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2.1.2.6 Self- and Cross-Aldol Reactions of 2-Hydroxyacetaldehyde

DOI: 10.1055/sos-SD-215-00021

Clapés, P.Science of Synthesis: Biocatalysis in Organic Synthesis, (2015280.

d-Fructose 6-phosphate aldolase from E coli (FSA, described in Section 2.1.2.2) can also utilize 2-hydroxyacetaldehyde (glycolaldehyde, GA) as a nucleophilic aldol component and thus can catalyze a direct self-aldolization to furnish d-(−)-threose (42, R1 = H; R2 = OH) (Scheme 33). In the presence of other aldehydes, 2-hydroxyacetaldehyde invariably acts as a donor for the corresponding cross-aldol additions. In the competition between self addition and cross addition of 2-hydroxyacetaldehyde, the extent of the desired cross-aldol reaction depends on the substrate quality of the acceptor aldehyde, but can be further increased by maintaining a low 2-hydroxyacetaldehyde concentration in the reaction medium, e.g. by slow addition of this compound with the aid of a syringe pump.[‌116‌] Formation of cross-aldol adducts is also favored by the formation of stable cyclic hemiacetals or hemiaminals as occurs with the in examples Scheme 33. The exemplary reactions show high stereoselectivity (>99%) for (2S)-configured syn-adducts. As with other enzymatic aldol additions, these reactions are conveniently carried out at ambient conditions in water, or with up to 20% of dimethylformamide as cosolvent to enhance the solubility of hydrophobic aldehydes.

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