Asymmetric Henry reaction catalyzed by Cu(II)-based chiral amino alcohol complexes with C2-symmetry

Enantiomer separation of 2-halomandelic acids via diastereomeric salt formation with chiral N-substituted 2-amino-2-phenylethanols

Chiral N-substituted secondary 1,2-aminoalcohols have been developed for the enantioseparation of ortho-halomandelic acids (o-X-MAs) via diastereomeric salt formation. Two structural isomers, N-methyl-2-amino-1,2-diphenylethanol and N-benzyl-2-amino-2-phenylethanol, showed high separation abilities for o-X-MAs (X = Cl, Br, I). The chiral recognition mechanism was elucidated by crystallographic analysis of the less-soluble salts. The substituents on the nitrogen atom of the resolving agents and the inclusion of the crystallization solvent stabilized the salt and enhanced their separation ability.

Batch and Flow Nitroaldol Synthesis Catalysed by Granulicella tundricola Hydroxynitrile Lyase Immobilised on Celite R-633

Granulicella tundricola hydroxynitrile lyase (GtHNL) catalyses the synthesis of chiral (R)-cyanohydrins and (R)-β-nitro alcohols. The triple variant GtHNL-A40H/V42T/Q110H (GtHNL-3V) was immobilised on Celite R-633 and used in monophasic MTBE saturated with 100 mM KPi buffer pH 7 for the synthesis of (R)-2-nitro-1-phenylethanol (NPE) in batch and continuous flow systems. Nitromethane was used as a nucleophile. A total of 82% of (R)-NPE and excellent enantioselectivity (>99%) were achieved in the batch system after 24 hours of reaction time. GtHNL-3V on Celite R-633 was successfully recycled five times. During more recycling steps a significant decrease in yield was observed while the enantioselectivity remained excellent over eight cycles. The use of a flow system enabled the continuous synthesis of (R)-NPE. A total of 15% formation of (R)-NPE was reached using a flow rate of 0.1 mL min−1; unfortunately, the enzyme was not stable, and the yield decreased to 4% after 4 hours on stream. A similar yield was observed during 15 hours at a rate of 0.01 mL min−1. Surprisingly the use of a continuous flow system did not facilitate the process intensification. In fact, the batch system displayed a space-time-yield (STY/mgenzyme) of 0.10 g L−1 h−1 mgenzyme−1 whereas the flow system displayed 0.02 and 0.003 g L−1 h−1 mgenzyme−1 at 0.1 and 0.01 mL min−1, respectively. In general, the addition of 1 M nitromethane potentially changed the polarity of the reaction mixture affecting the stability of Celite-GtHNL-3V. The nature of the batch system maintained the reaction conditions better than the flow system. The higher yield and productivity observed for the batch system show that it is a superior system for the synthesis of (R)-NPE compared with the flow approach.

Asymmetric Henry Reaction of Nitromethane with Substituted Aldehydes Catalyzed by Novel In Situ Generated Chiral Bis(β-Amino Alcohol-Cu(OAc)2·H2O Complex

Novel chiral thiophene-2,5-bis(β-amino alcohol) ligands (L1–L5) were designed and synthesized from thiophene-2,5-dicarbaldehyde (3) with chiral β-amino alcohols (4a–e) in 4 steps with overall 23% yields. An in situ generated L-Cu(OAc)2·H2O catalyst system was found to be highly capable catalyst for the asymmetric Henry reaction of nitromethane (7) with various substituted aromatic aldehydes (6a–m) producing chiral nitroaldols product (8a–m) with excellent enantiomeric purity (up to 94.6% ee) and up to >99% chemical yields. 20 mol% of L4-Cu(OAc)2 catalyst complex in EtOH was effective for the asymmetric Henry transformation in 24 h, at ambient temperature. Ease of ligand synthesis, use of green solvent, base free reaction, mild reaction conditions, high yields and excellent enantioselectivity are all key factors that make this catalytic system robust and highly desirable for the access of versatile building block β-nitro alcohol in practical catalytic usage via asymmetric Henry reaction.

Enantioselective Henry Reaction Catalyzed by Copper(II) Complex of Bis(trans-cyclohexane-1,2-diamine)-Based Ligand

Copper(II) complex of the ligand possessing two enantiomerically pure trans-cyclohexane-1,2-diamine units proved to be an efficient catalyst for the enantioselective Henry reaction of aromatic aldehydes with nitromethane. The effect of various reaction conditions on yield and enantioselectivity of the Henry reaction was studied. The results suggest that only one cyclohexane-1,2-diamine unit is involved in catalysis of the Henry reaction.

Asymmetric Formal Synthesis of the Long-Acting DPP-4 Inhibitor Omarigliptin

A highly efficient asymmetric synthesis of the key tetrahydropyranol intermediate of DPP-4 inhibitor omarigliptin (1) is described. The successful development of a protecting-group- and precious-metal-free synthesis was achieved via the discovery of a practical asymmetric Henry reaction and the application of a one-pot nitro-Michael-lactolization-dehydration through-process. Other features of the synthesis include a highly efficient MsCl-mediated dehydration and a crystallization-induced dynamic resolution for exceptional ee and dr upgrade. The synthesis of this complex intermediate utilizes simple starting materials and proceeds in four linear steps.