Stereoselective synthesis of (R)- and (S)-2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acids, -carboxylates and -carboxamides

Asymmetric Chemoenzymatic One-Pot Synthesis of α-Hydroxy Half-Esters

A new chemoenzymatic one-pot strategy has been developed for the synthesis of α-hydroxy half-esters containing consecutive quaternary and tertiary stereocenters using asymmetric cascade catalysis. In this study, an asymmetric Ca²⁺-catalyzed [2,3]-Wittig rearrangement reaction was proven to be suitable for a combination with porcine liver esterase-mediated hydrolysis resulting in the enhanced enantiomeric purity of the obtained products in a one-pot synthesis compared to the stepwise method.

Synthesis of Chiral 2-Substituted 1,4-Benzoxazin-3-ones via Iridium-Catalyzed Enantioselective Hydrogenation of Benzoxazinones

An efficient iridium-catalyzed enantioselective hydrogenation of 2-alkylidene 1,4-benzoxazin-3-ones using our developed iPr-BiphPHOX as a ligand is reported. This method showed good functional group compatibility and delivered the corresponding reduced products in excellent yields (up to 99%) with excellent enantioselectivities (up to 99% ee). The reaction proceeded very well on a gram scale with low catalyst loadings (0.1 mol %), providing the product with no erosion in enantioselectivity. Additionally, three bioactive molecules can be easily obtained from the reduced products.

Multigram-scale HPLC enantioseparation as a rescue pathway for circumventing racemization problem during enantioselective synthesis of ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate

Racemic ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate is a key synthon for the design of promising therapeutic drugs. It is mainly synthesized from racemic ethyl 2,3-dibromopropionate and 2-aminophenol in presence of K₂ CO₃ in refluxed acetone. Unfortunately, synthesis of (R)- and (S)-enantiomers using the enantioselective version of this reaction, which should normally be performed with a double S_N 2 mechanism, is unsuitable due to a racemization process involving the dehydrobromination of enantiopure ethyl 2,3-dibromopropionate into ethyl 2-bromoacrylate. For the first time, the enantioselective version is studied (ee ≈ 55-66%), and the percentage of racemization process has estimated to around 34-46% after determination of the optimal experimental conditions for analytical HPLC enantioseparation of racemate. The influence of the experimental and purification conditions on the racemization rate is also studied. The results indicate that racemization occurs faster at the beginning of the reaction but the initiation of the double S_N 2 process takes place more faster to limit the racemization rate. The study of the influence of experimental conditions (reaction times, temperature, solvent or type of base, etc.) on the degree of racemization of the (R)- enantiomer is performed and shows that despite changes in the experimental conditions, the synthesis of the (R)- enantiomer is always accompanied by a racemization rate which is difficult in reducing. In parallel, (R)- and (S)-enantiomers are obtained in high enantiopurity (ee > 99.5%) by preparative HPLC enantioseparation of racemate on multigram scale and characterized by optical rotation measurements, ECD and UV spectra.

Phase-transfer-catalysed asymmetric synthesis of 2,2-disubstituted 1,4-benzoxazin-3-ones

1,4-Benzoxazin-3-one is a scaffold which is found in a variety of biologically active molecules. Because of its unique structure and drug-like activities, 1,4-benzoxazin-3-ones have been widely used in drug discovery. However, just a few methods have been developed to access these molecules by catalytic asymmetric synthesis. We report herein the phase-transfer-catalysed asymmetric alkylation of 2-aryl-1,4-benzoxazin-3-ones as a new way for the highly enantioselective synthesis of 2,2-disubstituted 1,4-benzoxazin-3-ones.

Phase-transfer catalyzed enantioselective α-alkylation of α-acyloxymalonates: construction of chiral α-hydroxy quaternary stereogenic centers

The enantioselective synthesis of α-acyloxy-α-alkylmalonates was developed as an efficient method for producing chiral α-tertiary alcohols, which are potentially valuable intermediates in the synthesis of natural products and pharmaceuticals.

Synthesis of Diverse Benzo[1,4]oxazin-3-one-Based Compounds Using 1,5-Difluoro-2,4-dinitrobenzene

This paper discusses the synthesis of benzo[1,4]oxazin-3-one-based compounds from 1,5-difluoro-2,4-dinitrobenzene (1), including benzo[1,4]oxazin-3-ones (5-11) and five novel benzo[1,4]oxazin-3-one-based tricycles: 6-hydroxy-4H-1-oxa-4,5,8-triazaanthracen-3-one (14), 3,8-dihydro-5-oxa-1,3,8-triazacyclopenta-[b]-naphthalene-7-one (15, 17, 21), 3,8-dihydro-5-oxa-1,2,3,8-tetraazacylopenta[b]-naphthalene-7-one (16, 20), 3,8-dihydro-1H-5-oxa-1,3,8-triazacyclopenta[b]-naphthalene-2,7-dione (18, 22), and 5,8-dihydro-4H-1-oxa-4,5,8-triazaanthracene-3,6,7-trione (19). Finally, a chemical library based on 15 was synthesized in parallel solution-phase reactions.

A One-Pot Synthesis of Pyrido[2,3-b][1,4]oxazin-2-ones

Pyrido[2,3-b][1,4]oxazin-2-ones are conveniently prepared in excellent yields by a one-pot annulation of N-substituted-2-chloroacetamides with 2-halo-3-hydroxypyridines with use of cesium carbonate in refluxing acetonitrile. The key transformation features a Smiles rearrangement of the initial O-alkylation product and subsequent cyclization.

Influence of chirality of the preceding acyl moiety on the cis/trans ratio of the proline peptide bond

We report that the cis/trans ratio of the proline peptide bond can be strongly influenced by the chirality of the acyl residue preceding proline. Acyl moieties derived from (2S)-2,6-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid (8) and (2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoic acid (5) in acyl-Pro molecules influence isomerization of the proline peptide bond constraining the omega dihedral angle to the trans orientation. Structures of benzyl (2S)-1-([(2S)-2,6-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazin-2-yl]carbonyl)-2-pyrrolidinecarboxylate (3) derived from 2D (1)H NMR conformational analysis and crystallographic data exhibit only the trans conformation of proline peptide bond. On the other hand the diastereomer 4, which contains an (R) acyl moiety, exhibits two sets of signals in (1)H NMR spectra. The signals were assigned to trans (72%) and cis (28%) conformers. Crystallographic analysis of 4 showed that only the cis conformation is present in the crystalline state. The (1)H NMR chemical shift pattern of three sets of signals observed in 2 was observed also in benzyl (2S)-1-[(2R/S)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoyl]-2-pyrrolidinecarboxylate. (R)-Carboxylic acid 5, after coupling with (S)-ProOBn, yielded benzyl (2S)-1-[(2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoyl]-2-pyrrolidinecarboxylate (6), which in DMSO-d(6) exhibited only the trans conformation of the proline peptide bond. These results suggest that in these particular cases acyl-Pro peptide bond isomerization is strongly influenced by the stereochemistry of the acyl residue preceding proline. (2S)-2,6-Dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid (8) and (2R)-3-methoxy-2-methyl-2-(4-methyl-2-nitrophenoxy)-3-oxopropanoic acid (5) are promising chiral peptidomimetic building blocks that can be used as acyl moieties to force the proline peptide bond into the trans conformation in a variety of acyl-Pro molecules.