Recent topics in the desymmetrization of meso -diols

Copper-catalyzed asymmetric allylic substitution of racemic/meso substrates

The synthesis of enantiomerically pure compounds is a pivotal subject in the field of chemistry, with enantioselective catalysis currently standing as the primary approach for delivering specific enantiomers. Among these strategies, Cu-catalyzed asymmetric allylic substitution (AAS) is significant and irreplaceable, especially when it comes to the use of non-stabilized nucleophiles (pK a > 25). Although Cu-catalyzed AAS of prochiral substrates has also been widely developed, methodologies involving racemic/meso substrates are highly desirable, as the substrates undergo dynamic processes to give single enantiomer products. Inspired by the pioneering work of the Alexakis, Feringa and Gennari groups, Cu-catalyzed AAS has been continuously employed in deracemization and desymmetrization processes for the synthesis of enantiomerically enriched products. In this review, we mainly focus on the developments of Cu-catalyzed AAS with racemic/meso substrates over the past two decades, providing an explicit outline of the ligands employed, the scope of nucleophiles, the underlying dynamic processes and their practical applications.

Asymmetric Total Synthesis of Alstrostine G Utilizing a Catalytic Asymmetric Desymmetrization Strategy

A highly effective enantioselective monobenzoylation of 1,3-diols has been developed for the synthesis of 1,1-disubstituted tetrahydro-β-carbolines. The chemistry has been successfully applied to the asymmetric total synthesis of (+)-alstrostine G, which also features a cascade Heck/hemiamination reaction enabling facile construction of the pivotal pentacyclic core.

General Synthesis of meso-1,4-Dialdehydes and Their Application in Ir-Catalyzed Asymmetric Tishchenko Reactions

A practical synthesis of meso-1,4-dialdehydes based on the oxidative cleavage of cyclobutanediol derivatives using polymer-supported periodate was developed. The meso-1,4-dialdehydes were obtained in up to >99% yield and subsequently employed in Ir-catalyzed asymmetric Tishchenko reactions to give the corresponding chiral lactones, which are versatile synthetic intermediates, in good yield with moderate enantiomeric excess. The catalytically active species was identified by means of cold-spray ionization mass spectrometry and 1H NMR spectroscopy.

Brønsted-Acid-Catalyzed Enantioselective Desymmetrization of 1,3-Diols: Access to Chiral β-Amino Alcohol Derivatives

Herein, we describe a Brønsted-acid-catalyzed enantioselective desymmetrization of 1,3-diols with alkynes through a hydroalkoxylation/hydrolysis process. The reaction leads to the atom-economical synthesis of valuable chiral β-amino alcohols under mild reaction conditions. Further synthetic transformations based on the β-amino alcohol moiety provide divergent approaches toward chiral N-containing heterocycles.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Catalytic Asymmetric β‐Oxygen Elimination**

A catalytic enantioselective β‐O‐elimination reaction is reported in the form of a zirconium‐catalyzed asymmetric opening of meso‐ketene acetals. Furthermore, a regiodivergent β‐O‐elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected cis‐1,2‐diols in good yield and enantiomeric excess. The combination with a Mitsunobu reaction or a one‐pot hydroboration/Suzuki reaction sequence then gives access to additional diol and aminoalcohol building blocks. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β‐elimination reactions.

Chiral Symmetry Breaking of Monoacylated Anhydroerythritols and meso-1,2-Diols through Crystallization-Induced Deracemization

We developed a chiral symmetry breaking method for monoacylated meso diols. The X-ray crystal structure analysis of monoacylated 1,4-anhydroerythritols, meso cyclic diols with a cis configuration, revealed that the O-(p-anisoyl) derivative crystallized as a racemic conglomerate of the P2₁ 2₁ 2₁ crystal system. It was confirmed that the substrate racemized by intramolecular transfer of the acyl group in the presence of a catalytic amount of base. Evaporating the solvent gradually from the solution or Viedma ripening to promote crystallization-induced deracemization efficiently led to enantiomer crystals. These results provide the first successful example of asymmetric expression and amplification by deracemization of sugar derivatives without an external chemical chiral source. Furthermore, we applied this methodology to acyclic meso-1,2-diols. Three O-monoacylated substrates were successfully deracemized to 99 % ee by Viedma ripening. We also developed asymmetric desymmetrization of meso-1,2-diols by combining acylation and crystallization-induced deracemization.

