Conformational Transmission of Chirality: The Origin of 1,4-Asymmetric Induction in Michael Reactions of Chiral Imines

Straightforward access to densely substituted chiral succinimides through enantioselective organocatalyzed Michael addition of α-alkyl-cyclic ketones to maleimides

A simple, cheap and unchanged organocatalytic system provides efficient access to densely substituted chiral succinimides bearing QCC–TCC vicinal stereocenters from unreactive α-alkyl cyclic ketones and maleimides.

Arynes and Cyclic Alkynes as Synthetic Building Blocks for Stereodefined Quaternary Centers

We report a facile method to synthesize stereodefined quaternary centers from reactions of arynes and related strained intermediates using β-ketoester-derived substrates. The conversion of β-ketoesters to chiral enamines is followed by reaction with in situ generated strained arynes or cyclic alkynes. Hydrolytic workup provides the arylated or alkenylated products in enantiomeric excesses as high as 96%. We also describe the one-pot conversion of a β-ketoester substrate to the corresponding enantioenriched α-arylated product. Computations show how chirality is transferred from the N-bound chiral auxiliary to the final products. These are the first theoretical studies of aryne trapping by chiral nucleophiles to set new stereocenters. Our approach provides a solution to the challenging problem of stereoselective β-ketoester arylation/alkenylation, with formation of a quaternary center.

Enantioselective Access to Robinson Annulation Products and Michael Adducts as Precursors

The Robinson annulation is a reaction that has been useful for numerous syntheses since its discovery in 1935, especially in the field of steroid synthesis. The products are usually obtained after three consecutive steps: the formation of an enolate (or derivative), a conjugate addition, and an aldol reaction. Over the years, several methodological improvements have been made for each individual step or alternative routes have been devised to access the Robinson annulation products. The first part of this Review outlines the most relevant developments towards the formation of monocarbonyl-derived Robinson annulation products (MRA products, MRAPs) and activated monocarbonyl-derived Robinson annulation products (AMRA products, AMRAPs). The following sections are then devoted to the diastereoselective and enantioselective synthesis of these products, while the last section describes the enantiomeric resolution of racemic mixtures.

First enantioselective total synthesis and configurational assignments of suberosenone and suberosanone as potential antitumor agents

The first enantioselective syntheses of (1R)-suberosenone and (1R)-suberosanone are achieved, leading to revision of the AC of a natural (1S)-suberosanone.

Liposome/DNA systems: correlation between hydrophobicity and DNA conformational changes

In a previous work, we found that liposome hydrophobicity could affect deoxyribonucleic acid (DNA) association efficiency. Now, we have focused on the possible correlation between liposome hydrophobicity and DNA conformation. DNA lyophilized with cationic vesicles with high hydrophobicity changes its conformation into a more condensed form, probably the C form. With noncharged vesicles, it changes its conformation from B to a partial A form. These results contribute to a better understanding of the interaction between DNA and lipids, suggesting there is direct relationship between hydrophobicity and DNA conformation changes: The higher the hydrophobicity factor, the more pronounced the changes in DNA form, to a more condensed form.

Enamides and enecarbamates as nucleophiles in stereoselective C-C and C-N bond-forming reactions

Because the backbone of most of organic compounds is a carbon chain, carbon-carbon bond-forming reactions are among the most important reactions in organic synthesis. Many of the carbon-carbon bond-forming reactions so far reported rely on nucleophilic attack of enolates or their derivatives, because those nucleophiles can be, in general, readily prepared from the corresponding carbonyl compounds. In this Account, we summarize the recent development of reactions using enamide and enecarbamate as a novel type of nucleophile. Despite their ready availability and their intrinsic attraction as a synthetic tool that enables us to introduce a protected nitrogen functional group, enamide and enecarbamate have rarely been used as a nucleophile, since their nucleophilicity is low compared with the corresponding metal enolates and enamines. A characteristic of enamides and enecarbamates is that those bearing a hydrogen atom on nitrogen are relatively stable at room temperature, while enamines bearing a hydrogen atom on nitrogen are likely to tautomerize into the corresponding imine form. Enamides and enecarbamates can be purified by silica gel chromatography and kept for a long time without decomposition. During the investigation of nucleophilic addition reactions using enamides and enecarbamates, it has been revealed that enamides and enecarbamates bearing a hydrogen atom on nitrogen react actually as a nucleophile with relatively reactive electrophiles, such as glyoxylate, N-acylimino ester, N-acylimino phosphonate, and azodicarboxylate, in the presence of an appropriate Lewis acid catalyst. Those bearing no hydrogen atom on nitrogen did not react at all. The products initially obtained from the nucleophilic addition of enamides and enecarbamates are the corresponding N-protected imines, which can be readily transformed to important functional groups, such as ketones by hydrolysis and N-protected amines by reduction or nucleophilic alkylation. In the nucleophilic addition reactions of enamides and enecarbamates to aldehydes, it was unveiled that the reaction proceeds stereospecifically, that is, (E)-enecarbamate gave anti product and (Z)-enecarbamate afforded syn product with high diastereoselectivity (>97/3). This fact can be rationalized by consideration of a concerted reaction pathway via a hydrogen-involved cyclic six-membered ring transition state. In the addition reactions to N-acylimino phosphonates, much higher turnover frequency was observed when enamides and enecarbamates were used as a nucleophile than was observed when silicon enolates were used. When silicon enolates were used, the intermediates bearing a strong affinity for the catalyst inhibited catalyst turnover, resulting in low enantioslectivity because of the dominance of the uncatalyzed racemic pathway. In the case of nucleophilic addition of enamides and enecarbamate, however, a fast intramolecular hydrogen transfer from the enecarbamate nitrogen may prevent the intermediate from trapping the catalyst for a long time, to afford the product with a high enantioselectivity. In conclusion, enamides and enecarbamates, although originally employed as just N-analogues to silicon enolates, have emerged as remarkably useful nucleophiles in a variety of Lewis acid-catalyzed reactions.

