Asymmetric Mannich-type reactions for the synthesis of aspartic acid derivatives from chiral N-tert-butanesulfinylimino esters

Diastereoselective synthesis of chiral 3-substituted isoindolinones via rhodium(III)-catalyzed oxidative C-H olefination/annulation

A new method for the direct and stereoselective synthesis of 3-substituted isoindolinones via Rh(iii)-catalyzed chiral N-sulfinyl amide directed asymmetric [4 + 1] annulation of benzamides with acrylic esters has been developed. The reaction proceeded through an oxidative C-H olefination and a subsequent cyclization by intramolecular aza-Michael addition, producing a series of diastereoisomeric chiral isoindolinones (20 examples) in generally good yields with a dr value up to 5.5 : 1. The absolute configurations of the newly formed C-stereocenters in the major and minor diastereomers of the catalysis product have been determined by X-ray crystal diffraction analysis to be S and R, respectively. The separation of the major diastereoisomers from the catalysis products and subsequent removal of the N-sulfinyl chiral auxiliary afforded enantiomerically pure (S)-isoindolinones. The application of the obtained (S)-isoindolinones in the synthesis of several biologically active isoindolinones such as (S)-PD172938, (S)-pazinaclone and (S)-pagoclone is presented.

Stereodivergent Mannich reaction of bis(trimethylsilyl)ketene acetals with N-tert-butanesulfinyl imines by Lewis acid or Lewis base activation, a one-pot protocol to obtain chiral β-amino acids

We report a one-pot synthesis of chiral β^2,2,3-amino acids by the Mannich addition of bistrimethylsilyl ketene acetals to N-tert-butanesulfinyl imines followed by the removal of the chiral auxiliary. The synthesis and isolation of pure β-amino acid hydrochlorides were conducted under mild conditions, without strong bases and this method is operationally simple. The stereoselective reaction was promoted by two different activation methods that lead to different stereoisomers: (1) Lewis Acid (LA) catalysis with boron trifluoride diethyl etherate and (2) Lewis Base (LB) catalysis with tetrabutylammonium difluorotriphenylsilicate. The reaction presented good diastereoselectivity with LB activation and moderate to good dr with LA catalysis. The exceptions in both protocols were imines with electron donating groups in the aromatic ring.

Preparation of Enantiomerically Pure Perfluorobutanesulfinamide and Its Application to the Asymmetric Synthesis of α-Amino Acids

A high yielding and practical two-step synthesis of enantiomerically pure perfluorobutanesulfinamide from Senanayake's 2-aminoindanol-derived sulfinyl transfer reagent was developed and carried out on a multigram scale. Straightforward condensation of this sulfinamide with ethyl glyoxylate provided the N-perfluorobutanesulfinyl imino ester. The utility of this activated N-sulfinyl imino ester was demonstrated for reactions that gave either no product or very low yields with the corresponding less electrophilic N-tert-butanesulfinyl derivative. Specifically, the Rh(III)-catalyzed C-H bond addition of aromatic compounds to the N-perfluorobutanesulfinyl imino ester provided arylglycines with very high diastereoselectivities for a range of directing groups including pyrrolidine amide, azo, sulfoximine, 1-pyrazole, and 1,2,3-triazole functionalities. Thermal asymmetric aza-Diels-Alder reactions also proceeded in good yields and with high selectivity, including for the substituted dienes (E)-1,3-pentadiene and (2E,4E)-2,4-hexadiene.

B(C6F5)3-Catalyzed Group Transfer Polymerization of n-Butyl Acrylate with Hydrosilane through In Situ Formation of Initiator by 1,4-Hydrosilylation of n-Butyl Acrylate

The group transfer polymerization (GTP) of n-butyl acrylate (nBA) using hydrosilane (R₃SiH) and tris(pentafluorophenyl)borane (B(C₆F₅)₃) has been studied, which did not need to use the initiator of a silyl ketene acetal (SKA) as the starting polymerization component. B(C₆F₅)₃ catalyzed the in situ 1,4-hydrosilylation of nBA by R₃SiH to generate the corresponding SKA prior to the polymerization of nBA, which was confirmed by the ¹H NMR measurement of the model reaction. The formed SKA performed as the initiator for the B(C₆F₅)₃-catalyzed GTP of nBA leading to well-defined polymers with targeted molar masses and low dispersities.

