Acid-catalyzed isomerization of the petrosterol side chain (1) proceeds stereospecifitally to yield the naturally occurring 26-dehydro-2Sepiaplysterol side chain (2); in addition, a 1.5-hydride shift leading to 22-dehydro-25-epiaplysteryl acetate (3) has also been observed. -Our postulate' that biosynthetic C-alkylation may also occur by isomerization of a cyclopropane intermediate together with our recent progress' on the acid-catalyzed ring-opening of steroidal cyclopropanes prompted us to examine both the stereo-and the regio-specificity of this process with a naturally occurring cyclopropane. For this purpose, we selected the acetate 1 of the sponge sterol petrostero13 (isolated from Petrosia ficiformis 3a) , since one of the possible products of acid-catalyzed* ring-opening should be the recently isolated4 "26-dehydroaplysterol" (cf. 2) of unknown configuration at C-25. Elucidation of its C-25 stereochemistry is important in order to demonstrate a conceivable biosynthetic relation between 1 and 2. We now report a successful resolution of both problems, which increases the likelihood that the postu-latedls5 biosynthetic intermediacy of cyclopropanes operates in nature. Petrosteryl acetate (1; m.p. 109-llO°C (MeOH); m/z 394.3596, 100, M-HOAC)~ was treated in the usual manner* with gaseous HCl in acetic acid for 150 min at room temperature. The complex reaction mixture was separated by reverse-phase HPLC (two Altex-Ultrasphere columns in series with absolute MeOH (3 mL/min) as the mobile phase) and 11 fractions, yielding together over 80%, were collected and each analyzed by mass spectrometry. All short retention time fractions (cl35 min) show acetoxylated or chlorinated side chain fragments in their mass spectra which are due to molecules generated by nucleophilic ring-opening of the initially protonated steroidal cyclopropane.2 The two "true" isomers 1. and 3, having both retention times >150 min on HPLC, were isolated in 23% yield. The 'H-NMR spectrum (see Table) of the main component (m/z 394.3587, 100, M-HOAC)~ showed it to be the anticipated isomerization product 2 which was identical with "26dehydroaplysterol" isolated 4 from the sponge Petrosia ficiformis. The constitution of the side chain of the minor component 2 (m/z 394.3599, 100, M-HOAC)~ was established initially by comparison of its 'H-NMR spectrum (see
Table) with that of crinosteryl acetate (i)* which differs 2063