Catalytic conjugate additions of carbonyl anions under neutral aqueous conditions

The conjugate addition of carbonyl anions catalyzed by thiazolium salts that is fully operative under neutral aqueous conditions has been accomplished. The combination of alpha-keto carboxylates and thiazolium-derived zwitterions produces reactive carbonyl anions in a buffered protic environment that readily undergo conjugate additions to substituted alpha,beta-unsaturated 2-acyl imidazoles. The scope of the reaction has been examined and found to accommodate various alpha-keto carboxylates and beta-aryl substituted unsaturated 2-acyl imidazoles. The optimal precatalyst for this process is the commercially available thiazolium salt 5, a simple analogue of thiamin diphosphate. In this process, no benzoin products from carbonyl anion dimerization are observed. The corresponding 1,4-dicarbonyl compounds can be efficiently converted into esters and amides by way of activation of the N-methylimidazole ring via alkylation.     

Carbamate‐Catalyzed Enantioselective Bromolactamization

A highly facile, efficient, and enantioselective bromolactamization of olefinic amides was effected by a carbamate catalyst and ethanol additive. The amide substrates underwent N‐cyclization predominantly to give a diverse range of enantioenriched bromolactam products containing up to two stereogenic centers.     

13C assignments of the carbon atoms in the aromatic rings of lignin model compounds of the arylglycerol beta-aryl ether type

The 13C NMR signals from the aromatic ring carbons in a series of lignin model compounds of the arylglycerol beta-aryl ether type in DMSO solution have been assigned. The model compounds investigated are representative of the erythro and threo forms of differently substituted arylglycerol beta-aryl ethers.     

Enantioselective synthesis of 2-substituted 2-phenylethylamines by lithiation-substitution sequences: synthetic development and mechanistic pathway

The lithiation and asymmetric substitution of N-(2-phenylethyl)isobutyramide (2) with selected electrophiles, under the influence of (-)-sparteine, provides benzylically substituted products in 58-90% yields with enantiomeric ratios (ers) from 72:28 to 91:9. Syntheses of enantioenriched dihydroisoquinolines (S)-18 and (S)-19 and a tetrahydroisoquinoline (4S)-20 provide examples of synthetic applications. Mechanistic investigations suggest the enantiodetermining step at -78 degrees C is a dynamic thermodynamic resolution.     

Catalytic asymmetric epoxidation of alpha,beta-unsaturated amides: efficient synthesis of beta-aryl alpha-hydroxy amides using a one-pot tandem catalytic asymmetric epoxidation-Pd-catalyzed epoxide opening process

The catalytic asymmetric epoxidation of alpha,beta-unsaturated amides using Sm-BINOL-Ph3As=O complex was succeeded. Using 5-10 mol % of the asymmetric catalyst, a variety of amides were epoxidized efficiently, yielding the corresponding alpha,beta-epoxy amides in up to 99% yield and in more than 99% ee. Moreover, the novel one-pot tandem process, one-pot tandem catalytic asymmetric epoxidation-Pd-catalyzed epoxide opening process, was developed. This method was successfully utilized for the efficient synthesis of beta-aryl alpha-hydroxy amides, including beta-aryllactyl-leucine methyl esters. Interestingly, it was found that beneficial modifications on the Pd catalyst were achieved by the constituents of the first epoxidation, producing a more suitable catalyst for the Pd-catalyzed epoxide opening reaction in terms of chemoselectivity.     

Bifunctional Brønsted Base Catalyzes Direct Asymmetric Aldol Reaction of α‐Keto Amides

The first enantioselective direct cross‐aldol reaction of α‐keto amides with aldehydes, mediated by a bifunctional ureidopeptide‐based Brønsted base catalyst, is described. The appropriate combination of a tertiary amine base and an aminal, and urea hydrogen‐bond donor groups in the catalyst structure promoted the exclusive generation of the α‐keto amide enolate which reacted with either non‐enolizable or enolizable aldehydes to produce highly enantioenriched polyoxygenated aldol adducts without side‐products resulting from dehydration, α‐keto amide self‐condensation, aldehyde enolization, and isotetronic acid formation.     

Enantioselective cyanation/Brook rearrangement/C-acylation reactions of acylsilanes catalyzed by chiral metal alkoxides

New catalytic enantioselective cyanation/1,2-Brook rearrangement/C-acylation reactions of acylsilanes (4) with cyanoformate esters (7) are described. The products of the reaction are fully substituted malonic acid derivatives (8). Catalysts for this transformation were discovered via a directed candidate screen of 96 metal-ligand complexes. Optimization of a (salen)aluminum complex revealed significant remote electronic effects and concentration effects. The scope of the reaction was investigated by using a number of aryl acylsilanes and cyanoformate esters. Chemoselective reduction of the reaction products (8) afforded new enantioenriched alpha-hydroxy-alpha-aryl-beta-amino acid derivatives (32-34) and beta-lactams (35 and 36). This reaction provides a simple method for the construction of new nitrogen-containing enantioenriched chiral building blocks.     

