Enantioselective ring cleavage of dioxane acetals mediated by a chiral Lewis acid: application to asymmetric desymmetrization of meso-1,3-diols

[reaction: see text]. Phenylalanine-derived B-aryl-N-tosyloxazaborolidinones selectively activate one of two enantiotopic oxygen atoms in prochiral anti dioxane acetals derived from meso-1,3-diols, leading to enantioselective formation of ring-cleavage products. The reaction is utilized as a key step in asymmetric desymmetrization of meso-1,3-diols.     

Investigation of the asymmetric ionic Diels-Alder reaction for the synthesis of cis-decalins

The ionic Diels-Alder reaction, whereby an alpha,beta-unsaturated acetal in combination with a Lewis or Bronsted acid forms an equilibrium concentration of an activated dienophile, has been developed to provide an enantioselective synthesis of cis-decalins. Cyclohex-2-enone type chiral acetals of (2R,3R)-butane-2,3-diol have been screened against Lewis and Br?nsted acids with a variety of dienes and are efficient for the synthesis of a limited subset of cis-decalin structures. Diastereoselectivities of 73% and 82% have been found for the asymmetric ionic Diels-Alder reaction between the chiral acetal derivatives of cyclohex-2-enone (6) and 2-methylcyclohex-2-enone (18) with 2,3-dimethyl-1,3-butadiene (7). Terminal substituents on the diene partner in general render the system unreactive. However a synthetically useful cis-decalin 31, derived from the reaction of 2-methylcyclohex-2-enone and Z-3-t-butyldimethyl-silyloxypenta-1,3-diene has been prepared in enantiomerically pure form in 74% isolated yield using this asymmetric ionic Diels-Alder protocol.     

Palladium-catalyzed asymmetric synthesis of axially chiral (allenylmethyl)silanes and chirality transfer to stereogenic carbon centers in S(E)' reactions

[reaction: see text] Novel stereoselective reactions of 4-substituted-1-trimethylsilyl-2,3-butadienes ((allenylmethyl)silanes) were developed. The axially chiral (allenylmethyl)silanes were prepared from (3-bromopenta-2,4-dienyl)trimethylsilane by a Pd-catalyzed asymmetric reaction with soft nucleophiles with up to 88% enantioselectivity. The (allenylmethyl)silanes reacted with acetals in the presence of a TiCl(4) promoter to give 1,3-diene derivatives via an S(E)' pathway. The 1,3-dienyl products have (E)-geometry exclusively and up to 88%( )()chirality transfer from the axially chiral allenes to the centrally chiral 1,3-dienes was observed in the S(E)' reaction.     

Iridium‐Catalyzed Regio‐ and Enantioselective Allylic Substitution of Trisubstituted Allylic Electrophiles

The first Ir‐catalyzed enantioselective allylation of trisubstituted allylic electrophiles has been developed. Through modification of the leaving group of allylic electrophiles, we found that trisubstituted allylic phosphates are suitable electrophiles for asymmetric allylation. The reaction of allylic phosphates with enol silanes derived from dioxinones gave allylated products in good yields with high enantioselectivities.     

1,3‐Difunctionalization of Aminocyclopropanes via Dielectrophilic Intermediates

We report an oxidative ring‐opening strategy to transform acyl, sulfonyl or carbamate protected aminocyclopropanes into 1,3‐dielectrophilic carbon intermediates bearing a halide atom (Br, I) and a N,O‐acetal. Replacing the alkoxy group of the N,O‐acetal can be achieved under acidic conditions through an elimination–addition pathway, while substitution of the halides by nucleophiles can be done under basic conditions through a S_N2 pathway, generating a wide range of 1,3‐difunctionalized propylamines. A proof of concept for asymmetric induction was realized using a chiral phosphoric acid (CPA) as catalyst, highlighting the potential of the method in enantioselective synthesis of important building blocks.     

Carbene-Catalyzed Dynamic Kinetic Resolution and Asymmetric Acylation of Hydroxyphthalides and Related Natural Products

A catalytic dynamic kinetic resolution and asymmetric acylation reaction of hydroxyphthalides is developed. The reaction involves formation of a carbene catalyst derived chiral acyl azolium intermediate that effectively differentiates the two enantiomers of racemic hydroxyphthalides. The method allows quick access to enantiomerically enriched phthalidyl esters with proven applications in medicine. It also enables asymmetric modification of natural products and other functional molecules that contain acetal/ketal groups, such as corollosporine and fimbricalyxlactone C.     

