Rafael Pedrosa

Novel Bifunctional Chiral Urea and Thiourea Derivatives as Organocatalysts: Enantioselective Nitro-Michael Reaction of Malonates and Diketones

The ability of chiral hydrogen-bond donors to catalyze useful enantioselective transformations constitutes an increasing area of interest, and diverse types of structures have been described to this end. An important class of species are urea and thiourea compounds which have frequently been used in several transformations, such as Henry or aza-Henry, Mannich, Strecker, and Friedel–Crafts reactions or Michael and nitro-Michael additions. The modular design of these types of catalysts requires the possibility to introduce different urea and thiourea moieties and the modification of the structure with the chiral information. Most of the described catalysts until now are aryl or diaryl ureas or thioureas with electron withdrawing groups, although some relatively electron rich derivatives have proved to catalyze a variety of enantioselective transformations. The chiral information in the catalyst has been placed at both the nitrogen terminus in the urea or thiourea or in the central chiral core. In this respect, a few structures are used in the preparation of the catalysts, namely chiral diamines, both enantiomers of trans cyclohexane-1,2-diamine, binaphthylamines, diamines derived from cinchona alkaloids, amino alcohols, and very recently, sugars. In our opinion, the development of novel chiral scaffolds to be incorporated into urea and thiourea derivatives, capable to act as bifunctional organocatalyts, is necessary. Because, in general, 1,2-diamines are the structures that lead to the best results, we planned to prepare these compounds taking into account three facts: i) The starting material would have be commercially available and cheap; ii) both enantiomers of the amine must be accessible; and iii) the synthesis should be able to provide the ability to fine-tune the substituents at the nitrogen atoms. We envisioned that natural a-amino acids would be the starting material of choice and we describe here the synthesis of chiral diamines derived from them and their use as organocatalysts in nitro-Michael additions of stabilized carbanions. The generality of the synthesis was demonstrated in the preparation of catalysts 4a–i in three steps from commercially available N-Boc protected a-amino acids. The nature of the substituent at the stereocenter is dictated by the election of the starting amino acid, the substituents at the nitrogen atom can be varied in the formation of the aamino amide, and the structure of the urea–thiourea component is selected depending on the isocyanate–isothiocyanate used in the last step. In this way, N-Boc protected l-valine, l-isoleucine, l-phenylalanine, and l-tert-leucine were converted into 1a–h by reaction with the corresponding amines, which were transformed into 2a–h by deprotection with TFA in methylene chloride. Lithium aluminum hydride reduction to 3a–g and condensation with the corresponding isocyanate or isothiocyanate yielded the final urea or thiourea derivatives 4a–h in good yields. The same protocol allowed the preparation of ent-4a starting from d-valine, and 4 i, regioisomer of 4a, was obtained from l-valinamide hydrochloride by dimethylation, lithium aluminum hydride reduction and reaction with 3,5-bis(trifluoromethyl)phenyl isothiocyanate (Scheme 1). The catalytic activity of 4a–i was first evaluated in the reaction of trans b-nitrostyrene 5a with diethyl malonate 6a in the presence of 10 mol% of catalyst at room temperature, and the results are collected in Table 1. A set of five different solvents was tested as reaction media (entries 1–5 in Table 1) showing that the reactions occurred in good yields and moderate to good ee. Only in the case of methanol (entry 1) the enantioselectivity decreased to 40% ee, probably because the competitive establishment of hydrogen activation of the solvent with the catalyst, and the enantioselectivity increased when less polar solvents were used (compare entries 1–5), or if the reaction was carried out without [a] Dr. J. M. Andr>s, R. Manzano, Prof. Dr. R. Pedrosa Centro de InnovaciAn en QuCmica y Materiales Avanzados ACHTUNGTRENNUNG(CINQUIMA) and Departamento de QuCmica OrgEnica Facultad de Ciencias, Universidad de Valladolid Dr. Mergelina s/n, 47011 Valladolid (Spain) Fax: (+34)983423211 pedrosa@qo.uva.es Supporting information for this article is available on the WWW under http://www.chemistry.org or from the author.

Enantioselective Conjugate Addition of Nitro Compounds to α,β‐Unsaturated Ketones: An Experimental and Computational Study

A series of chiral thioureas derived from easily available diamines, prepared from α-amino acids, have been tested as catalysts in the enantioselective Michael additions of nitroalkanes to α,β-unsaturated ketones. The best results are obtained with the bifunctional catalyst prepared from L-valine. This thiourea promotes the reaction with high enantioselectivities and chemical yields for aryl/vinyl ketones, but the enantiomeric ratio for alkyl/vinyl derivatives is very modest. The addition of substituted nitromethanes led to the corresponding adducts with excellent enantioselectivity but very poor diastereoselectivity. Evidence for the isomerization of the addition products has been obtained from the reaction of chalcone with [D(3)]nitromethane, which shows that the final addition products epimerize under the reaction conditions. The epimerization explains the low diastereoselectivity observed in the formation of adducts with two adjacent tertiary stereocenters. Density functional studies of the transition structures corresponding to two alternative activation modes of the nitroalkanes and α,β-unsaturated ketones by the bifunctional organocatalyst have been carried out at the B3LYP/3-21G* level. The computations are consistent with a reaction model involving the Michael addition of the thiourea-activated nitronate to the ketone activated by the protonated amine of the organocatalyst. The enantioselectivities predicted by the computations are consistent with the experimental values obtained for aryl- and alkyl-substituted α,β-unsaturated ketones.

