Chiral Acyloxy Borane Research Articles

Stereoselectivity in (Acyloxy)borane-Catalyzed Mukaiyama Aldol Reactions

The origin of diastereo- and enantioselectivity in a Lewis acid-catalyzed Mukaiyama aldol reaction is investigated using a combination of dispersion corrected DFT calculations and transition state force fields (TSFF) developed using the quantum guided molecular mechanics (Q2MM) method. The reaction proceeds via a closed transition structure involving a nontraditional hydrogen bond that is 3.3 kJ/mol lower in energy than the corresponding open transition structure. The correct prediction of the diastereoselectivity of a Mukaiyama aldol reaction catalyzed by the conformationally flexible Yamamoto chiral (acyloxy) borane (CAB) requires extensive conformational sampling at the transition structure, which is achieved using a Q2MM-derived TSFF, followed by DFT calculations of the low energy conformational clusters. Finally, a conceptual model for the rationalization of the observed diastereo- and enantioselectivity of the reaction using a closed transition state model is proposed.

ChemInform Abstract: Stereoselective Total Synthesis of the Pseudopterolide Kallolide A.

A total synthesis of the pseudopterolides, kallolide A (36) and kallolide A acetate (35), has been achieved. The racemic forms were prepared from the syn adduct 20 of furan 13 and stannane 17 (BF(3).OEt(2)-promoted addition) via the 15-membered propargylic allylic ether 25. [2,3]Wittig ring contraction led to the cis, anti, cis product 26. Alcohol 26 was transformed to butenolide 34 with net retention of configuration by Pd(PPh(3))(4)-catalyzed carbonylation of the mesylate 27 and ensuing AgNO(3)-catalyzed cyclization of the derived allenic acid 29. Solvolysis of the SEM ether (at C2) 34 in acetic acid or aqueous t-BuOH afforded racemic kallolide A acetate (35) and kallolide A (36), respectively, with inversion of stereochemistry by an S(N)1 process. Key elements of stereocontrol, including the steric outcome of the [2,3]Wittig ring contraction, the allenic ester isomerization, and the solvolysis reactions, were predicted from molecular mechanics calculations. The C8 and C2 epimers 45 and 46 of the cis, anti, cis kallolide A SEM ether precursor 34 were also prepared. The synthetic route originated from the anti adduct 19 of aldehyde 13 and the allylic chloride 16 and CuCl (cat), HSiCl(3), and i-Pr(2)NEt. Adduct 19 was subjected to the same sequence as the syn counterpart 20 to produce the trans, anti, cis [2,3]Wittig ring contraction product 42. The derived allenoate 44 afforded a 1:1 mixture of butenolides 45 and 46 upon sequential treatment with TBAF and AgNO(3). Finally, the natural enantiomer (+)-36 of kallolide A was synthesized from the enantioenriched syn adduct (+)-20, prepared by addition of allylic stannane 17 to aldehyde 13 promoted by a modified chiral acyloxyborane Lewis acid.

Asymmetric total syntheses of (+)-cheimonophyllon e and (+)-cheimonophyllal.

The highly enantiocontrolled total syntheses of natural (+)-cheimonophyllon E (5) and (+)-cheimonophyllal (6), biologically intriguing oxygenated bisabolane-type sesquiterpenoids, have been completed. The present synthetic strategy featured the use of an asymmetric aldol-type reaction for preparing in the first synthetic step an optically active 6-C-substituted 3-methyl-2-cyclohexenone derivative. Thus, a Mukaiyama aldol reaction of 1-methyl-3-silyloxy-1,3-cyclohexadiene 31 with alpha,beta-unsaturated aldehyde 11 in the presence of a chiral (acyloxy)borane (CAB)-type Yamamoto catalyst 33 proceeded with high levels of both diastereo- and enantioselectivities. The predominant aldol adduct, syn-9, was transformed into gamma,delta-epoxy allylic alcohol 8 by a nine-step sequence, including the substrate-controlled 1,2-reduction of enone, syn-12, also the epoxidation of allylic alcohol 15. Epoxy-alcohol 8 underwent 5-exo-cyclization in a high regioselective manner under acidic conditions to produce a bicyclic key intermediate (+)-7, which was eventually efficiently converted to (+)-cheimonophyllon E (5) or (+)-cheimonophyllal (6).

