Intramolecular Sulfur Transfer in N-Enoyl Oxazolidine-2-thiones Promoted by Brønsted Acids. Practical Asymmetric Synthesis of β-Mercapto Carboxylic Acids and Mechanistic Insights

The ability of Brønsted acids alone to efficiently promote the sulfur transfer process in N-enoyl oxazolidine-2-thiones to give beta-mercapto carbonyl derivatives is demonstrated. The reactions proceed with essentially perfect diastereocontrol for a range of alkyl-substituted N-enoyl oxazolidine-2-thiones (d.r. regularly above 98:2) and high selectivity for most aryl-substituted counterparts (d.r. typically above 92:8). Importantly, the reaction works remarkably well in beta,beta-disubstituted N-enoyl oxazolidine-2-thiones as well, giving rise to quaternary C-S stereocenters in selectivities usually above 95:5. The relative efficiency of a range of acids (trifluoroacetic, difluoroacetic, acetic, triflic) is assessed showing TFA and TfOH as the most efficient and acetic acid as a totally inefficient reaction promoter. The new procedure complements the Lewis acid promoted reaction previously described by our group in two aspects: First, stereodivergent results are obtained for the Lewis acid or Brønsted acid promoted reactions of beta,beta-disubstituted enoyl compounds. Second, while the Brønsted acid promoted reactions are stereospecific, providing a good correlation between the substrate E/Z configuration and products stereochemistry, the reactions mediated by Lewis acids (BF3/OEt2) provide invariant d.r. values regardless of the E/Z composition of the starting olefin. The synthetic value of the method is illustrated by (a) removal of the oxazolidinone moiety from the rearranged products under reducing conditions (NaBH4, H2O-THF) which yields beta-mercapto alcohols and (b) treatment with Sm(OTf)3 in MeOH which affords the corresponding beta-mercapto carboxylic esters, both categories of compounds being isolated in up to 97% ee. Remarkably, the method constitutes the first general approach to highly enantioenriched building blocks bearing a quaternary C-S stereocenter. On the other hand, spectroscopic and inhibition experiments are carried out that demonstrate the participation of protons also in the Lewis acid promoted reactions. Finally, the computational studies carried out at the B3LYP/6-31G* level give support for an activation of the substrate enoyl by complexation with two molecules of either the Brønsted or Lewis acid and serve to explain the stereochemical outcome of the reactions.

Catalytic enantioselective aza-Henry reaction with broad substrate scope

In situ generated azomethines from readily available precursors react with nitromethane in the presence of 120 mol % of CsOH.H2O and 12 mol % of quinine- and cinchonidine-derived quaternary ammonium chlorides to provide the corresponding aza-Henry adducts in good yields and very high selectivities. It represents the first general enantioselective aza-Henry method for azomethines derived from enolizable aldehydes, giving rise to enantiomeric excesses above 94%. In addition, the reactions with nitroethane afforded high diastereo- and enantioselectivities (syn:anti up to 95:5; up to 98% ee for syn).

Highly Enantioselective Friedel−Crafts Alkylations of Pyrroles and Indoles with α‘-Hydroxy Enones under Cu(II)-Simple Bis(oxazoline) Catalysis

Remarkably high and regular enantioselectivities are obtained in Friedel-Crafts alkylation reactions involving alpha'-hydroxy enone templates and Cu(II)-bis(oxazoline) complexes as catalysts. The simple elaboration of adducts provides a route to enantioenriched aldehydes, carboxylic acids, and ketones containing the pyrrole and indole frameworks.

A practical total synthesis of hapalosin, a 12-membered cyclic depsipeptide with multidrug resistance-reversing activity, by employing improved segment coupling and macrolactonization

A practical total synthesis of hapalosin, a compound with multidrug resistance-reversing activity, has been carried out using an unprecedented macrolactonization strategy. One of the features of the new approach is the straightforward and fully stereocontrolled access to the key gamma-amino beta-hydroxy carboxylic acid subunit via an efficient acetate aldol addition reaction with N-methyl alpha-aminoaldehydes, which relies on a camphor-derived chiral lithium acetate enolate reagent. The scope of this aldol reaction is investigated and its potential application to the synthesis of other structurally related, biologically relevant compounds illustrated. Remarkably, the chiral tether in the resulting gamma-amino aldol adducts sterically protect the carbonyl group, thus avoiding intramolecular cyclization during the amino group deprotection and the subsequent segment coupling event. After successful segment coupling and smooth, clean release of the chiral auxiliary, a new macrolactonization protocol, based on the principle of double activation of both reactive sites, is applied, which leads to the 12-membered macrolactone hapalosin in unprecedented chemical efficiency.

Catalytic Enantioselective Conjugate Addition of Carbamates

Catalytic, asymmetric conjugate addition of carbamates to enoyl systems has been realized for the first time, providing a two-step access to virtually enantiopure N-protected beta-amino acids.

