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2.1.2.5.2 Sequential Two-Step Aldol Additions of Acetaldehyde to Aldehydes

DOI: 10.1055/sos-SD-215-00021

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

When the aldol adduct formed in the first aldol reaction step does not form a stable cyclic hemiacetal, a second addition of an acetaldehyde nucleophile may take place to furnish lactols 41 (Scheme 32). For the design of sequential aldol multimerization, the starting aldehyde acceptor 40 must therefore be carefully selected or strategically protected to prevent the reaction from stopping after the first addition step because of the formation of the stable cyclic compound. Furthermore, higher-order sequences are made impossible after the second addition because of a pyranose cyclization involving the first aldol hydroxy group. Such cascade aldol reactions normally require extended reaction times and high substrate and enzyme concentrations on a preparative scale. Because the wild-type RibA is inactivated by high aldehyde concentrations, commercial applications required the development of an engineered enzyme mutant, which is currently used in the industrial-scale synthesis of the statin precursor (3R,5S)-6-chloro-2,4,6-trideoxyhexapyranose (41, R1 = Cl) from chloroacetaldehyde and acetaldehyde.[‌193‌]

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