Highly enantioselective Michael addition of ketones to nitroolefins catalyzed by (S)-pyrrolidine arenesulfonamide

A new type of catalyst for the asymmetric Michael addition reaction has been designed and synthesized. This catalyst, (S)-pyrrolidine arenesulfonamide 1, resulted in high yields (up to 93%), excellent diastereoselectivities (syn/anti ratio up to 99/1), and excellent enantioselectivities (ee up to 99%) for various cyclic ketones and nitroolefins.     

Spontaneous and diastereoselective aldol reactions of cyclic β-amino ketones in the presence of water

Direct aldol reactions of 4-methyl-1-piperidone, tropinone and granatanone (pseudopelletierine) take place spontaneously in the presence of water without any catalyst or additional reagents. The anti/syn-diastereoselectivity of the aqueous aldol reaction depends on the amount of water used. The syn-selectivity of the reaction is probably due to the thermodynamic control. Excellent atom economy and low E factors together with anti-selectivity as high as 98:2 for the tropinone aldol were obtained.     

Highly stereoselective anti-aldol reactions catalyzed by simple chiral diamines and their unique application in configuration switch of aldol products

Chiral derivatives of trans-1,2-diaminocyclohexane with different N,N-dialkyl groups in well-defined orientations have been synthesized, and applied as catalysts for the asymmetric aldol reaction between a variety of aldehydes and ketones. Enantiomeric catalyst 1j catalyzed the reaction in ethanol and provided excellent diastereoselectivity and enantioselectivity. Significantly, simple replacement of organic solvents with water switched the products of the aldol reactions from anti to syn configuration. Such catalytic reactions led to the products with anti to syn diastereoselectivity up to 99:1 in ethanol, while in water gave the products with syn to anti diastereoselectivity up to 99:1.     

Reactions of complex ligands part 106. Heterobimetallic fused arenes: Chromium-templated benzannulation of a ferrocene-type metal carbene

The heterobimetallic iron cyclopentadienyl(indenyl)carbene chromium complex 3 was synthesized from 5-bromoindene by via metalation with C₅H₅Fe(CO)₂I, photodecarbonylation and chromium carbene functionalization of the π-bromoindenyl(cyclopentadienyl)iron intermediate 2. The chromium-templated benzannulation with 3-hexyne afforded syn- and anti-diastereomers of heterobimetallic hydroquinoid arene Cr(CO)₃ complexes 4a and 4b which were characterized by X-ray, spectroscopic and electrochemical means.     

Synthesis of helical [2.2]paracyclophanes containing carbocyclic and heterocyclic five-membered rings

The preparation of a [2.2]indenoparacyclophane-based diene is described. Diels-Alder cycloadditions of this diene with N-methylmaleimide, N-phenylmaleimide and maleic anhydride occurred in good yields and anti-diastereoselectively only under high pressure conditions. Heterohelicenophanes were prepared by dehydrogenation of the Diels-Alder products with 10% Pd/C catalyst. A new helicenophanequinone was obtained by the reaction between the diene and 1,4-naphthoquinone.     

Chiral 1,2-diaminocyclohexane as organocatalyst for enantioselective aldol reaction

A simple and commercially available chiral 1,2-diaminocyclohexane as catalyst, hexanedioic acid as co-catalyst could efficiently catalyze the asymmetric aldol reaction in MeOH-H₂O. Cyclic ketones as aldol substrates gave the anti-β-hydroxyketone products with moderate to good yields, diastereoselectivity and enantioselectivity (up to 78% yield, >20:1 anti/syn, 94% ee). Hydroxyacetone as aldol substrate afforded the syn-α, β-dihydroxyketones as major products in up to 85% yield with good enantioselectivity (up to >20:1 syn/anti, 93% ee).     

Utilization of heterobimetallic complexes as Lewis acids

The lithium-containing multifunctional heterobimetallic catalysts such as LaLi₃tris((R)-6,6′-dibromobinaphthoxide) showed moderate Lewis acidity in non-polar solvents. Asymmetric Diels–Alder reactions proceeded efficiently in the presence of the heterobimetallic catalyst. This is the first application of heterobimetallic asymmetric catalysts solely as Lewis acids.     

Titanium superoxide: a heterogeneous catalyst for anti-Markovnikov aminobromination of olefins

A new facile procedure for the aminobromination of olefins in high yields has been described using p-toluene sulfonamide (p-TsNH₂) and N-bromosuccinimide (NBS) as nitrogen and bromine sources, respectively, and titanium superoxide as a truly heterogeneous catalyst. The formation of anti-Markovnikov product exclusively in all the cases studied possibly proceeding through a free radical reaction pathway is remarkable.     

