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2.14.7 Self- and Cross-Aldol Reactions of Glycolaldehyde

DOI: 10.1055/sos-SD-202-00363

Clapés, P.; Fessner, W. -D.Science of Synthesis: Stereoselective Synthesis, (20112729.

d-Fructose 6-phosphate aldolase from E. coli (FSA, described in Section 2.14.2) can also utilize 2-hydroxyacetaldehyde (glycolaldehyde; GA) as a nucleophilic aldol component and thus can catalyze a direct self-aldolization to furnish d-()-threose (Scheme 40). 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.[‌172‌] Formation of cross-aldol adducts is also favored by the formation of stable cyclic hemiacetals or hemiaminals as occurs with the examples 86 shown in Scheme 40. 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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