Copper-catalyzed synthesis of enantioenriched tetraarylethanes

Herein we report the asymmetric synthesis of 1,2-dipyridyl-1,2-diarylethanes via an unusual Cu(I)-catalyzed dimerization reaction. Subjection of a variety of enantioenriched substituted 2-pyridyl alcohols to a one-pot protocol generates the desired products in good yields and diastereoselectivities and with ee's up to >99%.     

One-Pot Synthesis of Enantioenriched β-Amino Secondary Amides via an Enantioselective [4+2] Cycloaddition Reaction of Vinyl Azides with N-Acyl Imines Catalyzed by a Chiral Brønsted Acid

A catalytic enantioselective synthesis of β-amino secondary amides was achieved using vinyl azides as the enamine-type nucleophile and chiral N-Tf phosphoramide as the chiral Brønsted acid catalyst through a five-step sequential transformation in one pot. The established sequential transformation involves an enantioselective [4+2] cycloaddition reaction of vinyl azides with N-acyl imines as the key stereo-determining step that is efficiently accelerated by a chiral N-Tf phosphoramide catalyst in a highly enantioselective manner in most cases. Further generation of the iminodiazonium ion intermediate through ring opening of the cycloaddition product and subsequent skeletal rearrangement involving Schmidt-type 1,2-aryl group migration followed by recyclization of the resulting nitrilium ion were also initiated by the same acid catalyst. Final acid hydrolysis of the recyclized products in the same pot gave rise to enantioenriched β-amino amides through C−C bond formation at the α-position of the secondary amides.     

Asymmetric carbon-carbon bond formations in conjugate additions of lithiated N-Boc allylic and benzylic amines to nitroalkenes: enantioselective synthesis of substituted piperidines,pyrrolidines, and pyrimidinones

(-)-Sparteine mediated lithiations of N-Boc-allylic and benzylic amines provide configurationally stable intermediates which on conjugate additions to nitroalkenes provide highly enantioenriched enecarbamate products in good yields, and with high diastereoselectivities. Straightforward transformations of these adducts offer general routes to substituted 3,4-substituted piperidines, 3,4-substituted pyrrolidines, and 4,5-substituted pyrimidinones. Diastereoselective substitutions of intermediate lactams followed by reduction provide 3,4,5-substituted piperidines and 3,4-trisubstituted pyrrolidines. Lithiation adjacent to nitrogen of 3,4-substituted piperidines and pyrrolidines followed by diastereoselective substitution opens a route to 2,4,5- and 2,4,5,6-substituted piperidines as well as 2,3,4- and 2,3,4,5-substituted pyrrolidines. The enantiomers of the enecarbamate and 3,4-substituted piperidine products may be accessed by stannylation/transmetalation sequences as well as by further manipulation of 4-substituted piperidones. The methodology is used to synthesize both enantiomers of an aspartic peptidase inhibitor intermediate, 3-hydroxy-4-phenylpiperidine, as well as the antidepressant (+)-femoxetine.     

Enantioselective Copper(I)‐Catalyzed Alkynylation of Oxocarbenium Ions to Set Diaryl Tetrasubstituted Stereocenters

An enantioselective, copper(I)‐catalyzed addition of terminal alkynes to isochroman ketals to set diaryl, tetrasubstituted stereocenters has been developed. The success of this reaction relies on identification of a Cu/PyBox catalyst capable of distinguishing the faces of the diaryl‐substituted oxocarbenium ion. This challenging transformation enables efficient conversion of readily available, racemic ketals into high‐value enantioenriched isochroman products with fully substituted stereogenic centers. High yields and enantiomeric excesses are observed for various isochroman ketals and an array of alkynes.     

Synthetic applications of lithiated N-Boc allylic amines as asymmetric homoenolate equivalents

Lithiation of N-(Boc)-N-(p-methoxyphenyl) allylic amines in the presence of (-)-sparteine provides asymmetric homoenolate equivalents which react with electrophiles to provide highly enantioenriched enecarbamates. Acidic hydrolysis of the enecarbamates can provide the corresponding beta-substituted aldehydes. A synthetic sequence that involves a stereocontrolled intramolecular nitrone-olefin dipolar cycloaddition has been developed for the preparation of enantioenriched 2-formyl-4-phenyl-1-aminocyclopentanes from one beta-allyl-substituted aldehyde. Further manipulations allow access to an enantioenriched beta-lactam. In another synthetic sequence, transmetalation of the lithiated intermediates and reactions with aldehyde electrophiles can be controlled to afford highly enantioenriched anti homoaldol products. Use of an anti aldehyde homoaldol product as the chiral electrophile in an iterative reaction provides a double homoaldol product containing four stereogenic centers with high diastereoselectivity and enantioselectivity. Reaction pathways are proposed to account for the observed products.     

