Enantioselective synthesis of (−)-cytoxazone and (+)-epi-cytoxazone via Rh-catalyzed diastereoselective oxidative C–H aminations

Stereoselective synthesis of (-)-cytoxazone and its unnatural congener (+)-5-epi-cytoxazone

An interesting protocol for stereoselective synthesis of (-)-cytoxazone and its unnatural stereoisomer (+)-5-epi-cytoxazone from d-4-hydroxyphenylglycine in overall yields of 10% and 16%, respectively, is described. The stereoselective addition of cyanide to an N-Boc protected aminoaldehyde (tert-butyl ((R)-1-(4-methoxyphenyl)-2-oxoethyl)carbamate) (5) constitutes the key step in this approach, producing a mixture of cyanohydrins 6a and b (1,2-anti and 1,2-syn tert-butyl (2-cyano-2-hydroxy-1-(4-methoxyphenyl)ethyl)carbamate) in 89% yield, with reasonable stereoselectivity (1.0:1.8) in favor of the anti-Felkin product (1,2-syn). A one-pot sequence of three successive steps from this mixture produced the oxazolidinone isomers 9a and b ((4R,5R)- and (4R,5S)-4-(4-methoxyphenyl)-2-oxooxazolidine-5-carboxylate). Chromatographic column separation and reduction of the ester function of both precursors led to (-)-cytoxazone and (+)-5-epi-cytoxazone.

One-pot synthesis of oxazolidinones and five-membered cyclic carbonates from epoxides and chlorosulfonyl isocyanate: theoretical evidence for an asynchronous concerted pathway

The one-pot reaction of chlorosulfonyl isocyanate (CSI) with epoxides having phenyl, benzyl and fused cyclic alkyl groups in different solvents under mild reaction conditions without additives and catalysts was studied. Oxazolidinones and five-membered cyclic carbonates were obtained in ratios close to 1:1 in the cyclization reactions. The best yields of these compounds were obtained in dichloromethane (DCM). Together with 16 known compounds, two novel oxazolidinone derivatives and two novel cyclic carbonates were synthesized with an efficient and straightforward method. Compared to the existing methods, the synthetic approach presented here provides the following distinct advantageous: being a one-pot reaction with metal-free reagent, having shorter reaction times, good yields and a very simple purification method. Moreover, using the density functional theory (DFT) method at the M06-2X/6-31+G(d,p) level of theory the mechanism of the cycloaddition reactions has been elucidated. The further investigation of the potential energy surfaces associated with two possible channels leading to oxazolidinones and five-membered cyclic carbonates disclosed that the cycloaddition reaction proceeds via an asynchronous concerted mechanism in gas phase and in DCM.

Stereoselective Syntheses and Application of Chiral Bi- and Tridentate Ligands Derived from (+)-Sabinol

A library of bidentate diols, as well as tridentate triols and aminodiols, derived from (+)-sabinol, was synthesized in a stereoselective manner. Sabinol was transformed into allylic trichloroacetamide via Overman rearrangement of the corresponding trichloroacetimidate. After changing the protecting group to Boc, the enamine was subjected to stereospecific dihydroxylation with OsO₄/NMO, resulting in the (1R,2R,3R,5R)-aminodiol diastereomer. The obtained primary aminodiol was transformed to a secondary analogue. The ring closure of the N-benzyl-substituted aminodiol with formaldehyde was investigated and regioselective formation of the spiro-oxazolidine ring was observed. Hydroboration or dihydroxylation of sabinol or its benzyl ether with OsO₄/NMO resulted in the formation of sabinane-based diols and triols following a highly stereospecific reaction. Treatment of sabinol with m-CPBA afforded O-benzoyl triol as a diastereoisomer of the directly dihydroxylated product, instead of the expected epoxy alcohol. The resulting aminodiols, diol, and triols were applied as chiral catalysts in the reaction of diethylzinc and benzaldehyde from moderate to good selectivity.

