The isolation, structure determination and synthesis of three novel sterols with unusual side chains is reported.
A consistent scheme is proposed which accounts for the biosynthesis of all major and minor sterols in this sponge.
Recently
, 3-5 we recorded the structure elucidation of several marine sterols with highly branched side chains, which represent the most exotic array of biomethylation sequences hitherto encountered in the steroid field. These structures are all consistent with a plausible biosynthetic scheme,6 starting with epicodisterol, which, however, has not yet been confirmed experimentally by tracer incorporation studies. We have now solved7 the technical problem of incorporating a sterol precursor (such as epicodisterol) into a sponge and, therefore, are searching for the best substrates in which the largest number of unusual biomethylations can be demonstrated experimentally in one animal. In this paper we report our isolation and structure eludication studies with a Pseudoaxinyssa sp. (Australian Great Barrier Reef) which appears to be an extraordinarily interesting candidate, since it is virtually devoid of conventional sterols. Hofheinz and Oesterhelt' reported that this sponge' contained only 24-isopropylcholesterol (1) and its 22-dehydro analog 2 -_. Together with 24-isopropylidenecholesterol (J)," these are the only natural marine sterols with a 24-isopropyl substituent. This raises interesting questions concerning their biosynthesis and biological function and we considered it worth while to look for trace sterols, which might represent biosynthetic markers. Separation of the sterol mixture by reverse phase HPLC (Altex Ultrasphere ODS-5 with 98:2 MeOH-H20) provided 47% of l_,' 51% of 2' and 1.5% of 3' as well as three new trace sterols, 11 whose properties and synthesis _ are herewith reported. 24-Ethyl-24-methylcholesterol (4) [rel. ret. time: GC, 2.23 (3% OV-17); HPLC, 1.25 (ODS-2)] is one of the few known sterols 3-5 with a quaternary center at C-24. The 'H NMR data (Table 1) are consistent with the assigned side chain substitution pattern. The characteristic mass spectral peaks at m/z 428.403 (M+, 12 of the --C30H520), 273, 255, 231 and 213 demonstrate the presence conventional cholesterol nucleus and the absence 13 of unsaturation in the side chain. The complete structure (except for C-24 stereochemistry) was proved by synthesis involving LDA-promoted ' and Liaoning Institute of Materia (1981).