J. Am. Chem. Soc. https://pubs.acs.org/doi/full/10.1021/jacs.1c07872 (2021)
The first step in the designed conversion scheme makes use of a CO2-hydroboration reaction catalysed by 1 mol% of Fe(H)2(dmpe)2 that leads to the formation of a bis(boryl)-acetal (BBA, pictured). Such compounds are known, although they generally tend to be unstable, unless complex substituents are installed on the boron centres. Intriguingly, the team found that simple mesityl moieties could stabilize the bis(boryl)-acetal rendering it a viable surrogate for formaldehyde. In fact, the compound can slowly release formaldehyde in the presence of stoichiometric amounts of water, with an optimal reactivity in water/tetrahydrofuran (THF) solutions. In previous studies, the group of Bontemps had shown that related BBA-reagents are compatible with carbene-catalysed C–C bond formation chemistry and can be used to assemble simple compounds like glycolaldehyde. In this case, in turn, they planned to take advantage of the stereocontrol offered by enzymes, which requires a good compatibility with aqueous buffer solutions. Remarkably, BBA performs well in triethanolamine (TEA) buffer/THF solutions facilitating its further utilization in a chemoenzymatic scheme. Incubating BBA with a combination of formolase (FLS) — an engineered lyase capable of assembling formaldehyde into dihydroxyacetone — and FSA A129S — a mutant of d-fructose-6-phosphate aldolase capable of linking formaldehyde and dihydroxyacetone via an enantioselective aldol condensation — afforded l-erythrulose (35% yield), an important specialty chemical with applications in the cosmetic industry.
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