Rare Ketose Synthesis from Glycolaldehyde by C2 Elongation of Aldoses Catalyzed by Transketolase Variants

The enzyme thiamine diphosphate-dependent transketolase (TK) efficiently catalyzes the C2 elongation of aldoses using β-hydroxypyruvate (HPA) as a nucleophile for the synthesis of Cn+2 ketoses by stereoselective C–C bond formation. This synthetic approach is flawed by the tedious, low-yield chemical route to HPA. We report here an alternative transketolase-catalyzed reaction, with glycolaldehyde (GoA) as a nucleophile, for the efficient upgrading of several aldoses (C3–C5) as electrophiles, eliminating the release of carbon dioxide from the commonly used HPA. We found that the syn-acyloin condensation of GoA catalyzed by wild-type Escherichia coli transketolase (wt-TK_eco) carried out under an inert atmosphere reduced byproduct formation and potentiated erythrulose (ERY) formation, giving ERY with a 78% isolated yield from 25 g·L^–1 of GoA. The same reaction conditions were extended to the cross-acyloin condensation of GoA as a nucleophile with different aldoses as electrophiles, d-glyceraldehyde, l-glyceraldehyde, l-threose, d-ribose, and 5-deoxy-d-ribose, in stoichiometric amounts. Following this one-pot procedure, the corresponding Cn+2 ketoses of biological interest, d-xylulose, l-ribulose, l-sorbose, d-sedoheptulose, and 7-deoxy-d-sedoheptulose were obtained with moderate-to-high yields (38–89%) and high diastereoselectivities, showing that the TK-catalyzed reaction equilibrium was most often shifted toward the expected products. TK-catalyzed cross-acyloin reaction from GoA removes the need for HPA in the reaction, affording efficient atom economy, with product yields similar to or higher than obtained with HPA as a nucleophile.