The "Non-Oxidative" pummerer reaction: conclusive evidence for S(N)2-type stereoselectivity, mechanistic insight, and synthesis of enantiopure L-alpha-trifluoromethylthreoninate and D-alpha-trifluoromethyl-allo-threoninate(1)

Unusual Regio- and Chemoselectivity in Oxidation of Pyrroles and Indoles Enabled by a Thianthrenium Salt Intermediate

A dearomative oxidation of pyrroles to Δ3-pyrrol-2-ones is described, which employs a sulfoxide as oxidant, in conjunction with a carboxylic acid anhydride and a Brønsted acid additive. 3-substituted pyrroles undergo regioselective oxidation to give the product isomer in which oxygen has been introduced at the more hindered position. Regioselectivity is rationalized by a proposed mechanism that proceeds by initial thianthrenium introduction at the less-hindered pyrrole α-position, followed by distal attack of an oxygen nucleophile and subsequent elimination of thianthrene. The same reaction conditions are also able to effect a chemoselective oxidation of indoles to indolin-3-ones and additionally of indolin-3-ones to 2-hydroxyindolin-3-ones. Here again, the regio- and chemoselectivities are rationalized through the intermediacy of a thianthrenium salt.

Synthesis and Immunological Evaluation of Pentamannose-Based HIV-1 Vaccine Candidates

Dense glycosylation and the trimeric conformation of the human immunodeficiency virus-1 (HIV-1) envelope protein limit the accessibility of some cellular glycan processing enzymes and end up with high-mannose-type N-linked glycans on the envelope spike, among which the Man₅GlcNAc₂ structure occupies a certain proportion. The Man₅GlcNAc₂ glycan composes the binding sites of some potent broadly neutralizing antibodies, and some lectins that can bind Man₅GlcNAc₂ show HIV-neutralizing activity. Therefore, Man₅GlcNAc₂ is a potential target for HIV-1 vaccine development. Herein, a highly convergent and effective strategy was developed for the synthesis of Man₅ and its monofluoro-modified, trifluoro-modified, and S-linked analogues. We coupled these haptens to carrier protein CRM197 and evaluated the immunogenicity of the glycoconjugates in mice. The serological assays showed that the native Man₅ conjugates failed to induce Man₅-specific antibodies in vivo, while the modified analogue conjugates induced stronger antibody responses. However, these antibodies could not bind the native gp120 antigen. These results demonstrated that the immune tolerance mechanism suppressed the immune responses to Man₅-related structures and the conformation of glycan epitopes on the synthesized glycoconjugates was distinct from that of native glycan epitopes on gp120.

Total Syntheses of Aromatic Abietane Diterpenoids Utilizing Advances in the Pummerer Rearrangement

The first total syntheses of triptobenzene T, vitexifolin C, 4- epi-triptobenzene L, triptobenzene L, and nepetaefolin F have been accomplished through an enantioselective, common intermediate approach and have enabled the confirmation and/or establishment of the absolute stereochemistry of each natural product synthesized. Application of three new and/or underutilized Pummerer reaction pathways proved critical to the synthetic work. A proline sulfonamide-catalyzed Yamada-Otani reaction was used to access the highly functionalized cyclohexane A ring core, including the C₁₀ all-carbon quaternary stereocenter. Additionally, the importance of the A ring unsaturation for controlling the stereoselectivity during the C₄ alkylation is showcased.

Highly diastereoselective synthesis of α-difluoromethyl amines from N-tert-butylsulfinyl ketimines and difluoromethyl phenyl sulfone

The first highly efficient and stereoselective difluoromethylation of structurally diverse N-tert-butylsulfinyl ketimines has been achieved with an in situ generated PhSO(2)CF(2)(-) anion, which provides a powerful synthetic method for the preparation of a variety of structurally diverse homochiral α-difluoromethyl tertiary carbinamines, including α-difluoromethyl allylic amines and α-difluoromethyl propargylamines. The stereocontrol mode of the present diastereoselective difluoromethylation of ketimines was found to be different from that of other known fluoroalkylations of N-tert-butylsulfinyl aldimines, which suggests that a cyclic six-membered transition state may be involved in the reaction.

