Rh(I)-catalyzed conjugate addition of alkenylzirconocene chloride: stereoselective formation of carbocycles through cascade reaction

Stereoselective formation of carbocycles was carried out through [RhCl(cod)]₂-catalyzed reactions of alkenylzirconocene chloride to ω-carbonyl-α,β-ene carbonyls (ketone, ester, and amide) or to bis-enone derivatives. The Rh(I)-catalyzed reaction of alkenylzirconocene chloride with ω-carbonyl-α,β-enoyl amide, which is derived from Oppolzer’s sultam, showed high diastereoselectivity to yield an carbocycle (95% yield, >95% de).     

Silver(I)-promoted asymmetric halomethoxylation of chiral α,β-unsaturated carboxylic acid derivatives: enantioselective synthesis of N-protected syn-β-methoxy-α-amino acids

Asymmetric halomethoxylation of chiral α,β-unsaturated carboxylic acid derivatives was performed with halogens (Br₂/I₂) promoted by silver(I) salts with high regio- and anti-selectivity and moderate to good diastereoselectivity. Reagent controlled diastereoselectivity was observed for N-cinnamoyl-2-oxazolidinone substrates especially for cinnamoyl and electron-deficient cinnamoyl substrates, when Ag₂O was used as a promoter instead of AgNO₃. Enoyl substrates containing Oppolzer’s sultam chiral auxiliary are independent of the counter ion of the Ag(I) salt. This method was applied to a short synthesis of both enantiomers of N-protected syn-β-methoxyphenylalanine, and N- and O-protected syn-β-methoxytyrosine, unusual amino acid components of biologically active cyclic peptide and depsipeptide antibiotics.     

Highly regio- and stereoselective asymmetric bromoazidation of chiral alpha,beta-unsaturated carboxylic acid derivatives: scope and limitations

Lewis acid catalyzed asymmetric bromoazidation of chiral alpha,beta-unsaturated carboxylic acid derivatives was performed using N-bromosuccinimide (NBS) and trimethylsilyl azide (TMSN3) as the bromine and azide sources. Among the Lewis acids, Yb(OTf)3 was found to be the best catalyst. Regio- and anti-selectivity of 100% and moderate to good diastereoselectivity (up to 89:11) with good yields were obtained when Oppolzer's bornane sultam chiral auxilairy was used. Diastereoselectivity of >95:05 was observed when (2S,5S)-2,5-diphenylpyrrolidine was used as the chiral auxiliary.     

Diastereoselective electrochemical carboxylation of chiral alpha-bromocarboxylic acid derivatives: an easy access to unsymmetrical alkylmalonic ester derivatives

The diastereoselective electrochemical carboxylation of chiral N-(2-bromoacyl)oxazolidin-2-ones has been studied. This reaction was carried out by cathodic reduction of the C-Br bond, in the presence of carbon dioxide, followed by treatment with diazomethane. The yields and the diastereomeric ratio of the two epimeric alkylmalonic acid derivatives are strongly affected by various factors: solvent-supporting electrolyte system, temperature, electrode material, electrolysis conditions, oxazolidinone moiety. The higher yields (88%) were obtained starting from N-(2-bromopropionyl)-4R-phenyloxazolidin-2-one 1a, but with poor diastereoselectivity (61:39). The two epimers were easily separated by flash chromatography. The best results were achieved using a different chiral auxiliary: Oppolzer's camphor sultam. Starting from 1j a good yield in carboxylated product was obtained (80%) with excellent diastereoselectivity (98:2). These chiral alkylmalonic acid derivatives are valuable building blocks in the synthesis of molecules with biological activity and of chiral propane-1,3-diols derivatives.     