Asymmetric carbon-carbon bond formations by conjugate additions of lithiated N-boc allylic amines to nitroalkenes: enantioselective synthesis of functionalized cyclopentanoids

[reaction: see text] Allylic organolithiums generated by enantioselective deprotonation of N-Boc-N-(p-methoxyphenyl) allylic amines undergo conjugate additions with nitroalkenes to provide enecarbamates containing two contiguous stereogenic centers in good yields with high diastereoselectivities and enantioselectivities. Further elaboration of these adducts to enantioenriched substituted cyclopentanones and aminocyclopentanes is reported.     

Enantio‐ and Diastereoselective 1,2‐Additions to α‐Ketoesters with Diborylmethane and Substituted 1,1‐Diborylalkanes

The catalytic enantioselective synthesis of boronate‐substituted tertiary alcohols through additions of diborylmethane and substituted 1,1‐diborylalkanes to α‐ketoesters is reported. The reactions are catalyzed by readily available chiral phosphine/copper(I) complexes and produce β‐hydroxyboronates containing up to two contiguous stereogenic centers in up to 99:1 e.r. and greater than 20:1 d.r. The utility of the organoboron products is demonstrated through several chemoselective functionalizations. Evidence indicates the reactions occur via an enantioenriched α‐boryl‐copper‐alkyl intermediate.     

Synthesis and isomerization of N-α-aza-heteroaryl-β-lactams

The [2+2] carbonylative cycloaddition of N-α-aza-heteroaryl substituted imines with allyl bromide led partially to β-lactams, which underwent isomerization to the more stable α,β-unsaturated carbonyl compound. Pyrimidinone derivatives together with doubly unsaturated amides represent the remaining isolated products. The strong electron-withdrawing effect of the two α-aza-heterocycles linked to the nitrogen atom and to the C4 of the 2-azetidinone structure could give a ring expansion, through a 2-azetinone intermediate that affords the pyrimidinone compounds. The substituted amides, instead, should result from a ring-opening reaction of the β-lactam.     

Enantioselective Alkylation of N‐Arylhydrazones Derived from α‐Keto Esters and Isatin Derivatives through Asymmetric Phase‐Transfer Catalysis

The phase‐transfer‐catalyzed asymmetric alkylation reactions of N‐arylhydrazones derived from α‐keto‐esters and isatin derivatives afford enantioenriched azo compounds that bear a tetra‐substituted carbon stereocenter in good yields with high chemo‐ and enantioselectivity. The alkylation products can be readily converted into chiral amino esters, hydrazine derivatives, and aza‐β‐lactams without loss of enantiopurity.     

Highly Regio‐ and Enantioselective Synthesis of N‐Substituted 2‐Pyridones: Iridium‐Catalyzed Intermolecular Asymmetric Allylic Amination

The first iridium‐catalyzed intermolecular asymmetric allylic amination reaction with 2‐hydroxypyridines has been developed, thus providing a highly efficient synthesis of enantioenriched N‐substituted 2‐pyridone derivatives from readily available starting materials. This protocol features a good tolerance of functional groups in both the allylic carbonates and 2‐hydroxypyridines, thereby delivering multifunctionalized heterocyclic products with up to 98 % yield and 99 % ee.     

Organocatalytic Enantioselective Synthesis of Chiral Allenes: Remote Asymmetric 1,8‐Addition of Indole Imine Methides

An organocatalytic enantioconvergent synthesis of chiral tetrasubstituted allenes is disclosed. With suitable chiral phosphoric acid catalysts, a range of racemic indole‐substituted propargylic alcohols reacted with nucleophiles to provide efficient access to a series of enantioenriched allenes with high enantioselectivities. Control experiments suggested a mechanism involving remotely controlled asymmetric 1,8‐addition of the in situ generated indole imine methide via a bifunctional transition state.     

Preparation of 2-arylindole-4-carboxylic amide derivatives

A practical, highly efficient protocol has been developed for the synthesis of functionalized 2-arylindole-4-carboxylic amide derivatives. Commercially available methyl 2-methyl-3-nitrobenzoate gave substituted nitrostyrene benzoic acids by reaction with aromatic aldehydes in the presence of DBU in DMSO. Conversion of these products to the desired amides was followed by Pd-catalyzed reductive cyclization employing carbon monoxide as the terminal reductant to provide the 2-arylindole-4-carboxylic amide derivatives in excellent overall yield for the simple three-step sequence.     

DDQ-mediated direct oxidative coupling of amides with benzylic and allylic sp3 C–H bonds under metal-free conditions

A simple and efficient method for the direct oxidative coupling of amides with benzylic and allylic sp³ C–H bonds using DDQ as an oxidant is described. A range of amides including benzamide, benzyl carbamate, and substituted sulfonamides reacted efficiently with various benzylic and allylic substrates under metal free conditions to afford amidation products in good to excellent yields.