Stereocontrolling Element Research Articles

Cooperative Bifurcated Chalcogen Bonding and Hydrogen Bonding as Stereocontrolling Elements for Selective Strain-Release Septanosylation.

The exploitation of noncovalent interactions (NCIs) is emerging as a vital handle in tackling broad stereoselectivity challenges in synthesis. In particular, there has been significant recent interest in the harnessing of unconventional NCIs to surmount difficult selectivity challenges in glycosylations. Herein, we disclose the exploitation of an unconventional bifurcated chalcogen bonding and hydrogen bonding (HB) network, which paves the way for a robust catalytic strategy into biologically useful seven-membered ring sugars. Through 13C nuclear magnetic resonance (NMR) in situ monitoring, NMR titration experiments, and density functional theory (DFT) modeling, we propose a remarkable contemporaneous activation of multiple functional groups consisting of a bifurcated chalcogen bonding mechanism working hand-in-hand with HB activation. Significantly, the ester moiety installed on the glycosyl donor is critical in the establishment of the postulated ternary complex for stereocontrol. Through the 13C kinetic isotopic effect and kinetic studies, our data corroborated that a dissociative SNi-type mechanism forms the stereocontrolling basis for the excellent α-selectivity.

Crossing the ortho-hurdle: Ionic stereocontrol enables atroposelective Suzuki-Miyaura coupling

Atropisomerism—a unique stereochemical phenomena arising from a rotationally restricted single bond—is ubiquitous in natural products, functional materials, and pharmaceuticals. As a direct approach to atropoenantiomers of biaryls, catalyst-controlled asymmetric cross-coupling relies exclusively on differentiation between substrates’ ortho-substituents. Here, we describe the development of ionic interaction-directed atroposelective Suzuki-Miyaura coupling. Unprecedented stereocontrol is attainable through catalyst-substrate interactions at not only the ortho- but also the meta-positions. In particular, our study surmounts a formidable hurdle of decoupling the asymmetric induction from the intrinsic influence of proximal ortho-groups and demonstrates that high atroposelectivity is achievable by engaging distal meta-substituents as the dominant stereocontrol elements. The general adaptability of the new ionic catalyst system is attributable to the unconventional approach that engages strong electrostatic steering of substrates by a sterically symmetrical ligand, which could serve as the design principle of novel catalysts to unleash the vast potential of ionic interactions in catalysis.

Cooperative Stereoinduction in Asymmetric Photocatalysis.

Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.

Reactivity of Stabilized Vinyldiazo Compounds toward Alkenyl- and Alkynylsilanes under Gold Catalysis: Regio- and Stereoselective Synthesis of Skipped Dienes and Enynes

We report the gold-catalyzed reaction of vinyldiazo compounds and alkenylsilanes to produce skipped dienes, which are common structural motifs in an array of bioactive compounds. This carbon–carbon bond-forming transformation proceeds with complete regio- and stereoselectivity with the silyl group serving as a regio- and stereocontrolling element. Likewise, the use of alkynylsilanes as reaction partners yielded skipped enynes resulting from a C(sp)–C(sp3) coupling. Mechanistic experiments and DFT studies have provided support for a stepwise mechanism.

Enantioselective α-Allylation of Acyclic Esters Using B(pin)-Substituted Electrophiles: Independent Regulation of Stereocontrol Elements through Cooperative Pd/Lewis Base Catalysis.

Cooperation between a Lewis base and Pd catalyst enables the direct enantioselective α-functionalization of aryl and vinyl acetic acid esters using a bifunctional B(pin)-substituted electrophile. Critical to the success of this method was the recognition that both catalysts could control the necessary stereochemical aspects; the Lewis base catalyst controls the enantioselectivity of the reaction, whereas the Pd catalyst regulates alkenyl-B(pin) configuration. This is the first example of using cooperative catalysis to control both stereochemical features during Pd-catalyzed allylic alkylation.

Chiral Phosphoric Acid Catalyzed Enantioselective Ring Expansion Reaction of 1,3-Dithiane Derivatives: Case Study of the Nature of Ion-Pairing Interaction.