Selective Defluorination Approach to N-Cbz-3,3-difluoro-2-difluoromethylenepyrrolidine and Its Application to 3,3-Difluoroproline Dipeptide Synthesis

Mg-promoted defluorination of N-(p-methoxyphenyl)bis(trifluoromethyl)imine 1 gave perfluoroenamine 2, which was readily transformed to N-Cbz-2-trifluoromethyl-3,3-difluoropyrrolidine 10. Chemoselective defluorination from the trifluoromethyl group of 10 by LHMDS-promoted dehydrofluorination in THF provided 3,3-difluoro-2-difluoromethylenepyrrolidine 11. The product 11 was converted to 3,3-difluoroproline dipeptides 16 upon treatment with aminoesters.

Solvent-controlled asymmetric Strecker reaction: stereoselective synthesis of alpha-trifluoromethylated alpha-amino acids

[reaction: see text] Stereoselective approaches to alpha-trifluoromethylated alpha-amino acids (alpha-Tfm AAs) have been developed. The stereoconfigurations of the resulting alpha-Tfm AA precursors were well controlled by using different solvents. The optically active (S)-2-amino-2-phenyl-1,1,1-trifluoropropanoic acid was synthesized by this method.

Highly Stereoselective Asymmetric Pummerer Reactions That Incorporate Intermolecular and Intramolecular Nonbonded S···O Interactions

New chiral sulfoxides (R(S),S)-3, (S(S),S)-3, (R(S),S)-4, and (S(S),S)-4 and known chiral sulfoxides (R(S))-5, (R(S))-6, and (R(S))-7 were synthesized, and the stereochemistry of the new sulfoxides (R(S),S)-3 and (R(S),S)-4 was determined by X-ray crystallographic analysis. In their crystallographic structures, the intramolecular nonbonded S...O close contacts were recognized. Analyses of several sulfoxide complexes including rac-11 with N,N-dimethylacetamide (DMAC) or N-methyl-2-pyrrolidone (NMP) in a MeOH solution utilizing cold-spray ionization mass spectrometry provided, for the first time, direct information for intermolecular nonbonded S...O interactions between sulfoxides and amide (or lactam) in a solution. Highly diastereoselective and enantioselective Pummerer reactions based on the concept of intermolecular and intramolecular nonbonded S...O interactions were performed by treatment of several chiral sulfoxides (R(S), S)-3, (S(S), S)-3, (R(S), S)-4, (S(S), S)-4, (R(S))-5, (R(S))-6, and (R(S))-7 with acetic anhydride and trimethylsilyl triflate (TMSOTf) in DMAC, NMP, N,N-dimethylformamide, and N-formylpiperidine. Mechanistic studies on these facile stereoselective Pummerer reactions revealed the necessity for the amide/TMSOTf complex, such as 26 or 27, to be an efficient activation reagent for Ac(2)O and a trapping reagent for the released acetate ion, and that DMAC and NMP had a positive effect on this highly stereoselective chiral transfer reaction.

Catalytic Asymmetric Synthesis of α,α,α-Trifluoromethylamines by the Copper-Catalyzed Nucleophilic Addition of Diorganozinc Reagents to Imines

[reaction: see text] A copper-catalyzed asymmetric addition of diorganozinc reagents to N-phosphinoylimines has been developed for the synthesis of chiral alpha,alpha,alpha-trifluoromethylamines. The trifluoromethyl ketimines, generated in situ from the corresponding hemiaminals, led to the chiral amides in high yields (71-89%) and excellent enantiocontrol (91-99% ee).

