Convergent in situ Generation of Both Transketolase Substrates via Transaminase and Aldolase Reactions for Sequential One‐Pot, Three‐Step Cascade Synthesis of Ketoses

We describe an efficient three‐enzyme, sequential one‐pot cascade reaction where both transketolase substrates are generated in situ in a convergent fashion. The nucleophilic donor substrate hydroxypyruvate was obtained from l ‐serine and pyruvate by a transaminase‐catalyzed reaction. In parallel, three different (2 S )‐α‐hydroxylated aldehydes, l ‐glyceraldehyde, d ‐threose, and l ‐erythrose, were generated as electrophilic acceptors from simple achiral compounds glycolaldehyde and formaldehyde by d ‐fructose‐6‐phosphate aldolase catalysis. The compatibility of the three enzymes was studied in terms of temperature, enzyme ratio and substrate concentration. The efficiency of the process relied on the irreversibility of the transketolase reaction, driving a shift of the reversible transamination reaction and securing the complete conversion of all substrates. Three valuable (3 S ,4 S )‐ketoses, l ‐ribulose, d ‐tagatose, and l ‐psicose were obtained in good yields with high diastereoselectivity.