D-fructose-6-phosphate aldolase in organic synthesis: cascade chemical-enzymatic preparation of sugar-related polyhydroxylated compounds.

Novel aldol addition reactions of dihydroxyacetone (DHA) and hydroxyacetone (HA) to a variety of aldehydes catalyzed by D-fructose-6-phosphate aldolase (FSA) are presented. In a chemical-enzymatic cascade reaction approach, 1-deoxynojirimycin and 1-deoxymannojirimycin were synthesized starting from (R)- and (S)-3-(N-Cbz-amino)-2-hydroxypropanal, respectively. Furthermore, 1,4-dideoxy-1,4-imino-D-arabinitol and 1,4,5-trideoxy-1,4-imino-D-arabinitol were prepared from N-Cbz-glycinal. 1-Deoxy-D-xylulose was also synthesized by using HA as the donor and either 2-benzyloxyethanal or 2-hydroxyethanal as acceptors. In both cases the enzymatic aldol addition reaction was fully stereoselective, but with 2-hydroxyethanal 17 % of the epimeric product at C2, 1-deoxy-D-erythro-2-pentulose, was observed due to enolization/epimerization during the isolation steps. It was also observed that D-(-)-threose is a good acceptor substrate for FSA, opening new synthetic possibilities for the preparation of important novel complex carbohydrate-related compounds from aldoses. To illustrate this, 1-deoxy-D-ido-hept-2-ulose was obtained stereoselectively by the addition of HA to D-(-)-threose, catalyzed by FSA. It was found that the reaction performance depended strongly on the donor substrate, HA being the one that gave the best conversions to the aldol adduct. The examples presented in this work show the valuable synthetic potential of FSA for the construction of chiral complex polyhydroxylated sugar-type structures.