Asymmetric Synthesis of Functionalized α-Amino Acid Derivatives via the γ-Pyrone Carbaldimine-Based Organocatalytic Mannich Reaction

A powerful synthetic strategy for the asymmetric synthesis of enantiomerically enriched γ-functionalized α-amino acid derivatives based on the highly stereoselective proline-catalyzed Mannich-type reaction of pre-prepared or in situ-generated γ-pyrone-derived aldimines with carbonyl compounds and subsequent transformations of multifunctional reaction products has been developed. A significant positive nonlinear effect was detected for the key organocatalytic reaction. The developed strategy was applied for facile gram-scale preparation of (S)-γ-oxonorvaline, used for site-specific modification of proteins, and both enantiomers of amycolatolide A recently isolated from the lichen-derived actinomycete Amycolatopsis sp. YIM 130923.

Oxidative Ring-Opening Transformation of 5-Acyl-4-pyrones as an Approach for the Tunable Synthesis of Hydroxylated Pyrones and Furans

A selective and tunable approach for oxidation of 4-pyrones has been developed via ring-opening transformations leading to various hydroxylated oxaheterocycles. The first step of the strategy includes the base-catalyzed epoxidation of 5-acyl-4-pyrones in the presence of hydrogen peroxide for the effective synthesis of pyrone epoxides in high yields. The epoxides bearing the CO2Et group are reactive molecules that can undergo both pyrone and oxirane ring-opening via deformylation to produce hydroxylated 2-pyrones or 4-pyrones. The acid-promoted transformation led to 3-hydroxy-4-pyrones (24-76% yields), whereas the K2CO3-catalyzed ring-opening process of 2-carbethoxy-4-pyrone epoxides proceeded as an attack of alcohol at the C-3 position bearing the CO2Et group to give functionalized 6-acyl-5-hydroxy-2-pyrones (27-87% yields). The base-catalyzed reaction of 2-aryl-4-pyrone epoxides was followed by ring contraction and the dearoylation process to produce 3-hydroxyfuran-2-carbaldehydes in 42-80% yields. The transformation of 3-aroylchromone epoxides led to flavonols and 3-hydroxybenzofuran-2-carbaldehyde in the acidic and basic conditions, respectively. The prepared hydroxylated heterocycles demonstrated high reactivity for further transformations and low cytotoxicity and are promising fluorophores or UV filters.

Biocatalytic Synthesis of Homochiral 2-Hydroxy-4-butyrolactone Derivatives by Tandem Aldol Addition and Carbonyl Reduction

Chiral 2-hydroxy acids and 2-hydroxy-4-butyrolactone derivatives are structural motifs often found in fine and commodity chemicals. Here, we report a tandem biocatalytic stereodivergent route for the preparation of these compounds using three stereoselective aldolases and two stereocomplementary ketoreductases using simple and achiral starting materials. The strategy comprises (i) aldol addition reaction of 2-oxoacids to aldehydes using two aldolases from E. coli, 3-methyl-2-oxobutanoate hydroxymethyltransferase (KPHMT _Ecoli ), 2-keto-3-deoxy-l-rhamnonate aldolase (YfaU _Ecoli ), and trans-o-hydroxybenzylidene pyruvate hydratase-aldolase from Pseudomonas putida (HBPA _Pputida ) and (ii) subsequent 2-oxogroup reduction of the aldol adduct by ketopantoate reductase from E. coli (KPR _Ecoli ) and a Δ¹-piperidine-2-carboxylate/Δ¹-pyrroline-2-carboxylate reductase from Pseudomonas syringae pv. tomato DSM 50315 (DpkA _Psyrin ) with uncovered promiscuous ketoreductase activity. A total of 29 structurally diverse compounds were prepared: both enantiomers of 2-hydroxy-4-butyrolactone (>99% ee), 21 2-hydroxy-3-substituted-4-butyrolactones with the (2R,3S), (2S,3S), (2R,3R), or (2S,3R) configuration (from 60:40 to 98:2 dr), and 6 2-hydroxy-4-substituted-4-butyrolactones with the (2S,4R) configuration (from 87:13 to 98:2 dr). Conversions of aldol adducts varied from 32 to 98%, while quantitative conversions were achieved by both ketoreductases, with global isolated yields between 20 and 45% for most of the examples. One-pot one-step cascade reactions were successfully conducted achieving isolated yields from 30 to 57%.

Concise enantioselective synthesis of non-proteinogenic α-aminoacids via an organocatalytic Mannich-type reaction

Scalable asymmetric synthesis of non-proteinogenic (S)-aminoacids and their derivatives based on an organocatalyzed Mannich-type reaction involving allomaltol applicable to subsequent RuIII-catalyzed oxidative cleavage has been developed.