Efficient pi-facial control in the ene reaction of nitrosoarene,triazolinedione, and singlet oxygen with tiglic amides of the bornane-derived sultam as chiral auxiliary: an economical synthesis of enantiomerically pure nitrogen- and oxygen-functionalized acrylic acid derivatives

The ene reaction of 4-nitronitrosobenzene (ArNO), N-phenyl-1,2,4-triazoline-3,5-dione (PTAD), and singlet oxygen (1O2) with the optically active tiglic-acid derivatives of Oppolzer's bornane-derived sultam affords the respective ene products regioselectively in excellent diastereoselectivity (de up to 99%) and in good yield (55-90%). The enophiles ArNO and PTAD give with the methyl-substituted substrate exclusively the like-configured ene adduct, while 1O2 leads to an 83:17 diastereomeric mixture. With the sterically more demanding isopropyl-substituted derivative even the smallest enophile 1O2 forms exclusively the like diastereomer. The high diastereoselectivity is rationalized in terms of the proper conformational alignment of the substrate and a preferred enophilic attack from the C(beta)-re face of the double bond. This concept offers an efficient synthetic route to enantiomerically pure nitrogen- and oxygen-functionalized acrylic acid derivatives.     

Lewis acid catalyzed asymmetric halohydrin reactions of chiral α,β-unsaturated carboxylic acid derivatives with N-halosuccinimide (NXS) as the halogen source

Lewis acid catalyzed asymmetric halohydrin reactions—(halohydroxylation as well as halomethoxylation) of chiral α,β-unsaturated carboxylic acid derivatives were performed using N-halosuccinimide (NXS; X = Br, I) as the halogen source. Regio- and anti-selectivity of 100% and moderate to good diastereoselectivity with good yields were observed when Oppolzer’s sultam was used as the chiral auxiliary. Among the Lewis acids, Yb(OTf)₃ was found to be the best catalyst. Alkenoyl and cinnamoyl substrates smoothly underwent bromohydrin reactions and the more electron-rich cinnamoyl substrates preferred to undergo iodohydrin reactions. However, electron-deficient cinnamoyl substrates did not respond to this Lewis acid catalyzed halohydrin reaction with NXS (X = Cl, Br, I).     

Conjugate addition of lithium N-phenyl-N-(α-methylbenzyl)amide: application to the asymmetric synthesis of (R)-(-)-angustureine

The conjugate addition of lithium (R)-N-phenyl-N-(α-methylbenzyl)amide to a range of α,β-unsaturated 4-methoxyphenyl esters proceeds with excellent levels of diastereoselectivity to give the corresponding β-amino esters in good yield and as single diastereoisomers (>99:1 dr). The synthetic utility of this methodology has been demonstrated via the short and concise asymmetric synthesis of the tetrahydroquinoline alkaloid (R)-(-)-angustureine in six steps and 32% overall yield from commercially available oct-2-enoic acid.     

Asymmetric glyoxylate-ene reaction catalyzed by C2-symmetric chiral bis(oxazoline)–lanthanide complexes

Efficient C₂-symmetric chiral bis(oxazoline)-lanthanide catalysts are developed for the glyoxylate-ene reaction to afford the α-hydroxy esters in 85% yield and up to 54% ee. An enhanced level of diastereoselectivity (81% de) was obtained in the reaction between menthyl glyoxylate and alkenes catalyzed by bis(oxazoline)–Ln(OTf)₃ complex.     

Enantioselective synthesis of m-carboranylalanine, a boron-rich analogue of phenylalanine

The enantiomers of the highly lipophilic α-amino acid m-carboranyl-alanine [3-(1,7-dicarba-closo-dodecaborane(12)-1-yl)-2-aminopropanoic acid], a carborane containing analogue of phenylalanine, have been synthesised via hydroxyamination of the N-acyl derivative formed from 3-(m-carboranyl)propionic acid [3-(1,7-dicarba-closo-dodeca-borane(12)-1-yl)-2-propanoic acid] and Oppolzer's camphor sultam. The enantiomeric excess of both enantiomers of the amino acid was >98%. (S)-Configuration was assigned to the (+)-enantiomer (CH₃OH, 589 nm).     