Enantioselective reformatsky reaction induced by chiral β-amino alcohols

Abstract Reformatsky reagent derived from tert-butyl α-bromoacetate adds to carbonyl compounds in the presence of chiral amino alcohols leading to β-hydroxy tert-butyl esters with good e.e. The enantioface differentiation depends on the reaction conditions and on the structure of the chiral auxiliary. The best chemical yields and e.e. are obtained for aromatic aldehydes by using the C-2 symmetrical chiral bis-amino alcohol (5) derived from m-xylylene diamine.

Synthesis of the novel chiral 1,3-amino alcohol 8-N,N-bis(ferrocenylmethyl)amino-menthol and its use as catalyst in the enantioselective addition of diethylzinc to aldehydes

Abstract Optically active 8-N,N-bis(ferrocenylmethyl)aminomenthol 5, obtained by condensation of (−)-aminomenthol with ferrocenecarboxaldehyde followed by N-alkylation with (ferrocenylmethyl)trimethylammonium iodide and further reduction, was found to catalyze (5 mol%) the enantioselective ethylation of aromatic, aliphatic and organometallic aldehydes to secondary alcohols with high enantioselectivities (up to 95%) at room temperature.

Stereoselective cyanation of chiral α-amino aldehydes by reaction with Nagata's reagent: a route to enantiopure β-amino-α-hydroxy acids

Abstract Chiral α-dibenzylamino aldehydes react with diethylaluminum cyanide leading to anti -β-dibenzylamino-α-hydroxycyanides as the major diastereoisomers in good yields and diastereomeric excesses. Hydrolysis of the nitrile derivatives allows the synthesis of enantiopure β-amino-α-hydroxy acids.

A Simple Stereoselective Synthesis of Enantiopure 2-Substituted Pyrrolidines and Piperidines from Chiral (R)-Phenylglycinol-Derived Bicyclic 1,3-Oxazolidines

Chiral, nonracemic 2-substituted pyrrolidines and piperidines were prepared in high ee and moderate to good chemical yields in three steps from (R)-phenylglycinol and γ- or δ-chloroketones. The key step of the synthesis was the stereoselective reductive ring-opening of chiral bicyclic 1,3-oxazolidines prepared by condensation of (R)-phenylglycinol and the corresponding ketones.

Synthesis of Chiral, Non-racemic Aldols from Chiral β-Hydroxy-Weinreb Amides Prepared by Enantioselective Reformatsky-like Reaction Induced by Chiral β-Aminoalcohols

Abstract Chiral 1,2-aminoalcohols catalyze enantioselective Reformatsky addition of zinc derivatives of α-bromo Weinreb amides. Reaction of Grignard reagents with the resulting β-hydroxy N -methoxy- N -methyl amides allowed the preparation of β-hydroxy ketones regio- and enantioselectively.

Synthesis of both Enantiomers of Hemiesters by Enantioselective Methanolysis of Meso Cyclic Anhydrides Catalyzed by α-Amino Acid-Derived Chiral Thioureas†

Both ureas and thioureas derived from L- or D-valine act as bifunctional organocatalysts able to induce the enantioselective alcoholysis of mono-, bi-, and tricyclic meso anhydrides. The desymmetrization occurs in near quantitative yields and excellent enantiomeric ratios (up to >99:<1) under low catalyst loading. Both enantiomers of the hemiesters can be directly obtained by changing the configuration of the catalyst.

Stereoselective ring opening of chiral oxazolidines by reformatsky reagents: an enantioselective entry to β-amino esters.

Abstract Chiral oxazolidines obtained by condensation of aldehydes with (-).(R)- or (+).(S)-N-benzylphenylglycinol react with the Reformatsky reagent derived from ethyl bromoacetate, in mild reaction conditions (Et2O or CH2Cl2, 0°C, 15-60 min), leading to ethyl β-amino carboxylates in moderate to good diastereomeric excess (60-92%). These ring opening products are transformed into primary β-aminoesters, in one step, by debenzylation with H2/Pd on carbon without loss of their stereochemical integrity. In this way, ethyl β-amino carboxylates can be obtained in both enantiomeric forms, with chemical yields ranging 55-76% and moderate to good e.e. (60-92%).

Diastereoselective synthesis of enantiopure γ-amino-β-hydroxy acids by Reformatsky reaction of chiral α-dibenzylamino aldehydes

Abstract N,N-Dibenzylamino aldehydes 1 react with Reformatskys reagent leading to anti-γ-dibenzylamino-β-hydroxy esters 2 as the major stereoisomers. Treatment of 2 with TFA followed by hydrogenolysis on Pearlmans catalyst yields the corresponding γ-amino-β-hydroxy acids 10. Contrarily, some N-butoxycarbonyl (Boc) amino aldehydes lead to syn-γ-tert-butoxycarbonylamino-β-hydroxy esters as the major product.