No abstract

On the Depolymerization of Poly-(R)-3-hydroxy-butanoate (PHB) SummaryFrom cell-free PHB or from dried cells of Alcaligenes eutrophus H 16, which had been grown in an aqueous fructose solution, enantiomerically pure methyl, ethyl, butyl or P-methoxyethyl (R)-3-hydroxy-butanoates are obtained in yields ranging from 75-90% (0.1 to 70 g scale). The depolymerization is achieved by heating the PHB-containing materials to temperatures of 80-160" in the corresponding alcohol with or without the cosolvent dichloroethane in the presence of either sulfuric acid or tetraethoxytitanium catalyst. Since (S)-3-hydroxy-butanoates are also readily obtained (by yeast reduction of aceto-acetates), starting materials derived from 3-hydroxy-butyric acid now belong to the especially useful group of synthetic building blocks which are available in both enantiomeric forms.Der Einsatz von chiralen, enantiomerenreinen Synthesebausteinen des Typus A hat sich vielfach bewahrt [ 11, vor allem nachdem nicht nur Derivate A mit R'= H, sondern durch Alkylierung von doppelt deprotoniertem P-Hydroxy-bu ttersaureester (1) [ 2 ] auch solche mit R'=Alkyl, einfach herstellbar wurden. Bisher waren nur der (S)-konfigurierte /I-Hydroxyester 1, und damit die Bausteine A leicht zuganglich (Reduktion von Acetessigsaureathylester von der Re-Seite der Ketogruppe mit OR2

(Fig. 2 and 3).Enantiomerically pure 4,4,4-trichloro-and 4,4,4-trifluoro-3-hydroxybutanoic acid derivatives 1 and 2 are highly desirable chiral starting materials for organic synthesis. Thus, the p-lactone of (S)-4,4,4-trichloro-3-hydroxybutanoic acid was shown by Wynberg & Staring [I] to be accessible in 98% enantiomeric excess (ee) by quinidine-catalyzed asymmetric cycloaddition of ketene to chloral, and was hydrolyzed to (S)-malic acid, a source of numerous useful chiral building blocks [2]. In view of the interest in fluorinated analogues of biologically important natural products and drugs, an access to the chiral, non-racemic trifluoroester 2 would be even more important. Furthermore, it was interesting to find out, whether Prelog's rule [3] for the enantioselective hydrogenation of carbonyl compounds by NADH-dependant enzymes of microorganisms (see A-B) would be followed by the trihalomethyl precursors of 1 and 2, as it is for the 3-ketoesters which are reduced by baker's yeast (Saccharomyces cerevisiae) to the hydroxyester 3 [4][5][6], 4 [4], and 5 [7] of more than 90% ee') (ee N optical purity in the present paper).

Yeast reduction of ethyl acetoacetate: ( S )‐( + )‐ethyl 3‐hydroxybutanoate reactant: 20.0 g (0.154 mol) of ethyl acetoacetate product: (S)‐( + )‐ethyl 3‐hydroxybutanoate reactant: ethyl (S)‐( + )‐3‐(3′,5′‐dinitrobenzoyloxy)butanoate product: methyl 3‐hydroxybutanoate byproduct: (R‐)‐( − )‐Ethyl 3‐hydroxybutanoate