Theoretical Study of the Stereoselective Additions of Chiral Alcohols to Ketenes

In 1989, Larsen et al. at Merck discovered that the addition of chiral alcohols to ketenes provided enantiomerically enriched aryl propionic acids, compounds valued for their therapeutic antiinflammatory properties. The high 1,4-asymmetric induction observed (>99:1 dr in the addition, and up to 99% ee after hydrolysis to the acid) is rare. A quantitative model based on B3LYP density functional theory calculations accounts for the stereoselectivity in the addition of (S)-methyl lactate, (S)-3-methyl-2-butanol, and (S)-pantolactone to methylphenylketene. The conformational processes of the intermediates can impact the stereoselectivity of the process, and either the addition step, or the protonation of the enolate intermediate, may be stereoselectivity determining.

Transition states of amine-catalyzed aldol reactions involving enamine intermediates: theoretical studies of mechanism, reactivity, and stereoselectivity

The mechanisms, transition states, relative rates, and stereochemistries of amine-catalyzed aldol reactions involving enamine intermediates have been explored with density functional theory (B3LYP/6-31G*) and CPCM solvation models. Primary enamine-mediated aldol reactions involve half-chair transition states with hydrogen bonding leading to proton transfer. This leads to charge stabilization and low activation energies as compared to secondary enamine-mediated aldol reactions. Oxetane intermediates can be formed when C-C bond formation occurs without H-transfer in the transition state. The stereoselectivities of reactions of ketone enamines with aldehydes, including the facial stereoselectivity involving chiral aldehydes, were modeled and compared with experimental results. Transition states for the intramolecular aldol reactions leading to the formation of hydrindanone-beta-ketol and decalone-beta-ketol aldol products showed a preference for the formation of the cis-fused rings, in agreement with experimental results.

Preparation of 4'-substituted thymidines by substitution of the thymidine 5'-esters

tert-Butyl thymidylate 3 was prepared from thymidine 1 in six steps and 67% overall yield. When the lithium trianion of 3 (prepared by treatment of 3 with excess LDA and then excess tert-butyllithum) is reacted with electrophiles, trapping occurs stereoselectively from either the alpha- or beta-face depending on the electrophile (Scheme 1). Deuterioacetic acid in deuteriomethanol affords mainly the alpha-deuterated product (4a/4b = 2.4:1) while all other electrophiles, e.g., phenylselenenyl chloride, allyl bromide, and N-fluorobenzenesulfonimide (NFSI), give predominately (or completely) the products of attack from the beta-face (5bcd/4bcd = 3.7:1 to 100:0). The structures of the products were determined by coupling constant analysis of both the initial compounds and the diols 6bcd prepared by ester reduction and by formation of the acetonides 7bc. The methyl ester of the 3'-epimer of thymidylic acid 9 was also prepared from thymidine 1 in nine steps and 74% overall yield. When the lithium trianion of 9 (prepared by treatment of 9 with excess LDA and then excess tert-butyllithum) is reacted with electrophiles, trapping also occurs stereoselectively with attack on either the alpha- or beta-face depending on the electrophile (Scheme 2). Again, deuterioacetic acid in deuteriomethanol affords mainly the beta-deuterated product (11a/10a = 1.6:1) while all other electrophiles, e.g., phenylselenenyl chloride, methyl iodide, allyl bromide, and NFSI, gave predominately (or completely) the product of attack from the alpha-face (8.7:1 to 100: 0). Again, the structures of the products were determined by coupling constant analysis of both the initial compounds, and the diols 12b-e were prepared by reduction of the ester and by formation of the acetonides 13bcd. A rationale has been developed using molecular mechanics calculations to explain the diastereoselectivity that involves staggered axial attack on the sp(2) carbon opposite to the pseudoaxial alkoxy group in the most stable half-chair conformation of the enolates, as shown in Schemes 3-5.

Stereoelectronic, torsional, and steric effects on rates of enolization of ketones

The stereoselectivities of base-catalyzed enolizations of ketones have been studied by quantum mechanical methods. Transition structures of exo and endo deprotonation of camphor, norcamphor, and dehydronorcamphor have been located with two model bases. Stereoelectronic, torsional, and steric effects on activation energies were assessed. These calculations demonstrate the importance of torsional strain between partial bonds and vicinal bonds on the rates of deprotonation and on related reactions involving the formation of enolates or reactions of enols and enolates.