Formose reaction

The Formose reaction is formation of sugars from formaldehyde catalyzed by base and a divalent metal such as calcium hydroxide. Formose is a contraction of formaldehyde and aldose and discovered in 1861 by Aleksandr Butlerov ^{[1] [2]}. The intermediary reaction steps taking place are aldol reactions, reverse Aldol reactions and aldose-ketose isomerizations. Intermediates are glycolaldehyde, glyceraldehyde, dihydroxyacetone and tetrose sugars.

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A mechanism was proposed by Breslow in 1959 ^[3]

Two molecules condense to form glycolaldehyde 1 which further reacts in an Aldol reaction with another equivalent of formaldehyde to glyceraldehyde 2. An aldose-ketose isomerization of 2 forms dihydroxyketone 3 with can react with 2 to form ribulose 4 and through another isomerization ribose 5 or with formaldehyde to tetrulose 6 and then aldoltetrose 7. Intermediate 7 can split up into 2 in a retro-aldol reaction.

This reaction is of importance to the question of the Origin of Life as it clears the path from simple formaldehyde to complex sugars like ribose and from there to RNA. In addition, both formaldehyde and glycolaldehyde are known in outer space. In one experiment imitating early earth pentoses formed from mixtures of formaldehyde, glyceraldehyde and borate minerals such as colemanite Ca₂B₆O₁₁5H₂O or kernite Na₂B₄O₇ ^[4]

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