Aluminum enolates via retroaldol reaction: catalytic tandem aldol-transfer—Tischtschenko reaction of aldehydes with aldol adducts of ketones to ketones

Bidentate aluminum chelates derived from biphenol, binaphthol and catechol were found to be efficient catalysts for aldol-transfer reactions of ketone to ketone aldol adducts with aliphatic or aromatic aldehydes giving rise to the formation of aldol adducts of ketones to the aldehydes. In the presence of an excess of an aliphatic aldehyde, a catalytic tandem aldol-transfer—Tischtschenko reaction is observed. The tandem reaction produces monoesters of 1,3-diols with high anti selectivity and with modest to good chemical yield. 1,2-Unsaturated aldehydes are less reactive in the aldol-transfer reaction and require 2-4 times higher load of the catalyst to be used than aliphatic and aromatic aldehydes. Poor diastereoselectivity was observed in the formation of α-substituted aldols and 2-substituted monoesters of anti-1,3-diols indicating that the aldol-transfer reaction is not diastereoselective with the catalysts studied. The utility of the highly 1,3-anti selective formation of diolmonoesters was found to be limited by acyl migration.     

Construction of anti-1,2-diols bearing chiral tertiary alcohol moiety using free hydroxyacetone as aldol donor by imidazole-based prolineamide catalyst

A direct aldol reaction of free hydroxyacetone and activated ketone has been firstly achieved with a newly developed imidazole-based prolineamide catalyst. The adducts anti-1,2-diols bearing a chiral tertiary alcohol moiety were obtained in high yield (up to 92%) and stereoselectivity (up to 15:1 dr and 90% ee).     

Cs2.5H0.5PW12O40 catalyzed diastereoselective synthesis of β-amino ketones via three component Mannich-type reaction in water

A series of insoluble salts of Keggin heteropoly compounds were prepared and used as catalysts for the Mannich-type reaction of benzaldehyde, aniline, and cyclohexanone in water. Among them, Cs_2.5H_0.5PW₁₂O₄₀ showed excellent catalytic activity. Effects of surfactant, catalyst loading and temperature were studied to introduce the best reaction condition. The optimized reaction conditions were extended to Mannich reaction of various aldehydes, ketones, and amines in water. This rapid procedure afforded structurally divers β-amino ketones with major anti diastereoselectivity. Additionally, four new compounds were reported. The catalyst was recovered and reused for subsequent runs.     

Superior substrate control on diastereoselection in boric Lewis acid-promoted aldol reactions. Asymmetric synthesis of a 3,4-syn homologous series of ethyl 3,5-dihydroxy-2,4-dimethyl-5-phenylpentanoates

The BF₃·OEt₂-promoted aldol reaction of chiral syn- and anti-α-methyl-β-siloxy aldehydes with a silyl ketene acetal resulted in essentially complete syn Felkin selection. Even in the asymmetric aldol reaction using chiral oxazaborolidinones, the substrate control with respect to diastereoselection was found to overcome the promoter (catalyst) control which would normally occur depending on the stereocenter of the chiral boranes.     

A convenient approach to δ-amino-β-ketoesters by vinylogous Mannich reaction of masked acetoacetates

SiCl₄ is an efficient and selective catalyst for the vinylogous Mannich reaction of linear and cyclic synthetic equivalents of acetoacetate dianion, leading to δ-amino-β-ketoesters in moderate to high yields and complete γ-selectivity; anti-diastereoselectivity was observed by using a γ-methyl-substituted cyclic silyloxydiene.     

Ether-directed diastereoselectivity in catalysed Overman rearrangement: comparative studies of metal catalysts

Ether-directed diastereoselectivity in Overman rearrangement of δ-methoxy and δ-TBDMSO substituted allylic trichloroacetimidates has been explored using PtCl₂, PtCl₄, AuCl and AuCl₃ catalysts in comparison with commonly used Pd(II) catalysts. For both substrates the use of PtCl₂ catalyst gave notably improved anti/syn-ratio of 1,2-aminoalcohol derivatives (anti/syn=11:1 for δ-methoxy; 6:1 for δ-TBDMSO) compared to all metal catalysts known to promote Overman rearrangement. Formation of 2-trichloromethyloxazoline was observed as a dominant side reaction in the metal catalysed rearrangement of δ-methoxy substituted allylic trichloroacetimidates considerably reducing the yield of the desired product. This side reaction was suppressed when δ-TBDMS-ether was used as a directing group.     

