Oxazolidinone Chiral Auxiliary Research Articles

Evans’ Chiral Auxiliary‐Based Asymmetric Synthetic Methodology and Its Modern Extensions

AbstractAlthough the asymmetric catalysis has made a spurt of progress, the use of chiral auxiliaries remains crucial in asymmetric synthesis due to both the reliability and versatility of the methods, and the predictability of stereochemistry of the reactions. Up to date, Evans’ chiral non‐racemic oxazolidinone‐based asymmetric synthetic methodology is still widely used in asymmetric synthesis. More importantly, the Evans asymmetric synthetic methodology turned out to be a fruitful source of inspiration for the development of related asymmetric synthetic methodologies. However, although reviews on the application of Evans’ asymmetric synthetic methodology in both organic synthesis and medicinal chemistry continually to appear in the literature, a comprehensive review dedicating to the extensions of Evans’ chiral non‐racemic oxazolidinone‐based asymmetric methodology remains elusive. In this review, we summarize the extensions of the Evans asymmetric methodology, which cover: (1) the modification of the chiral oxazolidinone auxiliaries; (2) the extension of the Evans’ asymmetric aldol reaction from Evans’ syn‐aldol to other diastereomeric aldol adducts; (3) the extension of the asymmetric reaction types; (4) the extension of chiral imide‐type substrates to N‐alkenyl, N‐allenenyl and N‐alkynyl oxazolidinones; (5) the achiral oxazolidinone‐based asymmetric catalysis; (6) the catalytic transformation of Evans’ products; and (7) the straightforward transformations of Evans’ chiral products to other classes of compounds than the chiral carboxylic acids, esters, alcohols, and Weinreb amides.

Preparation and supporting on solid phase of chiral auxiliary (S)-4-(4hydroxybenzyl)oxazolidin-2-one from L-tyrosinol assisted by Microwaves

An oxazolidinone chiral auxiliary, (S)-(4-Hydroxybenzyl)1,3-oxazolidin-2-one 4 was prepared in 23-80 % yield from L-tyrosinol using different methodologies. In solution, compound 4 was protected with a benzyl group on the phenolic hydroxyl 5a, which allowed the optimization of the solid phase synthesis of 5b-5d. Chiral auxiliaries 5a and 5b reacted in solution with an α,β-unsaturated system to give (S)-4-(4-(benzyloxy)benzyl)-1,3-oxazolidin-2-one 6a. By contrast, in solid phase the same reaction afforded 6b. Both reactions were carried out under microwave activation. New compounds were characterized by 1H and 13C NMR, infrared spectroscopy and mass spectrometry.

Diastereoselective Electrophilic Trifluoromethylthiolation of Chiral Oxazolidinones: Access to Enantiopure α‐SCF3 Alcohols

AbstractLithium imide enolates featuring Evans’ chiral oxazolidinone auxiliary were involved in diastereoselective α‐trifluoromethylthiolation with electrophilic SCF3 donors. Diastereopure products were isolated and converted to enantiopure α‐SCF3 alcohols without racemisation.magnified image

Total Synthesis of (+)-Medicarpin.

(+)-Medicarpin has been synthesized asymmetrically for the first time in a linear scalable process with an overall yield of 11%. The two chiral centers were constructed in one step via condensation using a chiral oxazolidinone auxiliary. This method will likely accelerate research on medicarpin as an erythropoietin inducer for erythropoietin-deficient diseases.

A New Method for Asymmetric Synthesis of (－)-Arctigenin and Its Enantiomer

Arctigenin, the main active ingredient of traditional chinese medicine (TCM) arctii fructus, belongs to diben- zyl butyrolactone lignans. In order to study the structure-activity relationship of arctigenin, a new method for asymmetric syn- thesis of (-)-arctigenin and its enantiomer was developed. Phenylpropanoic acid derivate was used as starting material and the chiral center of beta butyrolactone was constructed by using oxazolidinone chiral auxiliary. The ees of R and S configurations are 98% and 96%, respectively. Then the second chiral center in the alpha position was constructed benefitting from the steric effect. After removal of protecting group, the target compounds were obtained in 58 % and 55% overall yield of (-)-arcti- genin and (+)-arctigenin with 97% and 96% ee, respectively. This work paved the way for further structural optimization of