Enantioselective Desymmetrization of 2-Aryl-1,3-propanediols by Direct O-Alkylation with a Rationally Designed Chiral Hemiboronic Acid Catalyst That Mitigates Substrate Conformational Poisoning

Enantioselective desymmetrization by direct monofunctionalization of prochiral diols is a powerful strategy to prepare valuable synthetic intermediates in high optical purity. Boron acids can activate diols toward nucleophilic additions; however, the design of stable chiral catalysts remains a challenge and highlights the need to identify new chemotypes for this purpose. Herein, the discovery and optimization of a bench-stable chiral 9-hydroxy-9,10-boroxarophenanthrene catalyst is described and applied in the highly enantioselective desymmetrization of 2-aryl-1,3-diols using benzylic electrophiles under operationally simple, ambient conditions. Nucleophilic activation and discrimination of the enantiotopic hydroxy groups on the diol substrate occurs via a defined chairlike six-membered anionic complex with the hemiboronic heterocycle. The optimal binaphthyl-based catalyst 1g features a large aryloxytrityl group to effectively shield one of the two prochiral hydroxy groups on the diol complex, whereas a strategically placed "methyl blocker" on the boroxarophenanthrene unit mitigates the deleterious effect of a competing conformation of the complexed diol that compromised the overall efficiency of the desymmetrization process. This methodology affords monoalkylated products in enantiomeric ratios equal or over 95:5 for a wide range of 1,3-propanediols with various 2-aryl/heteroaryl groups.

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane as a useful precursor to 2,3-dimethylene-1,4-dioxane for [4+2] cycloaddition reaction

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane readily prepared from biacetyl serves as a stable precursor to 2,3-dimethylene-1,4-dioxane which undergoes a [4+2] cycloaddition reaction with dienophiles to give functionalized cyclohexene derivatives. The cycloaddition adducts obtained by the present procedure are transformed into potentially useful intermediates for biologically important materials.

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane serves as a stable precursor to 2,3-dimethylene-1,4-dioxane which undergoes a cycloaddition with dienophiles. The adducts are transformed into useful intermediates for biologically important materials.

Breaking Molecular Symmetry through Biocatalytic Reactions to Gain Access to Valuable Chiral Synthons

In this review the recent reports of biocatalytic reactions applied to the desymmetrization of meso-compounds or symmetric prochiral molecules are summarized. The survey of literature from 2015 up to date reveals that lipases are still the most used enzymes for this goal, due to their large substrate tolerance, stability in different reaction conditions and commercial availability. However, a growing interest is focused on the use of other purified enzymes or microbial whole cells to expand the portfolio of exploitable reactions and the molecular diversity of substrates to be transformed. Biocatalyzed desymmetrization is nowadays recognized as a reliable and efficient approach for the preparation of pharmaceuticals or natural bioactive compounds and many processes have been scaled up for multigram preparative purposes, also in continuous-flow conditions.

Diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols via chirality transfer from a glycosyl donor

Chemical desymmetrization reactions of meso-diols are highly effective for the precise and efficient synthesis of chiral molecules. However, even though enzyme-catalyzed desymmetric glycosylations are frequently found in nature, there is no method for highly diastereoselective desymmetric chemical glycosylation of meso-diols. Herein, we report a highly diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols found in myo-inositol 1,3,5-orthoesters using a boronic acid catalyst based on predictions of regioselectivity by density functional theory (DFT) calculations. The enantiotopic hydroxyl groups of the meso-diols are clearly differentiated by the stereochemistry at the C2 position of the glycosyl donor with excellent regioselectivities. In addition, the present method is successfully applied to the synthesis of core structures of phosphatidylinositolmannosides (PIMs) and glycosylphosphatidylinositol (GPI) anchors, and common β-mannoside structures of the LLBM-782 series of antibiotics.