Synthesis of oxazolidinones: rhodium-catalyzed C-H amination of N-mesyloxycarbamates

N-Mesyloxycarbamates undergo intramolecular C-H amination reactions to afford oxazolidinones in good to excellent yields in the presence of rhodium(ii) carboxylate catalysts. The reaction is performed under green conditions and potassium carbonate is used, forming biodegradable potassium mesylate as a reaction by-product. This method enables the production of electron-rich, electron-deficient, aromatic and heteroaromatic oxazolidinones in good to excellent yields. Conformationally restricted cyclic secondary N-mesyloxycarbamates furnish cis-oxazolidinones in high yields and selectivity; DFT calculations are provided to account for the observed selectivity. trans-Oxazolidinones were prepared from acyclic secondary N-mesyloxycarbamates using Rh₂(oct)₄. The selectivity was reverted with a cytoxazone N-mesyloxycarbamate precursor using large chiral rhodium(ii) carboxylate complexes, affording the corresponding cis-oxazolidinone. This orthogonal selectivity was used to achieve the formal synthesis of (-)-cytoxazone.

Transition metal-catalyzed iodine(iii)-mediated nitrene transfer reactions: efficient tools for challenging syntheses

The main synthetic applications of catalytic C(sp³)–H amination and alkene aziridination reactions are discussed in the context of natural product synthesis. The examples highlight that these synthetic methods now firmly belong in the organic chemist's toolbox.

Synthesis Approaches to (-)-Cytoxazone, a Novel Cytokine Modulator, and Related Structures

(−)-Cytoxazone, originally isolated from cultures of a Streptomyces species has an oxazolidin-2-one 4,5-disubstituted ring. It is known that this natural product presents a cytokine modulator effect through the signaling pathway of Th2 cells (type 2 cytokines), which are involved in the process of growth and differentiation of cells. From this, the interest in the development of research aimed at the total synthesis of this molecule and its analogs has remained high, which can be confirmed by the large number of publications on the topic, more than 30 to date. This review focuses on the various creative methods for the synthesis of (−)-cytoxazone and its congeners. The assessment of the preparation of this oxazolidinone and related structures serves as a treatise on the efforts made in the synthesis of this important class of compound from its first total synthesis in 1999.

Stereoselective synthesis of 4-substituted-cyclic sulfamidate-5-carboxylates by asymmetric transfer hydrogenation accompanied by dynamic kinetic resolution and applications to concise stereoselective syntheses of (−)-epi-cytoxazone and the taxotere side-chain

DKR driven, asymmetric transfer hydrogenations of cyclic sulfamidate imine-5-carboxylates were developed.

Transition-metal-catalyzed aminations and aziridinations of C-H and C=C bonds with iminoiodinanes

Catalytic insertion or addition of a metal-imido/nitrene species, generated from reaction of a transition-metal catalyst with iminoiodanes, to C-H and C=C bonds offers a convenient and atom economical method for the synthesis of nitrogen-containing compounds. Following this groundbreaking discovery during the second half of the last century, the field has received an immense amount of attention with a myriad of impressive metal-mediated methods for the synthesis of amines and aziridines having been developed. This review will cover the significant progress made in improving the efficiency, versatility and stereocontrol of this important reaction. This will include the various iminoiodanes, their in situ formation, and metal catalysts that could be employed and new ligands, both chiral and non-chiral, which have been designed, as well as the application of this functional group transformation to natural product synthesis and the preparation of bioactive compounds of current therapeutic interest.

Catalytic C-H amination: recent progress and future directions

Recent developments in catalytic C-H amination are discussed in this feature article. The careful design of reagents and catalysts now provides efficient conditions for exquisitely selective intramolecular as well as intermolecular nitrene C-H insertion. The parallel emergence of C-H activation/amination reactions opens new opportunities complementary to those offered by nitrenes.

Dirhodium(II)-Catalyzed C-H Amination Reaction of (S)-3-(tert-Butyldimethylsilyloxy)-2-methylpropyl Carbamate: A Facile Preparation of Optically Active Monoprotected 2-Amino-2-methyl-1,3-propanediol

Dirhodium(II)-catalyzed C-H amination reaction of (S)-3-(tert-butyldimethylsilyloxy)-2-methylpropyl carbamate, which was easily prepared from methyl (S)-2-methyl-3-hydroxypropanoate, proceeded more smoothly than those of their 2-(methoxycarbonyl)propyl derivative to give the corresponding oxazolidinone in excellent yield. The resulting oxazolidinone was converted efficiently into both (R)-monoprotected and (S)-monoprotected 2-amino-2-methyl-1,3-propanediols.