Asymmetric Synthesis of β-Lactams Through the Staudinger Reaction and Their Use as Building Blocks of Natural and Nonnatural Products

In the last two decades, the better understanding of the mechanistic aspects of the beta-lactams' biological activity and their inhibition, and the chemical exploitation of beta-lactams as synthetic intermediates in organic chemistry, have experienced a continuous and somewhat complementary advance. A prerequisite for such a development has been the accessibility of enantiopure beta-lactams. The latter are now routinely prepared most often through the ketene-imine cycloaddition reaction, also termed the Staudinger reaction. This review accounts for the recent progress made in the asymmetric synthesis of beta-lactams (with special emphasis in the Staudinger reaction approach), as well as in their use as synthetic intermediates en route to natural products, including alpha- and beta-amino acids and peptides derived therefrom.

Current progress in the asymmetric aldol addition reaction

Control of stereochemistry during aldol addition reactions has attracted considerable interest over the years as the aldol reaction is one of the most fundamental tools for the construction of new carbon-carbon bonds. Several strategies have been implemented whereby eventually any single possible stereoisomeric aldol product can be accessed by choosing the appropriate procedure. With earlier methods, stoichiometric quantities of chiral reagents were required for efficient asymmetric induction, with the auxiliary most often attached covalently to the substrate carbonyl. Lewis acid catalyzed addition reactions of silyl enolates to aldehydes (Mukaiyama reaction) later opened the way for catalytic asymmetric induction. In the last few years, both chiral metal complexes and small chiral organic molecules have been found to catalyse the direct aldol addition of unmodified ketones to aldehydes with relatively high chemical and stereochemical efficiency. These techniques along with the more recent developments in the area are discussed in this tutorial review.

α‘-Hydroxy Enones as Achiral Templates for Lewis Acid-Catalyzed Enantioselective Diels−Alder Reactions

alpha'-Hydroxy enones react with dienes in the presence of (S,S)-[Cu(tBu-box)](OTf)2 or (S,S)-[Cu(tBu-box)](SbF6)2 (2 to 10 mol %) to afford the corresponding Diels-Alder adducts in high yield and selectivity. Isomeric ratios (regioselectivity, endo/exo or cis/trans) of up to >99:1 and ee values of up to >99% are obtained. Significantly, difficult dienes such as isoprene, 2,3-dimethyl butadiene and piperylene behave satisfactorily. Subsequent oxidative cleavage of the ketol in the resulting cycloadducts by treatment with cerium ammonium nitrate (CAN) yields the corresponding enantiopure carboxylic acids. Alternatively, carbonyl addition and subsequent diol cleavage with CAN produces the corresponding ketone adducts.

A chiral acrylate equivalent for metal-free Diels-Alder reactions: endo-2-acryloylisoborneol

The principle of metal-free activation of enones toward the Diels-Alder reaction with dienes is demonstrated by exploiting the capability of Brønsted acids to activate alpha'-hydroxyenones through hydrogen bonding. The diastereoselective application of such a principle is nicely realized by using a newly designed family of camphor-based chiral enones, which upon catalytic action of either trifluoroacetic or triflic acid lead to the corresponding cycloadducts with high chemical and stereochemical efficiency.

Design and synthesis of a novel class of sugar-peptide hybrids: C-linked glyco beta-amino acids through a stereoselective "acetate" Mannich reaction as the key strategic element

A new type of sugar-amino acid hybrid, which is comprised of a sugar unit (gluco-, galacto-, or mannopyranose) linked through a C-glycosidic linkage to the beta-position of an alpha-unsubstituted beta-amino acid unit, is presented. It is hypothesized that these new compounds, or the oligomeric peptides derived therefrom, might possess the structural features of beta-amino acid oligomers and the chemical and enzymatic resistance of C-glycosides to hydrolysis. The synthetic strategy is based on a new Mannich-type reaction between a chiral acetate enolate equivalent and alpha-amido sulfones derived from the corresponding sugar-C-glycoside aldehydes. While the sugar-C-glycoside aldehyde partner is prepared from well-established transformations on known sugar precursors, the lithium enolate derived from (1R)-endo-2-acetylisoborneol 3 is employed as the key element. This Mannich approach proceeds with essentially perfect diasteromeric control leading to the new beta-amino carbonyl adducts in good yields. Further, cleavage of the camphor auxiliary is smoothly performed by oxidative treatment with ammonium cerium nitrate (CAN). Complementarily, direct peptide-type coupling of the beta-amino carbonyl Mannich adducts with an alpha- or beta-amino acid residue and subsequent CAN-promoted detachment of the auxiliary yields dipeptide fragments bearing a sugar-containing aliphatic side chain and is a process that can be iterated. A preliminary conformational study based on the combination of experimental NMR data and molecular mechanics and molecular dynamics (MD) of one particular adduct is also provided.