High Syn Selectivity Research Articles

Reductive Ring-Opening 1,3-Difunctionalizations of Arylcyclopropanes with Sodium Metal

AbstractSodium dispersion promotes reductive ring opening of arylcyclopropanes. The presence of a reduction-resistant electrophile, such as methoxypinacolatoborane, epoxide, oxetane, paraformaldehyde, or chlorotrimethylsilane, during the reductive ring opening event leads to the formation of 1,3-difunctionalized 1-arylalkanes by immediate trappings of the resulting two reactive carbanions. In particular, the ring-opening 1,3-diborylations of arylcyclopropanes afford 1,3-diborylalkanes with high syn selectivity.

Gold-Catalyzed Hydroamination of Propargylic Alcohols: Controlling Divergent Catalytic Reaction Pathways To Access 1,3-Amino Alcohols, 3-Hydroxyketones, or 3-Aminoketones.

A versatile approach to the valorization of propargylic alcohols is reported, enabling controlled access to three different products from the same starting materials. First, a general method for the hydroamination of propargylic alcohols with anilines is described using gold catalysis to give 3-hydroxyimines with complete regioselectivity. These 3-hydroxyimines can be reduced to give 1,3-amino alcohols with high syn selectivity. Alternatively, by using a catalytic quantity of aniline, 3-hydroxyketones can be obtained in high yield directly from propargylic alcohols. Further manipulation of the reaction conditions enables the selective formation of 3-aminoketones via a rearrangement/hydroamination pathway. The utility of the new chemistry was exemplified by the one-pot synthesis of a selection of N-arylpyrrolidines and N-arylpiperidines. A mechanism for the hydroamination has been proposed on the basis of experimental studies and density functional theory calculations.

Efficient synthesis of syn-α-alkoxy epoxide via a diastereoselective Darzens reaction

A highly diastereoselective Darzens reaction was developed by using a MEM- or TBDPS-protected α-hydroxy aldehyde with di-tert-butyl bromomalonate. This method effectively provides a syn-α-alkoxy epoxide, and a possible reaction mechanism was proposed to explain the high syn selectivity.

Syn-Selective crotylation of aldehydes using bismuth-crotyl bromide-(1-butyl-3-methylimidazolium bromide) combination: some synthetic applications.

The Bi-[bmim][Br] combination has been found to offer high syn-selectivity in the crotylation of aldehydes with crotyl bromide using practically stoichiometric amounts of the reagents. The room temperature ionic liquid (RTIL), [bmim][Br], activated Bi metal in the presence of oxygen to produce crotylbismuthdibromide, which reacted with the aldehydes at room temperature. The major anti-syn diastereomeric product obtained from the crotylation of (R)-cyclohexylideneglyceraldehyde was utilized for the synthesis of dictyostatin and cryptophycin segments, and (+)-cis-aerangis lactone, using standard synthetic protocols.

ChemInform Abstract: anti‐Selective Asymmetric Nitro‐Michael Reaction of Furanones: Diastereocontrol by Catalyst.

Catalyst-controlled switching of diastereoselectivity from high syn-selectivity (>98/2 dr, syn) to anti-selectivity (up to 96/4 dr, anti) of the asymmetric nitro-Michael reaction of furanones is described. Anti-diastereoselectivity of the nitro-Michael reaction is very rare. With 0.1–5 mol % loadings of an epi-quinine catalyst, the reaction of 5-substituted 2(3H)-furanones with nitroalkenes smoothly proceeded to give the anti-Michael adducts in good yields (up to 95%) with excellent diastereo- and enantioselectivities (up to 96/4 dr, anti; up to 99% ee). DFT calculations support a model that accounts the high anti-diastereoselectivity.

ChemInform Abstract: Copper(I)‐Catalyzed Diastereoselective Borylative exo‐Cyclization of Alkenyl Aryl Ketones.

Copper(I)-catalyzed diastereoselective borylative exo-cyclization of alkenyl ketones with bis(pinacolato)diboron is reported. The reaction of alkenyl aryl ketones under a CuCl/Xantphos catalyst system provides four- or five-membered-ring syn-2-(borylmethyl)cycloalkanol derivatives in good yields with high syn selectivities. The utility of this method is demonstrated by further transformations of the cyclic products.

Anti-Selective Asymmetric Nitro-Michael Reaction of Furanones: Diastereocontrol by Catalyst.