Synthesis of chiral 4-substituted 2-hydroxypent-4-enoic acid derivatives via diastereoselective ene reaction promoted by ZnBr2

The diastereoselective carbonyl-ene reaction of various 1,1-disubstituted olefins with the chiral derivatives of glyoxylic acid, with Oppolzer’s sultam 1a and 8-phenylmenthol 1b auxiliaries, was studied. The reaction proceeds effectively under undemanding conditions in the presence of equimolar or catalytic amount of ZnBr₂ in good yield and at 72–94% de. Diastereoselectivities are usually slightly better with 8-phenylmenthyl glyoxylate 1b, however, the use of hemiacetal 1a is preferred because the products are often crystalline.     

Radical addition to oxime ethers for asymmetric synthesis of beta-amino acid derivatives

The diastereoselective alkyl radical addition to chiral oxime ethers was studied with a view to preparing enantiomerically pure alpha,beta-dialkyl-beta-amino acid derivatives. The phase transfer-catalyzed alkylation of Oppolzer's camphorsultam derivative of oxime ether proceeded smoothly to give the alkylated N-(beta-oximino)acyl derivatives. In the presence of BF3.OEt2, radical addition to the oxime ethers proceeded using triethylborane as the radical initiator to give alpha,beta-dialkyl-beta-amino acid derivatives with excellent diastereoselectivity.     

Enantioselective synthesis of 2-substituted-1,4-diketones from (S)-mandelic acid enolate and α,β-enones

An approach for the synthesis of chiral non-racemic 2-substituted-1,4-diketones from (S)-mandelic acid and α,β-enones has been developed. The reaction of lithium enolate of the 1,3-dioxolan-4-one derived from optically active (S)-mandelic acid and pivalaldehyde with α,β-unsaturated carbonyl compounds proceeds readily to give the corresponding Michael adducts in good yields and with high diastereoselectivities. The addition of HMPA (3 equiv) reverses and strongly enhances the diastereoselectivity of the reaction. A change in the reaction mechanism from a lithium catalyzed to the one where catalysis has been suppressed by coordination of HMPA to lithium is proposed to explain these results. Subsequent basic hydrolysis of the 1,3-dioxolan-4-one moiety yields the corresponding α-hydroxy acids (in hemiacetal form), which after decarboxylation with oxygen in the presence of pivalaldehyde and the Co(III)Me₂opba complex as catalyst give chiral 2-substituted 1,4-dicarbonyl compounds with very high enantiomeric excesses. In this approach, (S)-mandelic acid acts as an umpoled chiral equivalent of the benzoyl anion.     

Synthesis of dihydronaphthalenes via aryne Diels-Alder reactions: scope and diastereoselectivity

Novel aryne Diels-Alder reactions with functionalized acyclic dienes are reported. These give useful cis-substituted dihydronaphthalene building blocks in good yield which are not easily accessible via other means, as demonstrated in the synthesis of sertraline. The first asymmetric aryne cycloaddition with an acyclic diene is also reported, giving excellent diastereoselectivities with Oppolzer's sultam as a chiral auxiliary.     

Alkylation and aldol reactions of acyl derivatives of N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine: asymmetric synthesis of α-alkoxy-, α-substituted-β-alkoxy- and α,β-dialkoxyaldehydes

Treatment of a range of O-protected glycolate derivatives of the chiral auxiliary N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine with KHMDS in the presence of 18-crown-6 followed by addition of an alkyl halide generates α-substituted derivatives with very high levels of diastereoselectivity. Alternatively, reaction of the potassium enolate of a propanoate or O-protected glycolate derivative of N-1-(1′-naphthyl)ethyl-O-tert-butylhydroxylamine with a range of aldehydes gives syn-aldol products with high levels of diastereoselectivity. These adducts may be reductively cleaved with LiAlH₄ to give enantiopure α-alkoxy-, α-substituted-β-alkoxy- and α,β-dialkoxyaldehydes in good yield.     

Asymmetric Synthesis of Simplactones A and B

A new, simple, and short route for the synthesis of simplactones A ( 1 ) and B ( 2 ) was achieved from a synthetically prepared chiral auxiliary, i.e., the Oppolzer camphor‐derived sultam 4 , and (4‐methoxybenzyl)‐protected 3‐hydroxypropanal, in 52 and 48% overall yield, respectively, and with high diastereoselectivity (Schemes 2 and 3).     