Asymmetric allylation polymerization of bis(allylsilane) and dialdehyde containing Siphenyl linkage

Repetitive Sakurai–Hosomi allylation between dialdehyde and bis(allylsilane) yielded a polymer having stereogenic carbons in its main chain. In the presence of an enantiopure Lewis acid catalyst such as chiral (acyloxy)borane (CAB), the allylation polymerization proceeded in a stereoselective manner to give optically active polymers. We have prepared new monomers containing Siphenyl linkages for the asymmetric allylation polymerization. After polymerization, the Siphenyl linkages in the main chain could be cleaved easily to give the corresponding chiral homoallylic alcohol. The enantiomeric purity of the chiral polymer was then evaluated by chiral HPLC analysis of the alcohol obtained from the degradation.

Asymmetric Polymerization of Dialdehyde and Bis(allylsilane) in the Presence of Chiral (Acyloxy)borane Catalyst

We present a study of the asymmetric polymerization of dialdehyde and bis(allylsilane) based on the enantioselective addition of allylsilane to aldehyde in the presence of chiral Lewis acid. Chiral (acyloxy)borane (CAB) derived from enantiopure tartaric acid was used as a chiral Lewis acid to promote the asymmetric polymerization. Repetitive allylation reaction between these monomers in the presence of CAB yielded optically active polymers having asymmetric carbons in their main chain. The corresponding model asymmetric reaction of β-substituted allylsilane with benzaldehyde was also studied.

Asymmetric Allylation Polymerization of Bis(allylsilane) and Dialdehyde

The enantioselective addition of an allylsilane to an aldehyde is an excellent method to give optically active homoallyl alcohols. Applying this reaction to the asymmetric polymerization of bis(allylsilane) and dialdehyde, the synthesis of a new optically active polymer having chirality on the main chain was possible. Chiral acyloxyborane (CAB) was effective as a catalyst in this asymmetric allylation polymerization.

Experiments Directed Towards the Synthesis of Anthracyclinones. XXXIV Hetero-Diels - Alder Reactions Using Chiral Boron and Titanium Reagents

Reactions between benzaldehyde or o-anisaldehyde and a series of silyloxy dienes catalysed by the chiral acyl- oxyborane (CAB) complex (4) give high yields of enantioselective products from a Mukaiyama aldol rather than a hetero-Diels–Alder reaction. Attempts to effect a similar catalytic reaction with anthraquinone aldehydes were unsuccessful but use of 2 equiv. of the CAB complex (1) followed by cyclization promotes a formal hetero- Diels–Alder reaction between the aldehyde (7) and the diene (12) to give the dihydropyrone (24) in 45% yield and with a 79% e.e. in favour of the 2'R enantiomer. Hetero-Diels–Alder reactions between the aldehyde (7) and the diene (12) using the chiral titanium complexes Ti[(R)-BINOL]Cl 2 , Ti[(R,R)-TADDOL]Cl 2 and Ti[(R)-BINOL] 2 have been investigated. The first two complexes promote the reaction at −30 and −78° respectively but with low induced enantioselectivities.

Stereoselective Total Synthesis of the Pseudopterolide Kallolide A.

A total synthesis of the pseudopterolides, kallolide A (36) and kallolide A acetate (35), has been achieved. The racemic forms were prepared from the syn adduct 20 of furan 13 and stannane 17 (BF(3).OEt(2)-promoted addition) via the 15-membered propargylic allylic ether 25. [2,3]Wittig ring contraction led to the cis, anti, cis product 26. Alcohol 26 was transformed to butenolide 34 with net retention of configuration by Pd(PPh(3))(4)-catalyzed carbonylation of the mesylate 27 and ensuing AgNO(3)-catalyzed cyclization of the derived allenic acid 29. Solvolysis of the SEM ether (at C2) 34 in acetic acid or aqueous t-BuOH afforded racemic kallolide A acetate (35) and kallolide A (36), respectively, with inversion of stereochemistry by an S(N)1 process. Key elements of stereocontrol, including the steric outcome of the [2,3]Wittig ring contraction, the allenic ester isomerization, and the solvolysis reactions, were predicted from molecular mechanics calculations. The C8 and C2 epimers 45 and 46 of the cis, anti, cis kallolide A SEM ether precursor 34 were also prepared. The synthetic route originated from the anti adduct 19 of aldehyde 13 and the allylic chloride 16 and CuCl (cat), HSiCl(3), and i-Pr(2)NEt. Adduct 19 was subjected to the same sequence as the syn counterpart 20 to produce the trans, anti, cis [2,3]Wittig ring contraction product 42. The derived allenoate 44 afforded a 1:1 mixture of butenolides 45 and 46 upon sequential treatment with TBAF and AgNO(3). Finally, the natural enantiomer (+)-36 of kallolide A was synthesized from the enantioenriched syn adduct (+)-20, prepared by addition of allylic stannane 17 to aldehyde 13 promoted by a modified chiral acyloxyborane Lewis acid.

