Direct Catalytic Enantioselective α-Aminomethylation of Ketones

Metal- and Photocatalyst-Free, Visible-Light-Initiated C3 α-Aminomethylation of Quinoxalin-2(1H)-ones via Electron Donor-Acceptor Complexes

We report a metal- and photocatalyst-free C3 α-aminomethylation of quinoxalin-2(1H)-ones with N-alkyl-N-methylanilines. The reaction proceeds through the formation of a photoactivated electron donor-acceptor complex between quinoxalin-2(1H)-ones and N-alkyl-N-methylanilines. The present method provides a mild and environmentally friendly protocol that exhibits good atom economy and excellent functional group tolerance to obtain a library of biologically significant C3 α-aminomethylated quinoxalin-2(1H)-ones in good yields.

Direct Catalytic Enantioselective α-Aminomethylation of Aldehydes

The direct catalytic asymmetric alpha-aminomethylation of aldehydes is presented. The chiral amine and amino acid catalyzed reactions between unmodified aldehydes and a formaldehyde-derived imine precursor were fast and proceeded with high chemo- and enantioselectivities. The corresponding dibenzyl-protected gamma-amino alcohols were isolated in high yields with up to 98% ee after in situ reduction. The reaction is a novel entry to valuable beta(2)-amino acid derivatives.

Asymmetric catalysis in aqueous media

Lewis acid catalysis has attracted much attention in organic synthesis because of unique reactivity and selectivity attained under mild conditions. Although various kinds of Lewis acids have been developed and applied in industry, these Lewis acids must be generally used under strictly anhydrous conditions. The presence of even a small amount of water handles the reactions owing to preferential reactions of the Lewis acids with water rather than the substrates. In contrast, rare earth and other metal complexes have been found to be water-compatible. Several catalytic asymmetric reactions in aqueous media, including hydroxymethylation of silicon enolates with an aqueous solution of formaldehyde in the presence of Sc(OTf)3-chiral bipyridine ligand or Bi(OTf)3-chiral bipyridine ligand, Sc- or Bi-catalyzed asymmetric meso-epoxide ring-opening reactions with amines, and asymmetric Mannich-type reactions of silicon enolates with N-acylhydrazones in the presence of a chiral Zn catalyst have been developed. Water plays key roles in these asymmetric reactions.

Asymmetric multicomponent reactions (AMCRs): the new frontier

Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.

5-(Pyrrolidin-2-yl)-1H-tetrazole and 5-[(Pyrrolidin-2-yl)methyl]-1H-tetrazole: Proline Surrogates with Increased Potential in Asymmetric Catalysis

Catalytic enantioselective methodology has dramatically been enriched by the re-discovery of the simple amino acid proline as a chiral catalyst in the year 2000. Although no catalyst offers such a simple and broad access to quite complex reaction products, as does proline, its synthetic potential is not unrestricted, what is especially connected to its poor solubility in organic media. Exchange of the carboxylic moiety by a tetrazole unit leads to proline surrogates, that by far outperform proline with respect to yield, enantioselectivity, reaction time, substrate and solvent scope, catalyst loading, and stoichiometry of the compounds used in excess. These factors are discussed and critically compared with selected representative proline-catalyzed reactions.

Catalytic Asymmetric Mannich-Type Reactions Activated by ZnF2 Chiral Diamine in Aqueous Media

Catalytic asymmetric Mannich-type reactions of an alpha-hydrazono ester with silicon enolates in aqueous media have been developed by using ZnF2 and chiral diamines as catalysts. In these reactions, both Zn2+ and a fluoride anion were necessary to achieve high yields and enantioselectivities, suggesting a double activation mechanism, in which Zn2+ activates the alpha-hydrazono ester and the fluoride anion simultaneously activates the silicon enolate. When chiral diamine ligands bearing methoxy-substituted aromatic rings were employed, the reactions in aqueous THF were markedly accelerated. Furthermore, the use of these diamines facilitated the asymmetric Mannich-type reactions in water without any organic cosolvents. It is noteworthy that either syn or anti adducts were stereospecifically obtained from (E)- or (Z)-silicon enolates, respectively. Interestingly, these reactions proceeded smoothly only in the presence of water. On the basis of several experimental results, it can be concluded that the reaction mechanism is likely to be a fluoride-catalyzed one, in which the ZnF2 chiral diamine complex is regenerated from the Me3SiF formed during the reaction.

Acyclic Chiral Amines and Amino Acids as Inexpensive and Readily Tunable Catalysts for the Direct Asymmetric Three-Component Mannich Reaction

The direct three-component asymmetric Mannich reaction catalyzed by acyclic chiral amines or amino acids is presented. Simple acyclic chiral amines and amino acids--such as alanine-tetrazole (9), alanine, valine, and serine-catalyzed the three-component asymmetric Mannich reactions between unmodified ketones, p-anisidine, and aldehydes with high chemo- and stereoselectivity, furnishing the corresponding Mannich bases with up to >99 % ee. This study demonstrates that the whole range of amino acids in nature, as well as nonproteogenic amino acid derivatives, can be considered in the design and tuning of novel, inexpensive organocatalysts for the direct asymmetric Mannich reaction.

Multiple Component Approaches to C-Glycosyl β-Amino Acids by Complementary One-Pot Mannich-Type and Reformatsky-Type Reactions

The development of new methods for the preparation of C-glycosyl beta-amino acid libraries with chemical and stereochemical diversity levels was investigated and the results are described herein. Two complementary one-pot three-component Mannich-type and Reformatsky-type synthetic strategies have been developed for the construction of chiral 3-amino propanoate fragments (eventually bis-substituted at C-2) directly linked to the anomeric carbon of pyranose and furanose residues. Both methods involved as the initial step the coupling of a sugar aldehyde to p-methoxybenzylamine but differed in the nucleophile (a d(2) synthon equivalent) which was successively added: a ketene silyl acetal (Mannich route) or a bromozinc enolate (Reformatsky route). Individual C-glycosyl beta-amino esters were isolated as single 3R diastereoisomers in fair to excellent yield (60-90%) and their structure assigned by NMR spectroscopy (Riguera protocol) supported by X-ray crystallography. A tentative explanation of the observed stereochemical outcome based on transition-state models is provided. A preliminary study on the synthesis of alpha,alpha-difluoro C-glycosyl beta-amino acids via a more traditional Reformatsky route is also reported.