Carbene‐Catalyzed Dynamic Kinetic Resolution and Asymmetric Acylation of Hydroxyphthalides and Related Natural Products

A catalytic dynamic kinetic resolution and asymmetric acylation reaction of hydroxyphthalides is developed. The reaction involves formation of a carbene catalyst derived chiral acyl azolium intermediate that effectively differentiates the two enantiomers of racemic hydroxyphthalides. The method allows quick access to enantiomerically enriched phthalidyl esters with proven applications in medicine. It also enables asymmetric modification of natural products and other functional molecules that contain acetal/ketal groups, such as corollosporine and fimbricalyxlactone C.     

Catalytic enantioselective aldol reaction to ketones

An enantioselective aldol reaction between ketones and ketene silyl acetals is described using CuF-chiral phosphine as a catalyst. The key for high enantioselectivity was the development of a novel ligand derived from Taniaphos combined with the unique accelerative effect of PhBF3K. These conditions are applicable to various substrates such as aromatic, aliphatic, and heteroaromatic ketones. In the case of substituted nucleophiles, the reaction proceeds well. The diastereoselectivity is independent of ketene silyl acetal geometry. This is the first example of a catalytic enantio- and diastereoselective aldol reaction to ketones using ketene silyl acetals.     

Rhodium‐Catalyzed Asymmetric Cycloisomerization and Parallel Kinetic Resolution of Racemic Oxabicycles

While desymmetrizations by intermolecular asymmetric ring‐opening reactions of oxabicyclic alkenes with various nucleophiles have been reported over the past two decades, the demonstration of an intramolecular variant is unknown. Reported herein is the first rhodium‐catalyzed asymmetric cycloisomerization of meso‐oxabicyclic alkenes tethered to bridgehead nucleophiles, thus providing access to tricyclic scaffolds through a myriad of enantioselective C?O, C?N, and C?C bond formations. Moreover, we also demonstrate a unique parallel kinetic resolution, whereby racemic oxabicycles bearing two different bridgehead nucleophiles can be resolved enantioselectively.     

Ring-opening reactions of cyclic acetals and 1,3-oxazolidines with halosilane equivalents

Reactions of acetal and 1,3-oxazolidine rings were examined using two kinds of iodosilane equivalent reagents, a 1:2 mixture of Me3SiNEt2 and MeI (reagent 1a) and a 1:1 mixture of Et3SiH and MeI containing a catalytic amount of PdCl2 (reagent 1b). In the reactions of alkanone ethylene acetals with reagent 1a, a C-O bond in the acetal ring readily cleaved to give 2-(trimethylsiloxy)ethyl enol ethers. Similarly, the C-O bond of 1,3-oxazolidine rings cleaved to give ring-opened imine or enamine derivatives. The reactions of aromatic ketone ethylene acetals and cyclohexanone trimethylene acetal led to deprotection of the acetal unit to liberate free ketones. With reagent 1b, cycloalkanone ethylene acetal afforded a dimeric product with 2-iodoethyl alkenoate moieties, while aromatic ketone ethylene or trimethylene acetals produced deprotected ketones.     

Dynamic kinetic resolution via dual-function catalysis of modified cinchona alkaloids: asymmetric synthesis of alpha-hydroxy carboxylic acids

A highly enantioselective catalytic transformation of racemic alpha-hydroxy acids to optically active alpha-hydroxy acids is reported. A new procedure was developed for the condensation of racemic alpha-hydroxy acids with trichloromethyl chloroformate (diphosgene) at room temperature in the presence of activated charcoal to form 5-substituted-1,3-dioxolane-2,4-diones in 90-100% yield. An efficient dynamic kinetic resolution of 5-aryl dioxolanediones was realized via a modified cinchona alkaloid-catalyzed alcoholytic opening of the dioxolanedione ring, generating a variety of optically active alpha-hydroxy esters in 91-96% ee and 61-85% chemical yield. In this dynamic kinetic resolution, the modified cinchona alkaloid was found to serve dual catalytic roles, mediating both the rapid racemization of the 5-aryl dioxolanediones and the enantioselective alcoholytic ring opening of the 5-aryl dioxolanediones. Consequently, both enantiomers of the 5-aryl dioxolanediones were converted to highly enantiomerically enriched aromatic alpha-hydroxy esters in yields (61-85%), far exceeding the maximum of 50% for a normal kinetic resolution. This development not only represents an expansion of the scope of asymmetric acyl-transfer catalysis of synthetic catalysts but also provides a new approach for the development of efficient chemical dynamic kinetic resolutions promoted by a single catalyst. 5-Alkyl dioxolanediones were resolved by a conventional but highly enantioselective kinetic resolution to provide alpha-hydroxy acids and esters in high optical purity and good yields.     