Catalyst-controlled switching of diastereoselectivity from high syn-selectivity (>98/2 dr, syn) to anti-selectivity (up to 96/4 dr, anti) of the asymmetric nitro-Michael reaction of furanones is described. Anti-diastereoselectivity of the nitro-Michael reaction is very rare. With 0.1-5 mol % loadings of an epi-quinine catalyst, the reaction of 5-substituted 2(3H)-furanones with nitroalkenes smoothly proceeded to give the anti-Michael adducts in good yields (up to 95%) with excellent diastereo- and enantioselectivities (up to 96/4 dr, anti; up to 99% ee). DFT calculations support a model that accounts the high anti-diastereoselectivity.

A useful methoxyvinyl cation equivalent: α-t-butyldimethylsilyl-α-methoxyacetaldehyde

Described are the synthesis and application of α-t-butyldimethylsilyl-α-methoxyacetaldehyde as a formal methoxyvinyl cation equivalent. Addition of Grignard reagents to the title aldehyde, followed by treatment of the intermediate β-hydroxysilanes with KH, gives good yields of largely Z-methoxyvinylated products. Assuming a Peterson-like elimination mechanism, one can infer that the Grignard addition proceeds with high syn selectivity. These results are consistent with a chelation control model involving coordination to the α-methoxy group in the title aldehyde rather than an alternative stereoelectronic Felkin–Anh-type model. It must be noted that a steric Felkin–Anh model also accounts for the observed stereochemistry. All told, the title reagent can be employed to efficiently append a Z-configured methoxyvinyl group to an appropriate R–M species, in two steps.

Stereoselective TiCl4‐Mediated Aldol Reactions Starting from Acylsilanes

AbstractA simple and efficient method for the preparation of β‐hydroxyacylsilanes has been developed through a TiCl4‐mediated aldol reaction. A selection of Lewis acids has been assessed, and other parameters such as solvent and temperature have been optimized. These aldol reactions occur under mild conditions and furnish a selection of β‐hydroxy acylsilanes in fair‐to‐good yields with high syn selectivities (up to 99 %).

Stereocontrol in synthesis of homoallylic amines. Syn selective direct allylation of hydrazones with allylboronic acids.

Allylboronic acids directly react with acyl hydrazones, affording homoallylic amine derivatives. The reaction proceeds with very high syn selectivity, which is the opposite of the stereochemistry observed for allylboration of imines. The reaction can be carried out with both aromatic and aliphatic acyl hydrazones. Based on our studies the excellent syn stereochemistry can be explained by chelation control of the acyl hydrazone and the B(OH)(2) moiety.

DFT Studies of Mechanism and Origin of Stereoselectivity of Palladium-Catalyzed Cyclotrimerization Reactions Affording syn-Tris(norborneno)benzenes

Pd-catalyzed cyclotrimerization reactions of enantiopure halonorbornene derivatives furnished C3 or C3v symmetric syn-tris(norborneno)benzenes with high syn selectivity. To elucidate the reaction mechanism as well as the stereoselectivity of the present Pd-catalyzed cyclotrimerization, DFT calculations were carried out. The promising reaction pathway consists of (1) sequential olefin insertion followed by an HX elimination reaction of halonorbornene with the norbornenylpalladium intermediate, (2) electrocyclization of the trienylpalladium intermediate with a lower activation barrier than a triene compound, and (3) the β-elimination of HPdX of the cyclohexadienylpalladium intermediate. In addition, the stereoselectivity would be controlled by the regioselectivity in the olefin insertion process (homo and hetero positions) and the symmetry breaking in the palladacyclic intermediate.

Diastereoselective Synthesis of β-Heteroaryl syn-α-Methyl-β-Amino Acid Derivatives via a Double Chiral Auxiliary Approach

The addition of the SuperQuat enolate to five- and six-membered heterocyclic tert-butyl sulfinimines led to a high syn-selectivity of up to 99:1 in good to excellent yields. The reaction is tentatively proposed to proceed through an open-chain transition state with the presence of an α-heteroatom on the sulfinimine leading to high diastereoselectivities. The adducts were derivatized to β-amino esters and amides in a facile manner.