tert-Butanesulfinimines: structure, synthesis and synthetic applications

Since Ellman's seminal works, over the past ten years tert-butanesulfinimines have proved to be useful chiral amino intermediates for organic synthesis. Through highly stereoselective reactions, amongst which reductions, nucleophilic 1,2-additions and ylide condensations, a broad range of nitrogen-containing compounds has been synthesized. Although the stereoselectivity levels are high in most cases, the sense of the stereoinduction is generally not predictable. The object of this critical review is to present the models proposed to rationalize the stereochemical outcome of the reactions involving tert-butanesulfinimines and to point out an obvious lack of homogeneity amongst them (128 references).

An Easy Stereoselective Access to β,γ-Aziridino α-Amino Ester Derivatives via Mannich Reaction of Benzophenone Imines of Glycine Esters with N-Sulfonyl α-Chloroaldimines

Mannich-type addition of benzophenone imine glycinates across newly synthesized N-(p-toluenesulfonyl) alpha-chloroaldimines afforded gamma-chloro-alpha,beta-diamino ester derivatives with moderate diastereoselectivity as separable mixtures of anti and syn diastereomers. The gamma-chloro-alpha,beta-diamino esters were efficiently cyclized under basic conditions to the corresponding beta,gamma-aziridino alpha-amino ester derivatives, representing a new class of conformationally constrained heterocyclic alpha,beta-diamino acid derivatives. The relative configuration of the aziridines was determined via X-ray diffraction analysis. Mechanisms and intermediate transition states to explain the stereochemical outcome of the Mannich reaction with different substrates or under different conditions are proposed. The synthetic importance of the beta,gamma-aziridino alpha-amino ester derivatives is demonstrated by their conversion into the corresponding Boc-protected derivatives and ring opening reactions to alpha,beta-diamino esters and a gamma-amino alpha,beta-unsaturated amino ester.

Mild and Efficient Pentafluorophenylammonium Triflate (PFPAT)-Catalyzed C-Acylations of Enol Silyl Ethers or Ketene Silyl (Thio)Acetals with Acid Chlorides

A pentafluorophenylammonium triflate (PFPAT) catalyst (5 mol %) successfully promoted C-acylation of enol silyl ethers with acid chloride to produce various beta-diketones (12 examples; 62-92% yield). Similarly, C-acylation of ketene silyl acetals or ketene silyl thioacetals (i.e., crossed Claisen condensation) proceeded smoothly to provide not only alpha-monoalkylated beta-keto (thio)esters but also thermodynamically unfavorable (less accessible) alpha,alpha-dialkylated beta-keto (thio)esters in good to excellent yield (38 examples; 60-92% yield).

Stereoselective synthesis of 3-aminoindan-1-ones and subsequent incorporation into HIV-1 protease inhibitors

A new method for the stereoselective synthesis of 3-aminoindan-1-ones from triflates of salicylic sulfinyl imines and ethylene glycol vinyl ether has been developed. The reaction sequence starts with a regioselective Heck reaction followed by stereoselective Lewis acid mediated annulation. Acidic cleavage of the sulfinamides produced pure (R)- and (S)-3-aminoindan-1-ones, which were successfully isolated and incorporated into active HIV-1 protease inhibitors.

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.

Methods for the synthesis of polyhydroxylated piperidines by diastereoselective dihydroxylation: exploitation in the two-directional synthesis of aza-C-linked disaccharide derivatives

BACKGROUND

Many polyhydroxylated piperidines are inhibitors of the oligosaccharide processing enzymes, glycosidases and glycosyltransferases. Aza-C-linked disaccharide mimetics are compounds in which saturated polyhydroxylated nitrogen and oxygen heterocycles are linked by an all-carbon tether. The saturated oxygen heterocycle has the potential to mimic the departing sugar in a glycosidase-catalysed reaction and aza-C-linked disaccharide mimetics may, therefore, be more potent inhibitors of these enzymes.