Enzyme-catalyzed desymmetric glycosylations are often found in nature, however the corresponding chemical methods are lacking. Here, the authors report a highly diastereoselective desymmetric 1,2-cis-glycosylation of meso-diols found in myo-inositol 1,3,5-orthoesters using a boronic acid catalyst.

Selective Monoacylation of Diols and Asymmetric Desymmetrization of Dialkyl meso-Tartrates Using 2-Pyridyl Esters as Acylating Agents and Metal Carboxylates as Catalysts

With 2-pyridyl benzoates as acylating agents and Zn(OAc)₂ as a catalyst, 1,2-diols, 1,3-diols, and catechol were selectively monoacylated. Furthermore, the highly enantioselective desymmetrization of meso-tartrates was achieved for the first time, utilizing 2-pyridyl esters and NiBr₂/AgOPiv/Ph-BOX in CH₃CN or CuCl₂/AgOPiv/Ph-BOX in EtOAc catalyst systems (up to 96% ee). The latter catalyst system was also effective for the kinetic resolution of dibenzyl dl-tartrate.

Asymmetric Formal Synthesis of (+)-Catharanthine via Desymmetrization of Isoquinuclidine

Although (+)-catharanthine is an attractive alkaloid for both clinical research and organic synthetic chemistry, only a limited number of approaches for its catalytic asymmetric synthesis exist. Herein, we describe a novel strategy for synthesizing a chiral intermediate of (+)-catharanthine via phosphoric acid-catalyzed asymmetric desymmetrization of a meso-isoquinuclidine possessing a 1,3-diol unit that was synthesized by a formal amide insertion reaction.

Binuclear Double-Stranded Helicates and Their Catalytic Applications in Desymmetrization of Mesodiols

The ligand L₁ of 4-methyl-2,6-diformylphenol and L₂ of 4- tert-butyl-2,6-diformylphenol are synthesized through Schiff base condensation with rac-, ( R)-(+), or ( S)-(-)-1,1'-binaphthyl-2,2'-diamine (BNDA). As a result, the racemic L₁^rac, L₂^rac, and enantiopure L₁^RR, L₁^SS, L₂^RR, and L₂^SS ligands are obtained incorporating Cu(II) and Zn(II) salts by a simple one-pot metal template method. The series of dinuclear complexes of [M₂LX₂] (here, M = Cu²⁺, Zn²⁺; X = acetate ion, chloride ion; L = L₁^RR, L₁^SS, L₁^rac, L₂^RR, L₂^SS, L₂^rac) formulas are obtained in common. Among them, the single crystal X-ray structures for [Zn₂L₁^rac(OAc)₂] and [Zn₂L₁^SSCl₂] complexes are obtained. The detailed crystal structure and the chiroptical studies performed on these complexes dictates a self-sorting behavior in their self-assembly process and illustrate a chirality transfer from the ligand to the metal center on the complexes. The enantiopure dinuclear complexes [M₂L^RRX₂] and [M₂L^SSX₂] generate enantiopure ΛΛ and ΔΔ isomers, respectively, but the racemic complexes produce only homochiral ΛΛ and ΔΔ assemblies. The detailed studies based on UFLC (Ultra Fast Liquid Chromatography), CD, and single crystal X-ray structure together show the absence of heterochiral ΛΔ mesocate. All these complexes are adapted as catalysts for desymmetrization of various mesodiols, and the enantiopure complexes are found to give efficient enantioselectivity in desymmetrization of mesodiols with benzoyl chloride to monobenzoylated ester providing 98% yield and 92% ee.

Development of a new bicyclic imidazole nucleophilic organocatalyst for direct enantioselective C-acylation

A new chiral nucleophilic organocatalyst bearing a 5,6,7,8-tetrahydroimidazo[1,2-a]pyridine skeleton has been developed.

Carbene-catalyzed oxidative acylation promoted by an unprecedented oxidant CCl3CN

An unprecedented example of a NHC-catalyzed acylation reaction is promoted by CCl₃CN as an oxidant.

The dual reactivity of Weinreb amides applied to the late-stage divergent functionalisation of meso pyrrolidines

The dual reactivity of Weinreb amides was exploited to prepare diversified symmetrical and dissymmetrical 2,5-disubstituted pyrrolidines from simple building blocks.