A complete assignment of the 13C and 1H n.m.r. resonances of the lichen triterpenoids, pyxinol, 3,25-di-O-acetylpyxinol and 3,12,25-tri-O-acetylpyxinol has been achieved by using a combination of one- and two-dimensional n.m.r. data and T1 values. Hydrogen bonding between the 12β- hydroxy group and the carbonyl oxygen of the 25-acetoxy group of 3,25- di-O-acetylpyxinol leads to the furan ring system adopting a� conformation different from that adopted by 25-hydroxy analogues, hence differing chemical shifts are observed for some of the carbons of rings C and D and the furan ring.
Total syntheses of the lichen depsides glomelliferic acid (11, loxodellic acid (2), glomellic acid (3), 4-0-demethylglomelliferic acid (4), 4-0-demethylloxodellic acid (5), 4-0-demethylglomellic acid (6) and oxostenosporic acid (7) are reported.
IntroductionGlomelliferic acid (1) and its derivatives form a small group of lichen metabolites1-5 where one or both component rings contain a reactive benzyl ketone substituent in close proximity to the depside ester functional group. The interaction of these functionalities thwarted early attempts to synthesize this group of compounds6 but the total synthesis of several depsides containing an oxidized A-ring side chain have now been r e p~r t e d .~,~ Here, difficulties arising from the close proximity of functional groups were averted by using a 1,3-dithian-2-yl keto-protecting group. However, considerable difficulties were encountered in reproducing certain key steps in this approach and in extending this to depsides containing oxidized side chains in both Aand B-rings. We have since found that 1,3-dioxolan-2-yl keto-protecting groups are far superiorg and have now successfully used the appropriately substituted intermediates to complete the total synthesis of glomelliferic acid (I), loxodellic acid (2), glomellic acid (3), 4-0-demethylglomelliferic acid (4), 4-0-
The triterpenes 20α-acetoxyhopane-6α, 22-diol, 16β-acetoxyhopane-6α,22-diol,
6α-acetoxyhopane-16β,22-diol
and 6α,l6β -diacetoxyhopan-22-ol have
been isolated from chemical strains of Physcia aipolia. The structure of these triterpenes
followed primarily from the 13C n.m.r. spectra.
275ChemInform Abstract ((Ia)-(Ic), performed to clear some inconsistencies in previously published assignments, including those of Tanaka and Yahara (1978) for (Id)).
An efficient new procedure has been developed for the oxidation of the title compounds, utilizing one mole of bromine in the presence of acetic anhydride.
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