Asymmetric synthesis approach of enantiomerically pure spiro-indenoquinoxaline pyrrolidines and spiro-indenoquinoxaline pyrrolizidines

An efficient, one-pot, four-component procedure for the synthesis of a small library of novel chiral spiro-indenoquinoxaline pyrrolidines and chiral spiro-indenoquinoxaline pyrrolizidines with high regio-, diastereo- (up to 96 dr), and enantioselectivity (up to 99% ee), through a 1,3-dipolar cycloaddition reaction of azomethine ylides and an optically active cinnamoyl–crotonoyl oxazolidinone is described. This methodology was very simple and was utilized to construct complex products from simple starting materials. The process was carried out in aqueous ethanol as an eco-friendly solvent and in the absence of any Lewis acids catalysts. The oxazolidinone chiral auxiliary was removed through a non-destructive protocol. The reaction mechanism is discussed on the basis of the assignment of the absolute configuration of the cycloadducts and using quantum mechanical calculations. The regio- and stereoselectivity were described on the basis of transition states’ stabilities and global and local reactivity indices of the reactants. The results of the theoretical calculations are in agreement with the experimental outcomes.

N-Methylpyrrolidone Hydroperoxide/Cs2 CO3 as an Excellent Reagent System for the Hydroxy-Directed Diastereoselective Epoxidation of Electron-Deficient Olefins.

This report introduces N-methylpyrrolidone hydroperoxide (NMPOOH)/base as an excellent reagent system for hydroxy-directed syn selective epoxidation of electron-deficient olefins, characterized by high diastereoselectivity, short reaction times and remarkable chemoselectivity, especially in presence of oxidatively labile nitrogen or sulfur atoms. NMPOOH also proves efficient in the oxidation of electron-deficient aromatic aldehydes, in the removal of oxazolidinone chiral auxiliary, and in the functionalization of alkenes and alkynes, showing wide application potential.

Stereocontrolled Synthesis of the C1‐C10 Fragments of Monensin B and Laidlomycin

AbstractAn efficient synthetic route to the C1‐C10 fragments of laidlomycin and monensin B has been developed toward their total synthesis. The asymmetric carbons have been elaborated by a syn‐aldol reaction using the oxazolidinone chiral auxiliary for C6 and C7, an anti‐aldol reaction for C3 and C4, the Tishchenko–Evans reaction for C5, and a chiral building block for C2.

Completely Stereocontrolled Aldol Reaction of Chiral β-Amino Acids

A general protocol to independently access stereoisomerically pure β'-hydroxy-β-amino acid derivatives that is based on dibutylboron triflate-mediated aldol reaction of suitably protected β-amino acids bearing chiral oxazolidinone auxiliary is reported. The method smoothly afforded syn-aldol (α,β'-syn) products in pure form and excellent isolated yield. Both α,β-syn and α,β-anti isomers are readily accessible solely through the choice of the oxazolidinone chirality. This method allows for the preparation of stereoisomeric β'-hydroxy-β-amino acid derivatives that were previously unreported.

Opposing Auxiliary Conformations Produce the Same Torquoselectivity in an Oxazolidinone-Directed Nazarov Cyclization