RESULTS

The scope, limitations and diastereoselectivity of the dihydroxylation of stereoisomeric 2-butyl-1-(toluene-4-sulfonyl)-1,2,3,6-tetrahydro-pyridin-3-ols is discussed. In the absence of a 6-substituent on the piperidine ring, the Upjohn (cat. OsO4, NMO, acetone-water) and Donohoe (OsO4, TMEDA, CH2Cl2) conditions allow complementary diastereoselective functionalisation of the alkene of the (2R*,3R*) diastereoisomer. However, in the presence of a 6-substituent, the reaction is largely controlled by steric effects with both reagents. The most synthetically useful protocols were exploited in the two-directional synthesis of aza-C-linked disaccharide analogues. A two-directional oxidative ring expansion was used to prepare bis-enones such as (2R,6S,2'S)-6-methoxy-2-(6-methoxy-3-oxo-3,6-dihydro-2H-pyran-2-ylmethyl)-1-(toluene-4-sulfonyl)-1,6-dihydro-2H-pyridin-3-one from the corresponding difuran. Selective substitution of its N,O acetal was possible. The stereochemical outcome of a two-directional Luche reduction step was different in the two heterocyclic rings, and depended on the conformation of the ring. Finally, two-directional diastereoselective dihydroxylation yielded seven different aza-C-linked disaccharide analogues.

CONCLUSION

A two-directional approach may be exploited in the synthesis of aza-C-linked disaccharide mimetics. Unlike previous approaches to similar molecules, neither of the heterocyclic rings is directly derived from a sugar, allowing mimetics with unusual configurations to be prepared. The work demonstrates that highly unsymmetrical molecules may be prepared using a two directional approach. The deprotected compounds may have potential as inhibitors of oligosaccharide-processing enzymes and as tools in chemical genetic investigations.

Synthesis of Enantiopure Sulfinimines (Thiooxime S-Oxides) Catalyzed by Yb(OTf)3 from p-Toluenesulfinamide and Aldehydes in Mild Reaction Conditions

Enantiomerically pure sulfinimines as important building blocks in the asymmetric synthesis of amine derivatives are prepared in good to excellent yields from chiral p-toluenesulfinamide with aromatic, heteroaromatic, and aliphatic aldehydes. The unprecedented feature of the reported procedure is that the formation of the sulfinimines was achieved by the catalytic action of Yb(OTf)(3) in THF at room temperature. The reaction conditions were also applicable to Ellman's sulfinimines.

High Kinetic Resolution in the Addition of a Racemic Allenylzinc onto Enantiopure N-tert-Butanesulfinimines: Concise Synthesis of Enantiopure trans-2-Ethynylaziridines1

Enantiopure trans-ethynyl N-tert-butanesulfinylaziridines (R(S))-6 were prepared in good to excellent yields by the condensation of the racemic allenylzinc species 1 derived from 3-chloro-1-trimethylsilylpropyne onto the corresponding enantiopure N-tert-butanesulfinimines (R(S))-5. The absolute stereochemistry of enantiopure N-tert-butanesulfinylaziridines (R(S))-6 was shown to be (R(S),2R,3R) and results from a chelate-type transition state in which the zinc atom of allenylzinc 1 is coordinated by both the nitogen and the oxygen atoms of the imine. Further removal of the N-tert-butanesulfinyl auxiliary of alkyl 3-substituted and 3,3-disubstituted ethynyl N-tert-butanesulfinylaziridines (R(S))-6 could be achieved by treatment with HCl in MeOH affording the corresponding deprotected aziridines (2R,3R)-9 and (2R)-9 respectively as enantiomerically pure compounds.

Highly Diastereoselective Mannich-Type Reactions of Chiral N-Acylhydrazones

The Lewis acid-mediated addition of silyl enolates to easily accessible homochiral N-acylhydrazones derived from 3-amino-2-oxazolidinones proceeded in yields up to 71% and diastereomeric ratios of 99:1. In most cases, optimal reaction conditions entailed the simple use of ZnCl(2) in acetonitrile at room temperature. Hydrazones derived from phenyl-, isopropyl-, and benzyl-substituted 2-oxazolidinones were examined in the reaction in terms of yield and diastereoselectivity. The facile SmI(2)-mediated N-N bond cleavage of the formed hydrazines was demonstrated yielding a beta-amino acid derivative. Hence, the overall reaction sequence constitutes an efficient asymmetric Mannich-type reaction. The sense of diastereoselectivity was explained by a preferential attack on the less shielded Si face of the chiral hydrazones and confirmed by means of X-ray crystallography.