Racemic pentoses, synthesis

An acyclic precursor devised by Bogndr and Herczegh, prepared via oxidative cleavage of the furane nucleus, has been applied in the synthesis of racemic pentoses. 2-(2-Furyl)-4,4,S,S-tetramethyl-l,3-dioxolane 257 obtained in reaction of furfural with 2,3-dimethylbutane-2,3-diol was chosen as a substrate, owing to its stability in acidic media (at pH > 3). Oxidation of 257 with bromine water gave the unstable endialone derivative 258, which was reduced with sodium borohydride to 259 (isolated as crystalline dibenzoate). Epoxidation of 259 gave... 

A simple, divergent, asymmetric synthesis of the four stereomers of the 3-amino-2,3,6-trideoxy-L-hexose family has been proposed by Dai and co-workers [523] which is based on the Katsuki-Sharpless asymmetric epoxidation of allylic alcohols ( )-408. iV-Trifluoroacetyl-L-daunosamine, A-trifluoro acetyl-L-acosamine, A-benzoyl-D-acosamine, and A-benzoyl-D-nitrosamine have been derived from methyl sorbate via the methyl 4,5-epoxy-( )-hex-2-enoates obtained via a chemoenzymatic method [524]. Application of the Katsuki-Sharpless enantioselective epoxidation to racemic mono-O-benzylated divinylglycol has allowed us to prepare enantiomerically pure L-lyxo and D-/yro-pentoses and analogs [525,526,527, 528],... 

Proline is a stable, nontoxic, cyclic, secondary pyrrolidine-based amino acid with an increased pK value. Thus, proline is a chiral bidentate compound that can form catalytically active metal complexes (Melchiorre et al. 2008). Bidentate means that proline has not only one tooth but also a second one to bite and react. The greatest difference to other amino acids is a Lewis-base type catalysis that facilitates iminium and enamine-based reactions. It is especially noteworthy that cross-aldol condensations of unprotected glycoladehyde and racemic glyceralde-hyde in the presence of catalytic amounts of the Zn-(proline)2 gave a mixture of pentoses and hexoses (Kofoed et al. 2004). Again, proline seems to play the decisive role. The conditions are prebiotic the reaction proceeded in water for seven days at room temperature. It is remarkable that the pentose products contained ribose (34%), lyxose (32%), arabinose (21%), and xylose (12%) and that all are stable under the conditions. Thus, the diastereomeric and enantiomeric selection observed support the idea that amino acids have been the source of chirality for prebiotic sugar synthesis. 

Nakagawa and co-workers performed a synthesis of racemic and both enantiomeric forms of 2-deoxy-eo rliro-pentose from 3-hydroxy-4-pentenoic acid... 

The synthesis consisted of the reaction of 90 with N-bromosuccinimide (NBS) to give the bromolactone 91, substitution of the bromine atom for a hydroxyl group to form 92 and reduction of the lactone grouping in 92 with bis[2-(3-methylbutyl)] borane to give the desired product. Optic ly active forms of the acid 90, which served for the synthesis of enantiomeric 2-deoxy-erythro-pentoses, were readily obtained by resolution of the racemate with quinine. 

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