Most applications of chiral oxazolidinone auxiliaries in asymmetric synthesis operate through a common set of stereocontrol principles. That is, the oxazolidinone is made to adopt a specific, coplanar conformation with respect to the prochiral substrate, and reaction occurs preferentially at whichever stereoheterotopic face is not blocked by the substituents on the oxazolidinone. In contrast to these principles, we report here the discovery of an alternative mechanism of oxazolidinone-based stereocontrol that does not require coplanarity and is driven instead by allylic strain. This pathway has been uncovered through computational studies of an asymmetric Nazarov cyclization. Chiral oxazolidinone auxiliaries provide essentially complete control over the torquoselectivity of ring closure and the regioselectivity of subsequent deprotonation. Density functional theory calculations (M06-2X//B3LYP) reveal that in the transition state of 4π electrocyclic ring closure, the oxazolidinone ring and the cyclizing pentadienyl cation are distorted from coplanarity in a manner that gives two transition state conformations of similar energy. These two conformers are distinguished by a 180° flip in the auxiliary orientation such that in one conformer the oxazolidinone carbonyl is oriented toward the OH of the pentadienyl cation (syn-conformer) and in the other it is oriented away from this OH (anti-conformer). Surprisingly, both conformations induce the same sense of torquoselectivity, with a 3-5 kcal/mol preference for the C5-β epimer of the ring-closed cation. In both conformations, the conrotatory mode that leads to the C5-α epimer is disfavored due to higher levels of allylic strain between the oxazolidinone substituent and adjacent groups on the pentadienyl cation (R(4) and OH). The excellent torquoselectivities obtained in the oxazolidinone-directed Nazarov cyclization suggest that the allylic strain-driven stereoinduction pathway represents a viable alternative mechanism of stereocontrol for reactions of sterically congested substrates that lie outside of the traditional coplanar (N-acyloxazolidinone) paradigm.

A Facile Asymmetric Synthesis of (S)-14-Methyl-1-Octadecene, the Sex Pheromone of the Peach Leafminer Moth

An asymmetric synthesis of 14-methyl-1-octadecene, the sex pheromone of the peach leafminer moth has been achieved. The target molecule was synthesized in six linear steps and in 30.3% overall yield from commercially available hexanoyl chloride, (S)-4-benzyloxazolidin-2-one and 1,9-nonanediol. The hexanoyl chloride was connected with (S)-4-benzyloxazolidin-2-one, and with the induction of the chiral oxazolidinone auxiliary, after chiral methylation, LAH reduction and then tosylation gave the chiral key intermediate 5 in high stereoselectivity. 1,9-Nonanediol, was selectively brominated, THP protected and subjected to Li2CuCl4-mediated C-C coupling to afford a C12 intermediate. The target molecule, (S)-14-methyl-1-octadecene, was obtained after the two parts were subjected to a second Li2CuCl4-mediated C-C coupling. Our synthetic approach represents the first time a substrate-control asymmetric synthesis of (S)-14-methyl-1-octadecene has been reported.

Aromatic interactions as control elements in stereoselective organic reactions.

This Account describes how attractive interactions of aromatic rings with other groups can influence and control the stereoselectivity of many reactions. Recent developments in theory have improved the accuracy in the modeling of aromatic interactions. Quantum mechanical modeling can now provide insights into the roles of these interactions at a level of detail not previously accessible, both for ground-state species and for transition states of chemical reactions. In this Account, we show how transition-state modeling led to the discovery of the influence of aryl groups on the stereoselectivities of several types of organic reactions, including asymmetric dihydroxylations, transfer hydrogenations, hetero-Diels-Alder reactions, acyl transfers, and Claisen rearrangements. Our recent studies have also led to a novel mechanistic picture for two classes of (4 + 3) cycloadditions, both of which involve reactions of furans with oxyallyl intermediates. The first class of cycloadditions, developed by Hsung, features neutral oxyallyl intermediates that contain a chiral oxazolidinone auxiliary. Originally, it was thought that these cycloadditions relied on differential steric crowding of the two faces of a planar intermediate. Computations reveal a different picture and show that cycloaddition with furan takes place preferentially through the more crowded transition state: the furan adds on the same side as the Ph substituent of the oxazolidinone. The crowded transition state is stabilized by a CH-π interaction between furan and Ph worth approximately 2 kcal/mol. Attractive interactions with aromatic rings also control the stereoselectivity in a second class of (4+3) cycloadditions involving chiral alkoxy siloxyallyl cations. Alkoxy groups derived from chiral α-methylbenzyl alcohols favor crowded transition states, where a stabilizing CH-π interaction is present between the furan and the Ar group. The cationic cycloadditions are stepwise, while the Hsung cycloadditions are concerted. Our results suggest that this form of CH- π-directed stereocontrol is quite general and likely controls the stereoselectivities of other addition reactions in which one face of a planar intermediate bears a pendant aromatic substituent.

