites which include isonitrile [ 11 and mixed-biogenesis terpenoids [2], as well as guanidine-derived compounds [3]. In contrast, as regards novel steroids, of which the phylum Porifera is otherwise the most various producer [4], interest in the Axinellida has been so far concentrated on some A-ring contracted C,, sterols only [5].We expand here the interest towards the Axinellida steroids, and in general the sponge steroids, by reporting on two new pregnanes from a member of the Axinellida, the North-East-Atlantic sponge Axinella agnutu. This has also an interest per se in the area of steroids as these novel pregnanes have unique structural features.In fact, though pregnane steroids exert important hormonal functions in terrestrial organisms [6], only a few of them, not possessing any peculiar structural characteristic, have been so far isolated from marine organisms. Included are free pregnanes from the distant demosponge Haliclonu rubens (Ceractinomorpha, Haposclerida) [7], the alcyonacean corals Gersernia rubijormis [gal and Cupnellu sp. nov. and Cupnellu erecta [gb], and the gorgonian Telesto riisei [8c], other than aglycones of starfish saponins [9].The structural elucidation of steroids is not easily accomplished by NMR techniques due to many closely lying signals, particularly in the 'H-NMR spectrum even at high magnetic field. Two approaches are currently followed. One is to place much emphasis on "C-NMR data in comparison with reference steroids of known structure [lo]. The other approach mainly relies on 'H-NMR data [ll], thus requiring much time on a latest generation, high-field NMR spectrometer in order to carry out two-dimensional experiments at high resolution. This [l 11, under favourable circumstances, allows to assign all "C and ' H resonances, though large amounts of steroids are needed. Therefore, the second approach [ 1 I] has been used with known steroids as models.

Polyoxygenated steroids of marine origin

“…Two novel unusual pregnanes, agnatasterone A (108) and B (109), pregnatrienolones without much relationship on reference steroids, have recently been discovered in Axinella agnata109 collected off Roscoff, France, and in Northern area of lie de Batz. The pregnane structures of 105-109 immediately suggest steroid hormones, but the biogenesis and the role of these sponge-derived pregnane steroids have yet to be elucidated.…”

Sulfated Steroids: Ptilosteroids A−C and Ptilosaponosides A and B from the Solomon Islands Marine Sponge Ptilocaulis spiculifer

“…In recent years it has been found that steroids deriving from cholesterol are also quite generally present in marine invertebrates . Pregnane derivatives have been isolated from gorgonian octocorals, , soft corals, and a few sponges. − Marine sponges are a prolific source of interesting new pregnane steroids and their oxygenated derivatives, including steroids with rearranged main skeletons as well as rearranged side chains. In our ongoing research on the exhaustive isolation of new molecules from the Solomon Islands sponge Ptilocaulis spiculifer , it was found that this species contains other terpenes than previously isolated. , Purification of the methanolic extract by conventional methods afforded a series of five new modified steroids, named ptilosteroids A−C and ptilosaponosides A and B. Herein, we report the isolation and structural elucidation of these polyoxygenated steroids.…”

“…1 Pregnane derivatives have been isolated from gorgonian octocorals, 2,3 soft corals, 4 and a few sponges. [5][6][7][8] Marine sponges are a prolific source of interesting new pregnane steroids and their oxygenated derivatives, including steroids with rearranged main skeletons as well as rearranged side chains. In our ongoing research on the exhaustive isolation of new molecules from the Solomon Islands sponge Ptilocaulis spiculifer, it was found that this species contains other terpenes than previously isolated.…”

Four New Pregnane-10,2-carbolactones from an <i>Epipolasis</i> sp. Marine Sponge

“…The structures of these rare pregnane analogues were fully elucidated by interpretation of one-and two-dimensional NMR spectroscopic data. While a number of pregnane steroids have been described from marine sponges and other invertebrates, [17][18][19][20][21][22] pregnanes possessing a 10,2-carbolactone ring have only been reported twice, from Hawaiian collections of Myrmekioderma sp. 15) and Petrosia (Stongylophora) sp.…”