Highly selective palladium catalyzed kinetic resolution and enantioselective substitution of racemic allylic carbonates with sulfur nucleophiles: asymmetric synthesis of allylic sulfides,allylic sulfones,and allylic alcohols

We describe the highly selective palladium catalyzed kinetic resolutions of the racemic cyclic allylic carbonates rac-1 a-c and racemic acyclic allylic carbonates rac-3 aa and rac-3 ba through reaction with tert-butylsulfinate, tolylsulfinate, phenylsulfinate anions and 2-pyrimidinethiol by using N,N'-(1R,2R)-1,2-cyclohexanediylbis[2-(diphenylphosphino)-benzamide] (BPA) as ligand. Selectivities are expressed in yields and ee values of recovered substrate and product and in selectivity factors S. The reaction of the cyclohexenyl carbonate 1 a (>/=99 % ee) with 2-pyrimidinethiol in the presence of BPA was shown to exhibit, under the conditions used, an overall pseudo-zero order kinetics in regard to the allylic substrate. Also described are the highly selective palladium catalyzed asymmetric syntheses of the cyclic and acyclic allylic tert-butylsulfones 2 aa, 2 b, 2 c, 2 d and 4 a-c, respectively, and of the cyclic and acyclic allylic 2-pyrimidyl-, 2-pyridyl-, and 4-chlorophenylsulfides 5 aa, 5 b, 5 ab, 6 aa-ac, 6 ba and 6 bb, respectively, from the corresponding racemic carbonates and sulfinate anions and thiols, respectively, in the presence of BPA. Synthesis of the E-configured allylic sulfides 6 aa, 6 ab, 6 ac and 6 bb was accompanied by the formation of minor amounts of the corresponding Z isomers. The analogous synthesis of allylic tert-butylsulfides from allylic carbonates and tert-butylthiol by using BPA could not be achieved. Reaction of the cyclopentenyl esters rac-1 da and rac-1 db with 2-pyrimidinethiol gave the allylic sulfide 5 c having only a low ee value. Similar results were obtained in the case of the reaction of the cyclohexenyl carbonate rac-1 a and of the acyclic carbonates rac-3 aa and rac-3 ba with 2-pyridinethiol and lead to the formation of the sulfides 5 ab, 6 ab, and 6 bb, respectively. The low ee values may be ascribed to the operating of a "memory effect", that is, both enantiomers of the substrate give the substitution product with different enantioselectivities. However, in the reaction of the racemic carbonate rac-1 a as well as of the highly enriched enantiomers 1 a (>/=99 % ee) and ent-1 a (>/=99 % ee) with 2-pyrimidinethiol the ee values of the substrates and the substitution product remained constant until complete conversion. Similar results were obtained in the reaction of the cyclic carbonates rac-1 a, ent-1 a (>/=99 % ee) and ent-1 c (>/=99 % ee) with lithium tert-butylsulfinate. Thus, in the case of rac-1 a and 2-pyrimidinthiol and tert-butylsulfinate anion as nucleophiles the enantioselectivity of the substitution step is, under the conditions used, independent of the chirality of the substrate; this shows that no "memory effect" is operating in this case. Hydrolysis of the carbonates ent-1 a-c, ent-3 aa and ent-3 ba, which were obtained through kinetic resolution, afforded the enantiomerically highly enriched cyclic allylic alcohols 9 a-c (>/=99 % ee) and acyclic allylic alcohols 10 a (>/=99 % ee) and 10 b (99 % ee), respectively.     

Thermal epimerization of inositol 1,3-benzylidene acetals in the molten state

1,3-O-Benzylidene-2,4,5,6-tetra-O-substituted-myo-inositol derivatives obtained by the DIBAL-H reduction of the corresponding myo-inositol 1,3,5-orthobenzoate derivatives undergo epimerization at the acetal carbon on heating, in the molten state, just above their melting point. The same epimerization reaction does not proceed either in the crystalline state or in solution. DFT calculations suggest that the epimeric acetal obtained by this thermal process is relatively more stable than the starting acetal. Either of these acetals could not be obtained by the reaction of the corresponding inositol derived diol with benzaldehyde. These observations constitute a novel reaction solely in the molten state, which are rarely encountered in the literature. X-ray crystal structures of the epimeric acetals as well as their radical deoxygenation reaction are also reported.     

Regulated-stereoselective construction of thirteen stereogenic centers necessary for the frame of (+)-discodermolide, based on iterative Lewis acid-promoted aldol reactions

The segments C(1)-C(13) and C(15)-C(21) containing the 13 stereogenic centers required for the frame of (+)-discodermolide were synthesized in good to excellent enantio- and diastereoselectivities from a common racemic aldehyde, derived from 2-methyl-1,3-propanediol. The enantioselective aldol reactions of the racemic aldehyde with a silylketene acetal, derived from ethyl 2-bromopropionate, in the presence of chiral oxazaborolidinones, prepared in situ with N-p-toluenesulfonyl-(R)- and -(S)-valine and BH(3).THF, proceeded under kinetic control to give the stereotriads with a high degree of enantioselectivity. Enantioselective (chiral borane) and diastereoselective (BF(3).OEt(2) and TiCl(4)) aldol reactions with the silylketene acetal, coupled with diastereoselective radical debrominations (Bu(3)SnH, Et(3)B, with or without MgBr(2)), were used iteratively. This aldol reaction strategy for the construction of the polypropionate frame dramatically shortened the steps needed for the construction of the final segments.     