Synthesis of new enantiomerically pure spirooxindolopyrrolizidines via a three-component asymmetric 1,3-dipolar cycloaddition reaction of azomethine ylides derived from isatin

An efficient, one-pot, three-component procedure for the synthesis of a small library of new chiral spirooxindolopyrrolizidines with high regio-, diastereo-, and enantioselectivity, from the 1,3-dipolar cycloaddition of azomethine ylides and optically active cinnamoyl oxazolidinone is described. The process occurs at room temperature in aqueous ethanol as a green solvent and in the absence of any Lewis acids. The oxazolidinone chiral auxiliary is removed in a non-destructive manner. The reaction mechanism is discussed on the basis of the assignment of the absolute configuration of the cycloadducts, and on theoretical calculations.

Stereospecific Polymerization of Chiral Oxazolidinone-Functionalized Alkenes

Acryloyl and vinyl monomers functionalized with a chiral oxazolidinone auxiliary have been successfully polymerized in a stereospecific fashion to highly isotactic, optically active polymers, through either the previously established isospecific coordination polymerization (for acryloyl monomers) or a novel isospecific cationic polymerization (for vinyl monomers). Specifically, conjugated chiral acryloyloxazolidinones, N-acryloyl-(R or S)-4-phenyl-2-oxazolidinone [(R or S)-AOZ], are readily polymerized by chiral ansa-zirconocenium coordination catalysts, (R,R-, S,S-, or R,R/S,S)-[C2H4(η5-Ind)2]Zr+(THF)[OC(OiPr)═CMe2][MeB(C6F5)3]− (1), in an isospecific manner through a catalyst-site-controlled mechanism, producing the corresponding optically active chiral polymers, (R or S)-PAOZ. Owing to the nature of stereocontrol dictated by the chiral catalyst site, even the coordination polymerization of the parent AOZ, without the chiral side group, also affords PAOZ with nearly quantitative isotacticity. A series of experiments have shown that the chiral polymers (R or S)-PAOZ exhibit no chiral amplifications, despite having stereoregularly placed stereogenic centers in the main chain, and the optical activity of the polymers arises solely from their chiral auxiliary, a consequence of adopting a random-coil secondary structure and thus having a cryptochiral chain. In sharp contrast, the chiral isotactic polymers derived from nonconjugated chiral vinyl oxazolidinones, N-vinyl-(R)-4-phenyl-2-oxazolidinone [(R)-VOZ] and its p-hexyloxyphenyl derivative (R)-HVOZ (designed to solve the solubility issue of the resulting polymer), exhibit substantial chiral amplifications by virtue of adopting a solution-stable, one-handed helical conformation. The synthesis of such helical vinyl polymers has been accomplished by the development of a novel isospecific cationic polymerization using Lewis and Brønsted acids, such as [Ph3C][B(C6F5)4], BF3·Et2O, and [H(Et2O)2][B(C6F5)4], through a chiral auxiliary-controlled mechanism. Noteworthy is the combination of the near-quantitative isotactic placement of the stereogenic centers of the polymer main chain with the chiral side groups located near those stereocenters that renders one-handed helicity of (R)-PVOZ and (R)-PHVOZ. Significantly, this novel cationic polymerization process, operating at ambient temperature, effectively assembles two elements of polymer local chirality—side-chain chirality and main-chain chirality—into global chirality in the form of excess one-handed helicity. Furthermore, the resulting chiral helical vinyl polymers exhibit considerably higher thermal decomposition temperatures and polymer crystallinity, in comparison to the random-coil chiral acryloyl polymers, having a similarly high degree of main-chain stereoregularity.

ChemInform Abstract: Metal Ion Mediated Diastereoface-Selective 1,3-Dipolar Cycloaddition of Nitrile Oxides with Dipolarophiles Bearing an Oxazolidinone Chiral Auxiliary.

