Chemical and Biochemical Approaches for the Synthesis of Substituted Dihydroxybutanones and Di-, and Tri-Hydroxypentanones

<div>Polyhydroxylated compounds are building blocks for the synthesis of carbohydrates and other natural products. Their synthesis is mainly achieved by different synthetic versions of aldolcoupling reactions, catalyzed either by organocatalysts, enzymes or metal-organic catalysts. We have investigated the formation of 1,4-substituted 2,3-dihydroxybutan-1-one derivatives from para- and meta-substituted phenylacetaldehydes by three distinctly different strategies. The first involved a direct aldol reaction with hydroxyacetone, dihydroxyacetone or 2-hydroxyacetophenone, catalyzed by the cinchona derivative cinchonine. The second was reductive cross-coupling with methyl or phenyl glyoxal promoted by SmI2 resulting in either 5-substituted 3,4-dihydroxypentan-2-ones or 1,4 bis-phenyl substituted butanones, respectively.</div><div>Finally, in the third case, aldolase catalysis was employed for synthesis of the corresponding 1,3,4-trihydroxylated pentan-2-one derivatives. The organocatalytic route with cinchonine generated distereomerically enriched syn products (de = 60−99 %), with moderate enantiomeric excesses (ee = 43−56%), but did not produce aldols with either hydroxyacetone or dihydroxyacetone as donor ketones. The SmI2-promoted reductive cross-coupling generated product mixtures with diastereomeric and enantiomeric ratios close to unity. This route allowed for the production of both 1-methyl- and 1-phenylsubstituted 2,3-dihydroxybutanones, at yields between 40−60%. Finally, the biocatalytic approach resulted in enantiopure syn (3R,4S) 1,3,4-trihydroxypentan-2-ones.</div>