Carbohydrates “(CH2O)n” or sugars are the formal adducts of carbon (atoms) to water with a repeating unit that structurally resembles “H–C̈–OH” (hydroxymethylene, 3a). Although hydroxymethylene has been suggested as a viable building block for sugar formation, it is a reactive species that had escaped its detection until recently.
Here we demonstrate that formaldehyde (2a) reacts with its constitutional isomer hydroxymethylene to give glycolaldehyde (1a) in a nearly barrierless reaction. This carbonyl-ene-type transformation operates in the absence of base and solvent at cryogenic temperatures similar to those found in extraterrestrial environments or interstellar molecular clouds. Hydroxymethylene acts as a building block for an iterative sugar synthesis as we demonstrate through the formation of the triose glyceraldehyde (5a). The results, therefore, provide a link between the well-known formose (Butlerow) reaction and sugar formation under non-aqueous conditions.[3-4] High-level ab initio coupled cluster computations confirm our proposed nearly barrierless carbonyl-ene-type reaction.