Highly enantioselective organocatalysis of the Michael addition of benzyloxyacetaldehyde to nitroolefins

A novel category of di(N,N-dimethylbenzylamine)prolinol silyl ether catalyst, which when used in conjunction with an acidic co-catalyst, generates an ammonium salt supported organocatalyst. This catalytic system is shown to be very effective for the Michael reaction of benzyloxyacetaldehyde and various nitroolefins in isopropanol. Excellent enantioselectivities (up to 99%) and diastereoselectivities (syn/anti of 75:25) and short reaction times were obtained. The presence of the bulky OTMS group combined with the presence of two large N,N-dimethylbenzyl ammonium ion groups accounts for the effectiveness of this catalytic system.     

Water enables diastereodivergency in bispidine-based chiral amine-catalyzed asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones

Tuning diastereoselectivity is a great challenge in asymmetric catalysis for the inherent stereochemical bias of the substrates. Here, we report a diastereodivergent asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones catalyzed by a bispidine-based chiral amine catalyst, in which additional water switches the diastereoselectivity efficiently. Both chiral anti- and syn-benzosultams with potential anti-HIV-1 activity are obtained in excellent yields and good to excellent ee values. Control experiments and density functional theory (DFT) calculations were applied to study the diastereodivergent mechanism, which reveal that the diastereodivergent catalysis should be state-determined, and the water reverses the energies of states to realize the diastereodivergency. The findings are quite new and might inspire more diastereodivergent asymmetric synthesis.

A diastereodivergent asymmetric Mannich reaction of cyclic N-sulfonyl ketimines with ketones is realized by employing bispidine-based chiral amine as catalyst and additional water switching the diastereoselectivity.     

Stereoselective anti-SN2′ Mitsunobu reaction of α-hydroxy-α-alkenylsilanes

A novel silyl group-directed anti-S_N2′ reaction of allylic alcohols under Mitsunobu reaction conditions is described. The Mitsunobu reaction of α-hydroxy-α-alkenylsilanes with a TBS or TIPS group gave the anti-S_N2′ product, in which regio- and stereochemical outcomes of the reaction depended on the steric bulkiness of the silyl group.     

Pyrrolidine-linker-camphor assembly: bifunctional organocatalysts for efficient Michael addition of cyclohexanone to nitroolefins under neat conditions

A simple and convenient strategy was developed to synthesize a new class of pyrrolidinyl-camphor based bifunctional organocatalysts possessing varying functional linkers. Catalytic screening of these camphor-pyrrolidine linked derivatives for asymmetric Michael reaction of cyclohexanone with β-nitrostyrene was carried out. Various aryl- and heteroaryl-nitroolefins, ketones as well as aldehydes gave the corresponding Michael adducts in high chemical yields (up to 95%) and exceptionally high diastereo-(syn/anti up to 99:1) and enantioselectivity (up to 95%) using catalyst 6 under solvent-free conditions.     

Preparation and application of 2-(arylmethoxy)isopinocampheols for the asymmetric aldol reaction of 3,3,3-trifluoropropionates

The preparation of 2-(arylmethoxy)isopinocampheols from pinanediol via the DIABL-H reduction of the corresponding aryl aldehyde acetals has been described. A systematic examination of the asymmetric aldol reaction of 3,3,3-trifluoropropionates led to the double diastereoselective aldol reaction of 2-(arylmethoxy)isopinocampheyl 3,3,3-trifluoropropionates providing anti-α-trifluoromethyl-β-hydroxy esters in 63-85% yields, ?99% anti-selectivity and 80-96% de for the anti-isomer.     

Chiral 1,1′-binaphthylazepine derived amino alcohol catalyzed asymmetric Henry reaction

The catalytic asymmetric Henry reaction of nitromethane to various aldehydes has been developed using a chiral binaphthylazepine derived amino alcohol and Cu(OAc)₂·H₂O as the catalyst. High yields and good enantioselectivities (up to 97% ee) were obtained for both aromatic and aliphatic aldehydes. Moreover, this catalytic system also works well for the diastereoselective Henry reaction to afford the corresponding adducts in up to 95:5 syn/anti selectivity and 95% enantioselectivity.