Chiral counterion controlled asymmetric catalysis via an ion-pairing interaction has attracted immense attention in recent years. Despite a number of successful studies, the mechanistic elucidation of the stereocontrolling element in the ion-pairing interaction is rarely conducted and hence its nature is still far from being well understood. Herein we report an in-depth mechanistic case study of a newly developed enantioselective ring expansion reaction of 1,3-dithiane derivatives catalyzed by chiral phosphoric acid (CPA). An unprecedented enantioselective 1,2-sulfur rearrangement/stereospecific nucleophilic addition sequence was proven to be the stereoselective pathway. More importantly, by thorough investigation of the intrinsic nature of the stereospecific nucleophilic addition to the cationic thionium intermediate, we discovered that the key interaction in this process is the nonclassical C-H···O hydrogen bonds formed between the conjugate base of the CPA catalyst and the cationic intermediate. These C-H···O hydrogen bonds not only bind the catalyst to the substrates to form energetically favored states throughout the overall processes but also firmly maintain the relative positions of these fragments as the "fixed" contact ion pair to sustain the chiral information generated at the initial sulfur rearrangement step. This mechanistic case study provides a very clear understanding of the nature of the ion-pairing interaction in organocatalysis. The conclusion encourages the further development of the research field with the focus to design new organocatalysts and cultivate novel organocatalytic transformations.

Study of Stereocontrolling Elements in Chiral Phosphoric Acid Catalyzed Addition Reaction of Vinylindoles with Azlactones

DFT studies were carried out to clarify the reaction mechanism and the stereocontrolling elements in the addition reaction of vinylindoles with azlactones catalyzed by a chiral phosphoric acid. The results suggest that the reaction proceeds via a six-membered transition state that is composed of the vinyl group of vinylindole and the enol moiety of azlactone.

Application of a C-C Bond-Forming Conjugate Addition Reaction in Asymmetric Dearomatization of β-Naphthols.

A C-C bond-forming conjugate reaction was successfully applied to the enantioselective dearomatization of β-naphthols. A C(sp2)-C(sp3) bond is formed by using propargylic ketones as reactive partners. Good to excellent Z/E ratios and ee values were obtained by employing an in situ generated magnesium catalyst. Further transformations of the Z-configured C-C double bond in the products were achieved under mild reaction conditions. Moreover, the stereocontrolling element of this magnesium-catalyzed dearomatization reaction was explored by computational chemistry.

ChemInform Abstract: Asymmetric Synthesis of α‐Amino Acids via Homologation of Ni(II) Complexes of Glycine Schiff Bases. Part 3. Michael Addition Reactions and Miscellaneous Transformations

AbstractReview: 131 refs.

Asymmetric synthesis of α-amino acids via homologation of Ni(II) complexes of glycine Schiff bases. Part 3: Michael addition reactions and miscellaneous transformations.

The major goal of this review is a critical discussion of the literature data on asymmetric synthesis of α-amino acids via Michael addition reactions involving Ni(II)-complexes of amino acids. The material covered is divided into two conceptually different groups dealing with applications of: (a) Ni(II)-complexes of glycine as C-nucleophiles and (b) Ni(II)-complexes of dehydroalanine as Michael acceptors. The first group is significantly larger and consequently subdivided into four chapters based on the source of stereocontrolling element. Thus, a chiral auxiliary can be used as a part of nucleophilic glycine Ni(II) complex, Michael acceptor or both, leading to the conditions of matching vs. mismatching stereochemical preferences. The particular focus of the review is made on the practical aspects of the methodology under discussion and mechanistic considerations.

ChemInform Abstract: Synthesis of (.+‐.)‐Amathaspiramide F (I) and Discovery of an Unusual Stereocontrolling Element for the [2,3]‐Stevens Rearrangement.