A novel strategy toward the synthesis of N-(β-glycosyl)asparagines based on the alkylation of ethyl nitroacetate using N-(β-glycosyl)iodoacetamides

A conceptually novel strategy has been developed for the synthesis of N-(β-glycosyl)asparagine precursors in good yield by the alkylation of ethyl nitroacetate using six per-O-acetylated N-(β-glycosyl)iodoacetamides derived from mono- and disaccharides. The use of a chiral organocatalyst, N-(9-anthracenylmethyl)cinchoninium chloride (10 mol %), resulted in diastereoselective alkylation up to 64% de. Single crystal structure analysis of the purified major diastereomer of the Glc derivative revealed an absolute configuration of S at the α-carbon of the monosubstituted ethyl nitroacetate which is a precursor of the l-asparagine conjugate.     

Asymmetric 1,4-addition of alkenylzirconium reagents to α,β-unsaturated ketones catalyzed by chiral rhodium complexes

Highly enantioselective 1,4-addition of alkenylzirconocene chlorides to α,β-enones was found to be catalyzed by a chiral rhodium complex generated from [Rh(cod)(MeCN)₂]BF₄ and (S)-BINAP. The reaction can be applied to either cyclic or acyclic enones and the optical yield was up to 99% ee. The reaction mechanism would involve the transmetalation between the alkenylzirconocene chloride and the rhodium complex to give the alkenylrhodium species as a key intermediate.     

不对称双酸催化3,4-二氢-2H-吡喃的反电子Hetero-Diels-Alder反应

研究了双酸催化剂不对称催化烯醚和β,γ-不饱和α-酮酸酯的反电子Hetero-Diels-Alder (HDA)反应, 为手性合成3,4-二氢-2H-吡喃类化合物提供了一种新的催化合成方法. InBr₃与手性磷酸钙盐Ca(1c)₂组合的手性双路易斯酸催化体系能够有效催化3,4-二氢-2H-吡喃和β,γ-不饱和α-酮酸酯的反电子HDA反应, 反应给出优秀的产率(最高达98%), 中等到良好的非对映选择性(最高达89:11)和良好到优秀的对映选择性(最高可达94%). 并且该双酸催化体系也能成功实现其它烯醚(如: 2,3-二氢-2H-呋喃, 乙烯基乙醚)的HDA反应, 获得优秀的非对映选择性(>94:6)和良好的对映选择性.     

Reaction of α,β-unsaturated Fischer carbene complexes with allyl alkoxide

Optically enriched homo-binuclear Fischer chromium carbene complexes with planar chiral arene chromium complexes gave α-allyl β-arylpropionates up to 97% ee by reaction with allyl alkoxide and subsequent photo-oxidative demetalation. The chiral hetero-binuclear tungsten carbene complexes afforded anti α-allyl β-hydroxy β-arylpropionates as a major product up to 92/8 dr by the same reaction sequence. High diastereoselectivity in these reactions is contributed to the planar chirality of the arene chromium complex, even though the reaction was carried out under vigorous basic media. The reaction products, α-allyl β-arylpropionates were derived by 1,3-M(CO)₅ shift and subsequent [3,3]-sigmatropic rearrangement. Also, the corresponding chromium-uncomplexed α,β-unsaturated Fischer carbene complexes afforded α-allyl β-arylpropionates under the same conditions. Formation of β-allyl β-arylpropionates via 1,2-M(CO)₅ shift followed by [3,4]-sigmatropic rearrangement was not observed in both reactions of chromium-coordinated and the corresponding chromium-uncoordinated α,β-unsaturated Fischer carbene complexes with allyl alkoxide in the presence of base.     

Enantioselective synthesis of 3-substituted-4-aryl piperidines useful for the preparation of paroxetine

The asymmetric conjugate addition reaction between 4-fluorophenylmagnesium bromide and various chiral α,β-unsaturated esters and enoylsultam substrates was explored to prepare a key intermediate useful in the preparation of paroxetine. The most selective auxiliary was found to be Oppolzer's (1S)-(−)-camphorsultam. Interestingly, the diastereoselection was opposite to that reported for acyclic enoylsultams.