Abiotic aldol reactions of formaldehyde with ketoses and aldoses—Implications for the prebiotic synthesis of sugars by the formose reaction

The aldol reactions of formaldehyde is the essence of the formose reaction, considered the leading prebiotic pathway for accessing sugars on the early Earth. However, the formose reaction produces an intractable mixture, and efforts to tame the reaction to selectively and efficiently form aldose sugars have been unsuccessful. We have undertaken an NMR-mechanistic study of the aldol reactions of excess formaldehyde with glycolaldehyde, dihydroxyacetone, erythrulose, and erythrose under mild conditions and show that the reaction pathway is dominated by the formation linear ketoses and eventual accumulation of branched ketoses. Formation of C4 and higher aldo-sugars were not observed, implying that neither carbonyl migrations nor retroaldol reactions are occurring. Our results suggest that (1) controlling the aldol reaction of formaldehyde to selectively produce linear aldoses appears unfeasible; and (2) the concept of the formose reaction as a prebiotic source of ribose on early Earth needs serious reconsideration, and other models/options should be explored.