Stereoselective Addition of α-Sulfinyl Carbanions toN-p-tolylsulfinylketimines: Synthesis of Optically Pure 1,2,2‘-Trialkyl-2-aminoethanols

The reactions of lithium carbanions derived from both enantiomers of methyl (1) and ethyl p-tolyl sulfoxide (2) with (S)-N-arylsulfinylketimines 3 and 4 took place in a highly stereoselective manner and good isolated yields. The configuration of the carbon bonded to nitrogen relies exclusively on the N-sulfinylimine configuration. When ethyl p-tolyl sulfoxide (2) is use as nucleophile, two chiral centers are created simultaneously, where the configuration of the carbon bonded to the sulfur is mainly controlled by 2. The asymmetric induction increases with the temperature, being total at room temperature in the case of the matched pair of reactants. A non-oxidative Pummerer reaction on the obtained aminosulfoxides allows a straightforward synthesis of optically pure 1,2-ethanolamines with one or two chiral centers, including amino alcohols with a bulky quaternary carbon bonded to the amine group.

Chemical and enzymatic synthesis of fluorinated-dehydroalanine-containing peptides

Michael acceptors have long been recognized as reactive functionalities that may link a biologically active molecule to its cellular target. 1,2-Dehydro amino acids are potential Michael acceptors present in a large number of natural products, but their reactivity is modulated by the deactivating nature of the alpha-amino group engaged in an amide bond. We describe here the preparation of 3-fluoro-1,2-dehydroalanine moieties within peptides that significantly enhance the reactivity of the Michael acceptor. Two different routes were designed to access these compounds, one relying on chemical means to introduce the desired functionality and the second taking advantage of a peptide epimerase. In the chemical approach, the fluoro-Pummerer reaction of cysteine derivatives afforded 3-fluorocysteine residues that were oxidized to the corresponding sulfoxides, followed by thermolytic elimination to provide the desired 3-fluorodehydroalanines. The mechanism of the fluoro-Pummerer reaction was investigated and several possible pathways were ruled out. The enzymatic approach utilized the dipeptide epimerase YcjG from Escherichia coli. Difluorinated alanine was incorporated at the C terminus of a dipeptide by chemical means. The resulting peptide proved to be a substrate for YcjG, which catalyzed fluoride elimination to provide the 3-fluorodehydroalanine-containing peptide. Mechanistic investigations showed that fluoride elimination occurred faster than epimerization and at a rate close to that of epimerization of Ala-Ala.

Control of peptide conformation by the Thorpe-Ingold effect (C alpha-tetrasubstitution)

The preferred conformations of peptides heavily based on the currently extensively exploited achiral and chiral alpha-amino acids with a quaternary alpha-carbon atom, as determined by conformational energy computations, crystal-state (x-ray diffraction) analyses, and solution ((1)H-NMR and spectroscopic) investigations, are reviewed. It is concluded that 3(10)/alpha-helical structures and the fully extended (C(5)) conformation are preferentially adopted by peptide sequences characterized by this family of amino acids, depending upon overall bulkiness and nature (e.g., whether acyclic or C(alpha) (i) <--> C(alpha) (i) cyclized) of their side chains. The intriguing relationship between alpha-carbon chirality and bend/helix handedness is also illustrated. gamma-Bends and semiextended conformations are rarely observed. Formation of beta-sheet structures is prevented.

Enantioselective synthesis of fluorinated alpha-amino acids and derivatives in combination with ring-closing metathesis: intramolecular pi-stacking interactions as a source of stereocontrol

[reaction: see text]. Hydride reduction of C=N bonds stereocontrolled by intramolecular pi-stacking interactions of 1-naphthylsulfinyl and N-aryl groups, nonoxidative Pummerer rearrangement, and ring-closing metathesis are efficiently combined in a highly stereoselective entry to enantiomerically pure cyclic and acyclic fluorinated beta-amino alcohols and alpha-amino acid derivatives, respectively.