A Practical Synthesis of Chiral Oxazolines through a Highly Diastereoselective Coupling-Cyclization Reaction ofN-(Buta-2,3-dienyl)amides and Aryl Iodides

Process for (S)-Ketamine and (S)-Norketamine via Resolution Combined with Racemization

A concise, recyclable, and efficient process is presented for the preparation of (S)-ketamine (esketamine, (S)-1a) via classic resolution combined with the recycling of the undesired isomer. With commercially available ketone 2 as the starting material, this procedure features three steps including (1) an unique hydroxylation-ring expansion rearrangement, (2) mild amination via methanesulfonate, and (3) chiral separation using L-(+)-tartaric acid. The three simple steps are all performed in mild conditions and (S)-1a tartrate is obtained in 99.5% ee without recrystallization. Subsequently, racemization of the unwanted (R)-1a remained in resolution mother liquor was performed in the presence of a Lewis acid in quantitative yield with >99.0% chemical purity. This original and economical process afforded esketamine in 67.4% (28.9% without racemization) overall yield with two times recycling of the mother liquor without column purification. In addition, this procedure can also be applied to the preparation of (S)-norketamine, which is a safer potential antidepressant.

Allenation of Terminal Alkynes with Aldehydes and Ketones

So far, over 150 natural products and pharmaceuticals containing an allene moiety have been identified. During the last two decades, allenes have also been demonstrated as synthetically versatile starting materials in organic synthesis. In comparison to alkenes and alkynes, allenes are unique unsaturated hydrocarbons due to their axial chirality, which could be transformed to central chirality via chirality transfer to provide an irreplaceable entry to chiral molecules. Thus, methods for allene synthesis from readily available chemicals are of great interest. In 1979, Crabbé et al. reported the first CuBr-mediated allenation of terminal alkynes (ATA) reaction to form monosubstituted allenes from 1-alkynes and paraformaldehyde in the presence of diisopropylamine. During the following 30 years, the ATA reactions were limited to paraformaldehyde. This Account describes our efforts toward the development of ATA reactions in the last ten years. First, we improved the yields and scope greatly for the synthesis of monosubstituted allenes by modifying the original Crabbé recipe. Next we developed the ZnI₂-promoted or CuI-catalyzed ATA reactions for the synthesis of 1,3-disubstituted allenes from terminal alkyne and normal aldehydes. Furthermore, we first realized the CdI₂-promoted ATA reaction of ketones with pyrrolidine as the matched amine for the preparation of trisubstituted allenes. Due to the toxicity of CdI₂, we also developed two alternative approaches utilizing CuBr/ZnI₂ or CuI/ZnBr₂/Ti(OEt)₄. The asymmetric version of ATA reactions for the synthesis of optically active 1,3-disubstituted allenes has also been achieved in this group with two strategies. One is called "chiral ligand" strategy, using terminal alkynes, aldehydes, and nonchiral amine with the assistance of a proper chiral ligand. The other is the "chiral amine" strategy, applying terminal alkynes, aldehydes, and chiral amines such as ( S)- or ( R)-α,α-diphenylprolinol or ( S)- or ( R)-α,α-dimethylprolinol. Optically active 1,3-disubstituted allenes containing different synthetically useful functionalities such as alcohol, amide, sulfamide, malonate, carboxylate, and carbohydrate units could be prepared without protection with the newly developed CuBr₂-catalyzed chiral amine strategy. Recently, we have applied these enantioselective allenation of terminal alkyne (EATA) reactions to the syntheses of some natural allenes such as laballenic acid, insect pheromone, methyl ( R)-8-hydroxyocta-5,6-dienoate, phlomic acid, and lamenallenic acid, as well as some non-allene natural γ-butyrolactones such as xestospongienes (E, F, G, and H), ( R)-4-tetradecalactone, ( S)-4-tetradecalactone, ( R)-γ-palmitolactone, and ( R)-4-decalactone.

A catalytic highly enantioselective allene approach to oxazolines

A completely different approach for the asymmetric construction of oxazoline derivatives with an excellent enantioselectivity via the palladium-catalyzed coupling-cyclization of N-(buta-2,3-dienyl)amides with aryl iodides has been reported.

Oxazolines are a very important class of heterocyclic compounds. However, catalytic enantioselective syntheses are very limited. Here, a highly enantioselective palladium-catalyzed coupling-cyclization of readily available N-(buta-2,3-dienyl) amides with aryl or 1-alkenyl iodides has been developed for the asymmetric construction of oxazoline derivatives. Many synthetically useful functional groups are tolerated in this reaction. The absolute configuration of the chiral center in the products has been established by X-ray diffraction study. A model for prediction of the absolute configuration of the chiral center in the products from this cyclic enantioselective nucleophilic allylation has been proposed. The synthetic potentials based on the unique structure of the products formed have also been demonstrated.