Gorgosterol

Scheme 13. Biosynthesis of the gorgosterol side-chain in dinofiageilates...

The biosynthesis of gorgosterol (93) has been the subject of much discussion and speculation, yet it was not until very recently that its origin was elucidated [60]. This study was performed by feeding radiolabeled sterols to a cell-free extract of the heterotrophic dinoflagellate Crypthecodinium cohnii. As was predicted [55] in 1970, the gorgosterol side-chain (in 93a) is produced from dinosterol (94) as shown in Scheme 13. 

Establishment of the dietary or symbiotic origin of sterols could be a difficult task, unless the tracer sterol has an unusual structure and restricted occurrence. There is little doubt, for example, that petrosterol (59) found in the nudibranch Peltodoris atromaculata [112] arises from predation of the mollusc on the sponge Petrosia ficiformis, from which the sterol was first isolated [113]. However, the origin of gorgosterol (60) in Tridacna species has not yet been established [114] despite its unusual structure. 

Anderson, D. G., Gorgosterol biosynthesis localisation of squalene formation in the zooxanthellar component of various gorgonians, Comp. Biochem. Physiol., 81B, 423, 1985. 

Steroids containing cyclopropyl groups in the side chain, as exemplified by gorgosterol (17) from the gorgonia Plexaura flexnosa as well as the zoanthid Palythoa sp. . 

Other biogenetically distinct cyclopropane side chain substitution patterns are typified by the sponge sterols calysterol (25) isolated from Calyx nicaensis, and petrosterol (19) found in Petrosia. Unlike the biosynthesis of gorgosterol-type (17)... 

Cholesterol is a major sterol in Coelenterata. The predominant sterol is gorgosterol... 

Several polyhydroxy sterols were isolated from octocorals, alcyonaceans, gorgonians and other marine sources. Hippurin-I (58) [66], 3a-acetoxy-ll -hydroxy-24-methyl-22,25-epoxy-5a-furostan-18,20/S-lactone (59), 3a-acetoxy-24-methyl-ll/8,18 18,20/J 22,25-triepoxy-5a-furostane (60) and several of their analogues were isolated from Isis hippuris collected in Okinawa by Higa [67]. A polyhydroxy sterol (61) containing the gorgosterol side chain was also present [68] (Fig. 6). 

Acanthasterol [82] (acansterol) [83,84] (72), the isomer of gorgosterol, was isolated from Acanthaster planci by Scheuer. Gorgostanol [85] and demethyl-acanthasterol (73) [86] were also found in A. planci [86] (Fig. 8). Transformation from A - to A -sterol may be a common bioconversion in Echinodermata, because gorgonians are components of the diet of A. planci in which gorgosterol cannot be found. 

Attempts to use the secosteroid biosynthetic enzymes in a chemoenzymatic synthesis have recently proved successful. An acetone powder from P. americana transformed gorgosterol to 9(11)-secogorgosterol in a quantitative yield on a 5 mg scale [82]. The transformation was found to require both NAD and NADP. Interestingly, this chemoenzymatic process proved to be applicable to the production of a variety of known and novel 9( 11 )-secosterols indicating the lack of specificity of the enzymes involved and the utility as a synthetic tool [83], The secosteroids 48 - 51 were produced from their corresponding steroid in yields of 30 - 78%. 

Crystal structures and absolute configurations have been determined by X-ray methods for two marine sterols (8) and (9), related to gorgosterol, with a cyclopropane ring in the side-chain, for 8-aza-oestradiol, 3 -acetoxy-17a-iodoandrost-5-ene, and withanolide E, which is found to have the unusual 17a-configuration (10). Structure determinations of 2 -hydroxytestosterone... 

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