Abstract Magnesium ion-mediated cycloaddition reactions of nitrile oxides to chiral 3-acryloyl-2-oxazolidinones lead to highly diastereoselective formation of 2-isoxazolines. The magnesium ion serves to fix the α,β-unsaturated moiety through chelation to effect chiral shielding. These asymmetric reactions provide the first successful examples of Lewis acid-mediated stereocontrol of nitrile oxide cycloaddition reactions to electron-deficient dipolarophiles.

Hydrogenation of N‐Acylcarbamates and N‐Acylsulfonamides Catalyzed by a Bifunctional [Cp*Ru(PN)] Complex

Awakening of the Cp one: The bifunctional complex 1 facilitates the interaction with substrates bearing less electrophilic carbon atoms than ketones, epoxides, and imides. The title reaction was applicable to the reduction of Evans' asymmetric alkylation products to the chiral alcohols along with good recovery of the chiral oxazolidinone auxiliary. EWG = electron-withdrawing group.

Diels-Alder exo selectivity in terminal-substituted dienes and dienophiles: experimental discoveries and computational explanations.

The Diels-Alder reactions of a series of silyloxydienes and silylated dienes with acyclic alpha,beta-unsaturated ketones and N-acyloxazolidinones have been investigated. The endo/exo stereochemical outcome is strongly influenced by the substitution pattern of the reactants. High exo selectivity was observed when the termini of the diene and the dienophile involved in the shorter of the forming bonds were both substituted, while the normal endo preference was found otherwise. The exo-selective asymmetric Diels-Alder reactions using Evans' oxazolidinone chiral auxiliary furnished a high level of pi-facial selectivity in the same sense as their well-documented endo-selective counterparts. Computational results for these Diels-Alder reactions were consistent with the experimental endo/exo selectivity in most cases. A twist-asynchronous model accounts for the geometries and energies of the computed transition structures.

Asymmetric synthesis of trans-disubstituted cyclopropanes using phosphine oxides and phosphine boranes

The stereocontrolled synthesis of trans-disubstituted cyclopropylketones has been achieved from beta-alkyl, gamma-benzoyl phosphine oxides via a three-step cascade reaction incorporating an acyl transfer, phosphinoyl transfer and cyclisation to form the cyclopropane. Using Evans' chiral oxazolidinone auxiliary and by masking the phosphine oxide moiety as a phosphine borane we have extended the method to the synthesis of enantiomerically-enriched trans-disubstituted cyclopropyl ketones.

Practical and divergent synthesis of 1- and 5-substituted 3,9-diazaspiro[5.5]undecanes and undecan-2-ones

A divergent synthesis of 1- and 5-substituted 3,9-diazaspiro[5.5]undecanes and undecan-2-ones is described, in which the key step is an efficient Michael addition of a lithium enolate to a tetrasubstituted olefin acceptor. A variety of substituents (butyl, phenyl, and propoxyl) were introduced at C-1(5) in this manner. In addition, an asymmetric synthesis of one member of this series was achieved using an Evans oxazolidinone chiral auxiliary reagent.

Acylation, Diastereoselective Alkylation, and Cleavage of an Oxazolidinone Chiral Auxiliary

An introduction to the concepts and experimental techniques of diastereoselective synthesis using a chiral auxiliary is described. The 4-benzyl-2-oxazolidinone chiral auxiliary developed by Evans is acylated with propionic anhydride under mild conditions using DMAP as an acyl transfer catalyst. Deprotonation with NaN(TMS)2 at -78 °C leads to a rigidly chelated enolate that is alkylated by allyl iodide preferentially from the least hindered diastereoface with 98:2 selectivity. Chromatographic purification followed by hydrolytic cleavage of the auxiliary using LiOH/H2O2 gives 2-methyl-4-pentenoic acid in high enantiomeric purity. This exercise gives students the opportunity to gain proficiency with the techniques of modern organic synthesis, including the manipulation of moisture-sensitive reagents, low temperature reactions, column chromatography, TLC, and NMR analysis. The experiment also provides a context for the discussion of important reaction mechanisms, such as acylation and enolate alkylation, and serves to reinforce the critical conceptual distinction between absolute and relative stereochemical control.