1P6: d) Integrated synthesis of dihydroacetone phosphate from Glycerol and its utilization in situ for diastereoselective aldol additions

 Fessner and Sinerius²⁴ presented an efficient technique which integrates cofactor dependent enzymic phosphorylation and dehydrogenation into a single, closed-loop system by employing phosphoenolpyruvate as the sacrificial reagent for sequential ATP and NAIY recycling steps. Exemplary applications are developed for the synthesis of 6-phosphogluconate from glucose, and that of dihydroxyacetone phosphate from glycerol. The latter system is combined with exergonic diastereoselective aldol additions for the one-flask synthesis of a ketosugar (D-sorbose), thiosugar (L-threo-5-thiopentulose), or a sugar acid (L-threo-pent-4-ulosonic acid) starting from a mixture of glycerol and simple aldehydes. Dihydroxyacetone phosphate (DHAP, 14). Which is synthesized from the inexpensive source glycerol. DHAP is the essential substrate of a set of four stereo chemically distinct aldolases which catalyze highly stereo selective C-C bond formations between 14 and a broad range of aldehydic electrophiles.²⁵ This technique has recently received considerable attention because of its potential in the building-block type asymmetric synthesis²⁶ of sugars and related polyhydroxylated compounds such as glycosidase inhibiting alkaloids.²⁶ Dihydroxyacetone phosphate (DHAP, 14) when supplemented by a microbial rhamnulose-l-phosphate aldolase (RhuA, EC 4.1.2.19)²⁷ and a suitable aldehyde (Scheme 4). From the reaction with D-glyceraldehyde (15) the pyranoid D-sorbose-1-phosphate (16) was obtained in fair yield only (48%; unoptimized), possibly hampered by the less satisfactory redox equilibrium. Pure D-sorbose (17) was obtained in free form by enzymic dephosphorylation employing alkaline phosphatase (EC 3.1.3.1).

Engagement of the oxygen sensitive mercaptoacetaldehyde (18) in the combined enzymic process gave rise to the expected L-threo configurated thiopentulofuranose l-phosphate (19). In this case a slight precipitate was formed upon addition of 18, either due to the compound’s low solubility or to partial protein denaturation. Nevertheless, 19 (enantiomeric to the compound obtainable with rabbit muscle aldolase) were isolated in good yield. Its sensitivity required acid phosphatase (EC 3.1.3.2) for mild hydrolysis of the phosphate ester to provide the free thiosugar (20) which is of interest as a potential glycosidase inhibitor. In a third example, methyl glyoxylate (21) was applied as the aldol acceptor when it was discovered that the ester function became hydrolyzed during (or after) C-C bond formation. Since free glyoxylic acid and the carboxylic acid 14 formed upon aldol addition were not recognized and converted by the RhuA enzyme, complete conversion of the DHAP equivalent to the phosphorylated L-threo-pent-4-ulasonic acid (10: 1 equilibrium of the acyclic form 22a and a cyclic lactone hemiacetal 22b according to ¹H NMR) resulted through practically irreversible adduct formation. Author²⁴ considers that technique outlined in this paper will prove a valuable and practical extension to current enzymic methology of aldol reaction.

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24. W.-F. Fessner, and G. Sinerius, Bioorg. Med. Chem., 1994, 2, 639.
25. W.-D. Fessner, Kontakte (Darmstadt), 1992, 3, 3; 1993, (I), 23.
26. W.-D. Fessner, In Microbial Reagents in Organic Synthesis, pp. 43-55, Servi., S. Ed.; Kluwer Academic Publishers; Dordrecht, 1992.
27  W. D. Fessner, G. Sinerius, A. Schneider, M. Dreyer, G. E. Schulz, I. Badia, and J. Aguilar, Angew. Chem Int. Ed. Engl, 1991, 30, 555.

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