A two-step synthesis of allylic syn 1,3-diols via an intramolecular oxa-Michael reaction followed by a modified Julia olefination

A two-step process for the synthesis of allylic syn 1,3-diols is developed. An intramolecular oxa-Michael reaction onto vinyl heteroaromatic sulfones allows the diastereoselective formation of 1-sulfonyl 2,4-diols protected as benzylidene acetals. These sulfones are then engaged in a modified Julia olefination to furnish the olefins contiguous to the benzylidene acetal ring with good E/Z selectivity.     

推-拉电子体系环丙烷的不对称合成及其对映选择性开环/环化反应研究进展

含有推-拉电子(DA)体系的张力环化合物,例如DA环丙烷,是非常有用的合成砌块,被应用于天然产物全合成以及合成具有生物活性的分子的研究中。 近年来,本课题组利用手性铜、镍等配合物为催化剂,一方面发展了一系列高效合成手性DA环丙烷的新方法;另一方面陆续实现了仲胺、醇、硝酮、氮杂亚胺叶立德、烯醇硅醚、吲哚等多种亲核试剂与DA环丙烷的高对映选择性开环/环化反应。 本文结合我们课题组工作综述了DA环丙烷化合物的不对称合成、开环/环化反应以及动力学拆分方面的主要研究进展并进行了展望。     

Enantioselective Evans-Tishchenko reduction of β-hydroxyketone catalyzed by lithium binaphtholate

Lithium diphenylbinaphtholate catalyzed the enantioselective Evans-Tishchenko reduction of achiral β-hydroxyketones to afford monoacyl-protected 1,3-diols with high stereoselectivities. In the reaction of racemic β-hydroxyketones, kinetic optical resolution occurred in a highly stereoselective manner.     

Kinetic resolution and chemoenzymatic dynamic kinetic resolution of functionalized gamma-hydroxy amides

[reaction: see text] An efficient kinetic resolution of racemic gamma-hydroxy amides 1 was performed via Pseudomas cepacia lipase (PS-C)-catalyzed transesterification. The enzyme PS-C tolerates both variation in the chain length and different functionalities giving good to high enantioselectivity (E values of up to >250). The combination of enzymatic kinetic resolution with a ruthenium-catalyzed racemization led to a dynamic kinetic resolution. The use of 2,4-dimethyl-3-pentanol as a hydrogen source to suppress ketone formation in the dynamic kinetic resolution yields the corresponding acetates in good yield and good to high enantioselectivity (ee's up to 98%). The synthetic utility of this procedure was illustrated by the practical synthesis of the versatile intermediate gamma-lactone (R)-5-methyltetrahydrofuran-2-one.     

Indirect electrochemical α-methoxylation of aliphatic ethers and acetals - reactivity and regioselectivity of the anodic oxidation using tris(2,4-dibromophenyl)amine as redox catalyst

The technically important α-methoxylation of aliphatic ethers and acetals to form mixed acetals respectively aldehydes or ortho-esters can be performed electrochemically at low potentials in methanol solution using an undivided cell and tris(2,4-dibromophenyl)amine as redox catalyst. The regioselectivity is usually considerably higher as compared with direct electrolysis in the absence of a catalyst. Especially valuable is the method for the regioselective methoxylation of secondary carbon atoms in presence of primary or tertiary ones and of the acetal carbon in 1,3-dioxolanes. The redox catalyst is stable under the reaction conditions so that more than thousand turnovers could be obtained.     

Asymmetric cyclization of omega-formyl-1,3-dienes catalyzed by a zerovalent nickel complex in the presence of silanes

A nickel(0)-catalyzed asymmetric cyclization of omega-formyl-1,3-diene in the presence of silanes in which five- or six-membered carbocycles or pyrrolidine derivatives were afforded up to 86% ee by the use of (2R,5R)-2,5-dimethyl-1-phenylphospholane as a monodentate chiral ligand was investigated. The reaction course of this asymmetric cyclization can be explained for by two possible mechanisms: one in which the cyclization proceeds via a pi-allylnickel intermediate to produce a cyclized product having an internal olefin in the side chain, and one in which the cyclization proceeds via a sigma bond metathesis of silane and the nickel-oxygen bond of oxanickelacycle to produce a cyclized compound having a terminal olefin and/or an internal olefin in the side chain. It was speculated that both of these mechanisms operate in this asymmetric cyclization depending upon the nature of silanes and the reaction conditions.