Base-catalyzed three-component direct Mannich reaction of enolizable ketones with high syn-selectivities

The three-component direct Mannich reaction between aldehydes, p-toluenesulfonamide, and enolizable ketones was achieved for the first time with organic bases as the catalysts. The corresponding N-tosylated β-aminoketones were obtained in high yields and good to excellent diastereoselectivities using TMG as the catalyst. Through reduction of the ketone group, the reaction product may be converted into β-aminol with excellent diastereoselectivity.

Syn-Selective Vinylogous Kobayashi Aldol Reaction

The Kobayashi aldol reaction has become a prominent transformation in polyketide syntheses. This methodology takes advantage of the directing effects of the Evans auxiliary and allows the stereoselective incorporation of a four carbon segment with two additional methyl branches establishing an anti-relationship between the two newly formed chiral centers. So far this transformation was restricted to anti-aldol products. We present here a modified protocol that provides the corresponding aldol product with high syn-selectivity.

Mechanistic Variations in Ionic Hydrogenation of Unsaturated Phosphine and Amine Boranes

Internal hydride transfer occurs when tethered carbocations are generated from unsaturated phosphine or phosphinite boranes. 3-Methylenecyclohexyl-derived boranes 12 or 18 react with MsOH to give ionic hydrogenation products with high syn-selectivity. With unsaturated amine boranes, initial hydrogen evolution gives BH(2)(OMs) complexes, but IH occurs using excess MsOH in a slower second stage. A diastereoselective reaction occurs from 26b using camphorsulfonic acid (first stage) and MsOH (second stage), affording 33 (68% ee) after hydrolysis.

ChemInform Abstract: syn‐Allylstannation of N‐Acyliminium Intermediates by Tributyl[γ‐(silyloxy)allyl]stannanes: A Key Reaction for the Diastereoselective Synthesis of Polyhydroxypiperidines and Polyhydroxyazepanes.

AbstractThe allylstannation of N‐acyliminium intermediates affords the expected adducts with high syn‐selectivity.

Intermediate as Catalyst: Catalytic Asymmetric Conjugate Addition of Nitroalkanes to α,β-Unsaturated Thioamides

Catalytic asymmetric conjugate addition of nitroalkanes to α,β-unsaturated thioamides is promoted by a mesitylcopper/(R)-DTBM-Segphos precatalyst, affording γ-nitrothioamides in moderate to high syn-selectivity and excellent enantioselectivity. The intermediate Cu-thioamide enolate functions as a soft Lewis acid/hard Brønsted base cooperative catalyst to drive the catalytic cycle efficiently under proton transfer conditions.

Selectivity in the addition reactions of organometallic reagents to aziridine-2-carboxaldehydes: the effects of protecting groups and substitution patterns.

Good to excellent stereoselectivity has been found in the addition reactions of Grignard and organozinc reagents to N-protected aziridine-2-carboxaldehydes. Specifically, high syn selectivity was obtained with benzyl-protected cis, tert-butyloxycarbonyl-protected trans, and tosyl-protected 2,3-disubstituted aziridine-2-carboxaldehydes. Furthermore, rate and selectivity effects of ring substituents, temperature, solvent, and Lewis acid and base modifiers were studied. The diastereomeric preference of addition is dominated by the substrate aziridines' substitution pattern and especially the electronic character and conformational preferences of the nitrogen protecting groups. To help rationalize the observed stereochemical outcomes, conformational and electronic structural analyses of a series of model systems representing the various substitution patterns have been explored by density functional calculations at the B3LYP/6-31G* level of theory with the SM8 solvation model to account for solvent effects.

Direct Carbon−Carbon Bond Formation via Reductive Soft Enolization: A Kinetically Controlled syn-Aldol Addition of α-Halo Thioesters and Enolizable Aldehydes

The direct addition of enolizable aldehydes and α-halo thioesters to produce β-hydroxy thioesters enabled by reductive soft enolization is reported. The transformation is operationally simple and efficient and has the unusual feature of giving high syn-selectivity, which is the opposite of that produced for (thio)esters under conventional conditions. Moreover, excellent diastereoselectivity results when a chiral nonracemic α-hydroxy aldehyde derivative is used.

ChemInform Abstract: High Diastereofacial Selectivity in the Reaction of Silyl Enol Ethers with Chlorosulfides.

The chlorosulfides 1 or 2 react with silyl enol ethers in presence of anhydrous zinc bromide to give mainly the corresponding syn products 4 or 6 respectively. The high syn selectivity of these reaction is explained by nucleophilic addition to a Chelated Chiral thioniun ion.