No abstract

No abstract

The cis-or trans-arrangement of the hydroxyl groups in eight 2,3-transflavan-3,4-diol racemates has been established by consideration of their isopropylidene derivatives, their rates of reaction with lead tetra-acetate, and the proton magnetic resonance spectra of their diacetates. This necessitates a revision of the existing assignment of relative stereochemistry to the two racemates of 4'-methoxy-6-methyl-2,3-trans-flavan-3,4-diol. The most convenient preparative method for the 2,3-tran.s-flavan-3,4-cis-diols makes use of the reduction of 3-hydroxy-2,3-trans-flavan-4-ones with a mixture of lithium aluminium hydride and aluminium chloride.THE realisation 1 of the wide occurrence and importance of flavan-3,4-diols (leucoanthocyanidins) (I) in Nature has focused attention on the stereochemistry both of the natural diols and of those produced by various synthetical route^.^ Flavan-3,4-diol (I; R = R' = €3. ) can exist in eight stereoisomeric forms, which afford four racemates. The work described here is limited to 2,3-trans-flavan-3,4-diols obtained from 3-hydroxy-2,3-trans-fla\.an-4-ones, whose relative stereochemistry at the 2,3-positions is well e ~t a b l i s h e d . ~~~~Bognir and RBkosi 3b have shown that reduction of 2,3-trans-3-hydroxyflavan-4-one (11; R = R' = H) with lithium aluminium hydride, sodium borohydride, or hydrogen over palladium-charcoal leads to a flavan-3,4-diol of m. p. 145-146".Examination of the crude products from these reductions by chromatography on thin silica films5 has shown that they contain 3-hydroxyflavanone and its oxidation product, 3-hydroxyflavone,

Aus dem Diketon (I) wird nach Ketalisierung und Ringschluß mit Vinyläthylketon, Reduktion des Ketals (II) (Li/NH3) und Methylierung das entstandene Tetramethyldcrivat (III) reduziert (Li‐tert.‐butoxy‐A1‐hydrid), acetyliert und katalytisch hydriert zum Keto‐acetat (IV).