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  . The intermediary reaction steps taking place are aldol reactions, reverse Aldol reactions and aldose-ketose isomerizations. Intermediates are glycolaldehyde, glyceraldehyde, dihydroxyacetone and tetrose sugars.
A mechanism was proposed by Breslow in 1959 
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 
- Butlerov, A. C. R. Acad. Sci. 1861, 53, 145-147.
- Self-organizing biochemical cycles PNAS | November 7, 2000 | vol. 97 | no. 23 | 12503-1250 doi:10.1073/pnas.220406697.
- Breslow, R. Tetrahedron Lett. 1959, 1, 21, 22-26.
- Borate Minerals Stabilize Ribose A. Ricardo, et al. Science 303, 196 (2004); doi:10.1126/science.1092464