Asymmetric hetero Diels-Alder reaction catalyzed by stable and easily prepared CAB catalysts

A stable chiral (acyloxy)borane (CAB) complex is prepared in situ by mixing a tartaric acid derivative and different arylboronic acids at room temperature. A solution of the catalyst is effective in catalyzing hetero Diels-Alder reactions of electron rich dienes with conjugated aldehydes to produce dihydropyrone derivatives of high optical purities.

Asymmetric hetero Diels-Alder reaction catalyzed by stable and easily prepared CAB catalysts

A stable chiral (acyloxy)borane (CAB) complex is prepared in situ by mixing a tartaric acid derivative and arylboronic acids at room temperature. A solution of the catalyst is effective in catalyzing hetero Diels-Alder reactions to produce dihydropyrone derivatives of high optical purities.

Catalytic Asymmetric Aldol-Type Reactions Using a Chiral (Acyloxy)borane Complex

Abstract In the presence of 20 mol% of a chiral (acyloxy)borane (CAB) complex prepared from BH3·THF and a chiral mono-O-acylated tartaric acid, achiral silyl enol ethers or ketene silyl acetals react with achiral aldehydes to afford the corresponding aldol-type adducts in good yields with high enantio- and diastereoselectivities. Furthermore, the reactivity of aldol-type reactions can be improved without reducing the enantioselectivity by use of 10—20 mol% of the CAB complex prepared from 3,5-bis(trifluoromethyl)phenylboronic acid and chiral tartaric acid derivative. The enantioselectivity can also be improved without reducing the chemical yield by use of 20 mol% of the CAB complex prepared from o-phenoxyphenylboronic acid and chiral tartaric acid derivative. The observed erythro selectivities and re-face attack of nucleophiles on carbonyl carbon of aldehydes imply that the extended transition state model is applicable.

Asymmetric hetero Diels-Alder reaction catalyzed by stable and easily prepared chiral (acyloxy)borane (CAB) catalysts

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAsymmetric hetero Diels-Alder reaction catalyzed by stable and easily prepared chiral (acyloxy)borane (CAB) catalystsQingzhi Gao, Tohru Maruyama, Makoto Mouri, and Hisashi YamamotoCite this: J. Org. Chem. 1992, 57, 7, 1951–1952Publication Date (Print):March 1, 1992Publication History Published online1 May 2002Published inissue 1 March 1992https://doi.org/10.1021/jo00033a005RIGHTS & PERMISSIONSArticle Views595Altmetric-Citations63LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (291 KB) Get e-Alerts Get e-Alerts

Catalytic asymmetric aldol reactions. Use of a chiral (acyloxy)borane complex as a versatile Lewis-acid catalyst

In the presence of 20 mol% of a chiral (acyloxy)borane (CAB) complex prepared from (2R,3R)-2-O(2,6-diisopropoxybenzoyl)tartaric acid and borane-tetrahydrofuran, various allyltrimethylsilanes react with achiral aldehydes to afford the corresponding homoallylic alcohols in good yields with high diastereo- and enantioselectivities. Furthermore, the reactivity of allylation can be improved without reducing the enantioselectivity by using 10-20 mol% of the CAB complex prepared from 3,5-bis(trifluoromethyl)phenylboronic acid and chiral tartaric acid derivative. The observed selectivities and re-face attack of nucleophiles on the carbonyl carbons of aldehydes imply that the extended transition-state model is applicable

ChemInform Abstract: Asymmetric Intramolecular Diels‐Alder Reaction Catalyzed by a Chiral Acyloxyborane Complex.

AbstractThe methyldecatrienal (I) undergoes intramolecular cycloaddition reaction in the presence of a chiral acyloxyborane complex, derived from a monobenzoyltartaric acid, to form the tetrahydroindancarboxaldehyde (II) which is converted to the optically active acetal (IV).

ChemInform Abstract: Chiral (Acyloxy)borane (CAB): A Powerful and Practical Catalyst for Asymmetric Diels‐Alder Reaction.

AbstractReaction of the monoacylated enantiomeric tartaric acids (I) with borane (II) forms the chiral acyloxyboranes (III).

Chiral (acyloxy)borane (CAB): a powerful and practical catalyst for asymmetric Diels-Alder reactions

Chiral (acyloxy)borane (CAB): a powerful and practical catalyst for asymmetric Diels-Alder reactions

Asymmetric intramolecular Diels-Alder reaction catalyzed by chiral acyloxyborane complex

Intramolecular Diels-Alder reaction of 2-methyl-(E,E)-2,7,9-decatrienal catalyzed by chiral acyloxyborane complex proceeds with high stereo and enantioselectivities.