ChemInformVolume 45, Issue 10 Natural Products ChemInform Abstract: Synthesis of (.+-.)-Amathaspiramide F (I) and Discovery of an Unusual Stereocontrolling Element for the [2,3]-Stevens Rearrangement. Arash Soheili, Arash Soheili Dep. Biochem., Univ. Tex. Southwest. Med. Cent., Dallas, TX 75390, USASearch for more papers by this authorUttam K. Tambar, Uttam K. Tambar Dep. Biochem., Univ. Tex. Southwest. Med. Cent., Dallas, TX 75390, USASearch for more papers by this author Arash Soheili, Arash Soheili Dep. Biochem., Univ. Tex. Southwest. Med. Cent., Dallas, TX 75390, USASearch for more papers by this authorUttam K. Tambar, Uttam K. Tambar Dep. Biochem., Univ. Tex. Southwest. Med. Cent., Dallas, TX 75390, USASearch for more papers by this author First published: 21 February 2014 https://doi.org/10.1002/chin.201410213Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume45, Issue10March 11, 2014 RelatedInformation

ChemInform Abstract: C—H···O Non‐classical Hydrogen Bonding in the Stereomechanics of Organic Transformations: Theory and Recognition

AbstractReview: 90 refs.

Synthesis of (±)-Amathaspiramide F and Discovery of an Unusual Stereocontrolling Element for the [2,3]-Stevens Rearrangement

A formal total synthesis of (±)-amathaspiramide F through a tandem palladium-catalyzed allylic amination/[2,3]-Stevens rearrangement is reported. The unexpected diastereoselectivity of the [2,3]-Stevens rearrangement was controlled by the substitution patterns of an aromatic ring. This discovery represents a new stereocontrolling element for [2,3]-sigmatropic rearrangements in complex molecular settings.

C–H⋯O non-classical hydrogen bonding in the stereomechanics of organic transformations: theory and recognition

This manuscript describes the role of non-classical hydrogen bonds (NCHBs), specifically C-HO interactions, in modern synthetic organic transformations. Our goal is to point out the seminal examples where C-H···O interactions have been invoked as a key stereocontrolling element and to provide predictive value in recognizing future and/or potential C-H···O interactions in modern transformations.

Rational Design of Aldol Reactions That Proceed via Kinetic Resolution with Switchable Enantioselectivity

The stereoselectivity of aldol reactions of chiral reactants can be factorized into to three stereocontrol elements: the diastereoface selectivities of the ketone enol(ate) and aldehyde and the relative topicity of the coupling. Application of the multiplicativity rule to these elements leads to the prediction that kinetic resolution (KR) should be possible if all three stereocontrol elements are strongly biased. As a corollary, the enantioselectivity of the kinetic resolution should be switchable by a change in the sense of selectivity of any of the stereocontrol elements. This hypothesis was tested using aldehyde and ketone reactants with high diastereoface selectivities and developing reaction conditions that strongly favor either syn or anti relative topicity. The aldehyde 2 undergoes aldol reactions with near-exclusive Felkin diastereoface selectivity, and hydroxy-protected derivatives of ketone 1 (R = MOM, Et(3)Si, or Ac) undergo aldol reactions with high diastereoface selectivity to give 3,5-trans adducts. High levels of anti and syn relative topicity were obtained with dicyclohexylboron enolates and Ti(O(i)Pr)(4)Li "ate" enolates, respectively. Using these enolates, aldol reactions of (+/-)-2 with (+/-)-1 gave two of the eight possible diastereomeric adducts (3 from a diastereoselective like combination of reactant enantiomers and 4 from a diastereoselective unlike combination) predominantly (>95% of the adducts) in ratios of 0.05-20:1; boron enolates favored the like reaction (3:4, 15-20:1) and Ti "ate" enolates favored the unlike reaction (3:4, 1:10-20). Under these conditions, the ratio of like and unlike products is a measure of the mutual kinetic enantioselection (MKE) and reflects the ratio of the rate constants for the competing like and unlike reactions. For each of the four diastereomers of 1, the reactions with the highest MKEs in favor of the like (3) or unlike products (4) were repeated using highly enantioenriched ketone. These reactions occurred with the expected KR (s = 10-20) allowing selective access to enantioenriched diastereomers of 3 or 4 from (+/-)-2. These adducts are useful for polypropionate synthesis, and this design strategy for KR should be applicable to related processes.

Stereoselective Double Alkylation of the Acetoacetate Ester α-Carbon on a d-Glucose-Derived Template: Application to the Synthesis of Enantiopure Cycloalkenones Bearing an Asymmetric Quaternary Carbon

The previously developed D-glucose derivative, i.e., methyl 6-deoxy-2,3-di-O-(tert-butyldimethylsilyl)-a-D-glucopyranoside, served as a significant stereocontrolling element for the diastereoselective alkylation of the α-carbon in its acetoacetate at C-4 with two types of alkyl halides. The thus obtained doubly alkylated acetoacetate moiety bearing both methyl and allyl groups was efficiently converted into functionalized cycloalk-2-en-1-one derivatives by means of an intramolecular aldol strategy. Furthermore, the synthetic utility of the cycloalkenones was exemplified through the 1,4-addition to the thus obtained cyclopentenone derivative.

