Synthesis of Both Enantiomers of Conduritol C Tetraacetate and of meso‐Conduritol D Tetraacetate by Oxidation of Benzoquinone Bis(ethylene acetal)

Abstract

Epoxidation of p‐benzoquinone bis(ethylene acetal) (1) with m‐chloroperbenzoic acid or hydrogen peroxide/benzonitrile afforded corresponding monoepoxide 2, which was converted into p‐benzoquinone mono(ethylene acetal) monoepoxide 5 with perchloric acid. Dihydroxylation of 1 with osmium tetroxide or ruthenium trichloride/sodium periodate afforded corresponding cis‐diol 6, which was subsequently acetylated to give diacetate 7. One ethyleneacetal moiety in 7 could be selectively hydrolyzed with silica gel/ferric chloride under solvent‐free conditions to give ketone 8, which, upon reduction with sodium borohydride and subsequent acetylation of the formed alcohol group, afforded two diastereomeric triacetates 10. Hydrolysis of the remaining acetal functions in the two diastereomers 10, followed by reduction of the second carbonyl group as described above, afforded racemic conduritol C and meso‐conduritol D tetraacetates 12 and 13, respectively. Enzymatic resolution of the racemic arabino‐configured triacetate 10 with Lipozym failed, while the ribo‐configured counterpart reacted smoothly to give enantiomerically pure D‐ribo‐ and L‐ribo‐configured triacetates 10. The latter pair of enantiomerically pure triacetates were converted into both enantiomers of conduritol C tetraacetate 13 as described for the racemic compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Osmium-free direct syn-dihydroxylation of alkenes

“…These conditions were particularly successful for substrates that were difficult to oxidise with OsO 4 . 7 In recent years, Plietker has investigated the oxidation with RuO 4 and attempted to circumnavigate the different issues linked to the use of RuO 4 (over-oxidation/oxidative cleavage). 8 Mechanistic studies of the dihydroxylation led to the conclusion that the turnover-limiting step is the hydrolysis of the intermediate ruthenate ester (Fig.…”