Stereoselective synthesis of 2-dienyl-substituted piperidines using an η4-dienetricarbonyliron complex as the stereocontrolling element in a double reductive amination cascade

In the presence of NaBH(OAc)(3), a 1,5-keto-aldehyde, contained within a side-chain of an eta(4)-dienetricarbonyliron complex, undergoes a double reductive amination sequence with a series of primary amines, to provide the corresponding piperidine products in good to excellent yield. The dienetricarbonyliron complex functions as a powerful chiral auxiliary in this cascade process, exerting complete control over the stereoselectivity of the reaction, with the formation of a single diastereoisomeric product. The sense of stereoinduction has been confirmed by X-ray crystallography. Removal of the tricarbonyliron moiety can be effected with CuCl(2) to afford the corresponding 2-dienyl-substituted piperidine in excellent yield. Attempted extension of this cyclisation strategy to the corresponding azepane ring system using a 1,6-keto-aldehyde as the cyclisation precursor was unsuccessful; in this case, the reaction stopped after a single reductive amination on the aldehyde to provide an acyclic keto-amine product.

メリラクトンＡの全合成：遠隔不斉誘導と不斉非対称化

(-)-Merrilactone A [(-)-1], isolated from Illicium merrillianum in 2000, possesses neurite outgrowth activity in cultures of fetal rat cortical neurons, and, therefore, is expected to show therapeutic potential for the treatment of neurodegeneration associated with Alzheimer's and Parkinson's diseases. Apart from its biological aspects, the caged pentacyclic skeleton of 1 poses interesting synthetic challenges. In this account, we describe the total synthesis of (+)- and (-)-merrilactone A, based on two novel strategies. Both strategies employed, as a key step, a transannular aldol reaction of eight membered diketone to construct the core cis-bicyclo [3.3.0] octyl system of 1. In the first generation total synthesis, the 2, 6-bis (trifluoromethyl) benzyl group served as a long-range stereocontrolling element for synthesis of the enantiopure bicyclo [3.3.0] octane frame-work of (-)-1. On the other hand, asymmetric aldol reaction was developed in the second generation synthesis to build the fused core of (+)-1 in a more consice manner. The obtained key intermediates were utilized as a platform for the subsequent functional group manipulations necessary for construction of (-)- and (+)-1. Surprisingly, both the natural and unnatural enantiomers of 1 equally promoted neurite outgrowth in primary neuronal cultures.

Asymmetric Total Synthesis of (‐)‐Merrilactone A (I): Use of a Bulky Protecting Group as Long‐Range Stereocontrolling Element.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

The Synthesis of N,O-Ferrocenyl Pyrrolidine-Containing Ligands and Their Application in the Diethyl- and Diphenylzinc Addition to Aromatic Aldehydes

A facile route to a series of planar chiral N,O-ferrocenyl pyrrolidine-containing ligands with varying substituents at the nitrogen and oxygen donor atoms is described. The oxygen donor atom was introduced via a diastereoselective ortho-metalation of N-methylpyrrolidinyl and N-allylpyrrolidinyl ferrocene intermediates and was quenched with various ketones. The nitrogen substituent was varied through deallylation and subsequent derivatization of a secondary pyrrolidine. The efficacy of these novel ligands was investigated in the enantioselective addition of diethylzinc and diphenylzinc to aromatic aldehydes. The ligands proved highly effective in the diethylzinc addition to benzaldehyde that resulted in high yields of up to 99% and enantioselectivities (ee's) of up to 95%. The role of planar chirality was explored and the results indicated that the planar chirality, and not the central chirality, of the ferrocenyl ligands was the dominant stereo-controlling element. Employment of a mixed ethyl-phenylzinc reagent in the phenylation of aromatic aldehydes led to a mixture of the two additional products, and the phenylated product was obtained in up to 37% ee.