Intramolecular carbolithiation reactions for the preparation of Azabicyclo

Reactivity Umpolung of Tertiary Amide Enabled by Catalytic Reductive Stannylation

Reactivity umpolung is an important concept in organic chemistry. Established reactivity umpolung mainly focuses on the aldehyde and umpolung of amide carbonyl group is not known. In this report, we describe a process to obtain the umpolung reactivity of tertiary amide. This process hinges on the efficient reductive stannylation catalyzed by Ir/silane and facile Sn-Li exchange. By leveraging this umpolung reactivity, drug Fluoxetine was derivatized to 12 different analogues via reacting with various electrophiles and four biologically active molecules were prepared concisely. This unlocked umpolung reactivity of tertiary amide is expected to find applications to synthesize complex amines from amides.

Design and Synthesis of 2,3- trans-Proline Analogues as Ligands for Ionotropic Glutamate Receptors and Excitatory Amino Acid Transporters

Development of pharmacological tools for the ionotropic glutamate receptors (iGluRs) is imperative for the study and understanding of the role and function of these receptors in the central nervous system. We report the synthesis of 18 analogues of (2 S,3 R)-2-carboxy-3-pyrrolidine acetic acid (3a), which explores the effect of introducing a substituent on the ε-carbon (3c-q). A new synthetic method was developed for the efficient synthesis of racemic 3a and applied to give expedited access to 13 racemic analogues of 3a. Pharmacological characterization was carried out at native iGluRs, cloned homomeric kainate receptors (GluK1-3), NMDA receptors (GluN1/GluN2A-D), and excitatory amino acid transporters (EAAT1-3). From the structure-activity relationship studies, several new ligands emerged, exemplified by triazole 3p-d1, GluK3-preferring (GluK1/GluK3 K_i ratio of 15), and the structurally closely related tetrazole 3q-s3-4 that displayed 4.4-100-fold preference as an antagonist for the GluN1/GluN2A receptor ( K_i = 0.61 μM) over GluN1/GluN2B-D ( K_i = 2.7-62 μM).

2-Azanorbornane--a versatile chiral aza-Diels-Alder cycloadduct: preparation, applications in stereoselective synthesis and biological activity

The review presents the achievements in the field of preparation of chiral 2-azanorbornyl derivatives and their application in various stereoselective reactions as well as in biomimetic studies.

Generation, structure and reactivity of tertiary organolithium reagents

Tertiary organolithium reagents have great potential in natural product synthesis. Recent progress in organolithium generation has made them accessible. Their utility and stereoselectivity in synthesis is discussed.

Mechanistic insight into stereoselective carbolithiation

This article addresses the mechanistic features of asymmetric carbolithiation of β-methylstyrenes. While often the presence of functional groups is required to obtain high enantioselectivities in carbolithiation reactions, simple β-methylstyrene also gives high selectivities in (-)-sparteine-mediated addition of alkyl lithium compounds. Computational studies on the carbolithiation of β-methylstyrene with (-)-sparteine show that the observed selectivities are the result of repulsion effects in the diastereomeric transition states between the (-)-sparteine⋅alkyl lithium adduct and the β-methylstyrene, upon approximation of the two reactants. In contrast, for the ortho-amino β-methylstyrene (E)-benzyl(2-propenylphenyl)amine (4) X-ray structure analyses of intermediate lithium amides indicate a carbolithiation mechanism in which one side of the double bond is shielded by the amide moiety, leaving only one side free for approach of the chiral alkyl lithium adduct.

Fully substituted carbon centers by diastereoselective spirocyclization: stereoselective synthesis of (+)-lepadiformine C

Reductive lithiation of N-Boc alpha-amino nitriles generated alpha-amino alkyllithium reagents with unexpected selectivity. The intermediate radical prefers to align with the nitrogen lone pair, and this interaction leads to an A(1,3)-strain effect that biases the conformation of the radical. In cyclohexane rings with alpha-substituents the net effect is an inversion of configuration on reductive lithiation. In the presence of a tethered electrophile the alkyllithium cyclizes to produce a spiro compound, again with inversion of configuration. The overall result is retention of configuration in the cyclization reaction. The same overall selectivity is found with alpha-oxygen alkyllithium cyclizations, but in this case both steps proceed with retention. The difference can be explained by careful consideration of the intermediate geometries. The alpha-amino spirocyclization was utilized in a concise and stereoselective synthesis of lepadiformine C.

Barrier to Enantiomerization of Unstabilized, Chelated, and Dipole-Stabilized 2-Lithiopyrrolidines

Kinetics experiments have been used to establish the free energy, enthalpy, and entropy of activation for the enantiomerization of three structural classes of 2-lithiopyrrolidines. We find that alpha-aminoorganolithiums chelated by a N-methoxyethyl or N-Boc group have a barrier to enantiomerization (DeltaG++) 2-3 kcal/mol lower than that of unstabilized alpha-aminoorganolithiums at 273 K. Density functional calculations were performed to clarify possible ground state and transition structures and to identify possible pathways for inversion of these chiral organolithium species.

Rational design, synthesis, and pharmacological evaluation of 2-azanorbornane-3-exo,5-endo-dicarboxylic acid: a novel conformationally restricted glutamic acid analogue

The design and synthesis of conformationally restricted analogues of alpha-amino acids is an often used strategy in medicinal chemistry research. Here we present the rational design, synthesis, and pharmacological evaluation of 2-azanorbornane-3-exo,5-endo-dicarboxylic acid (1), a novel conformationally restricted (S)-glutamic acid (Glu) analogue intended as a mimic of the folded Glu conformation. The synthesis of 1 was completed in its racemic form in eight steps from commercially available starting materials. As a key step, the first facially selective hydroboration of a 5-methylidene[2.2.1]bicyclic intermediate was investigated. In this transformation, the catalytic methodology of Wilkinson's/catechol borane proved superior to stoichiometric borane or dialkyl borane reagents, in terms of higher diastereomeric excess and chemical yield. To our surprise (+/-)-1 did not show affinity in binding studies on native 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) (IC(50) > 300 microM, [(3)H]AMPA) or kainic acid (IC(50) > 160 microM, [(3)H]kainic acid) receptors nor in binding studies on the cloned iGluR5,6 subtypes (IC(50) > 300 microM, [(3)H]kainic acid).

Asymmetric synthesis of 2-alkenyl-1-cyclopentanols via tin-lithium exchange and intramolecular cycloalkylation

[reaction: see text] We report a method for the synthesis of chiral cyclopentanes using tin-lithium exchange and cycloalkylation reactions. The sec-butyllithium/(-)-sparteine-mediated deprotonation of an alkyl carbamate and subsequent substitution furnishes a highly enantioenriched stannane as a stable carbanion equivalent. It was transformed into suitable cyclization precursors, which underwent tin-lithium exchange and stereoselective cycloalkylation when reacted with n-butyllithium, giving highly enantioenriched cyclopentanes in very good yields. A kinetic resolution was observed with a higher substituted stannane.

Intramolecular carbolithiation reactions of chiral alpha-amino-organolithium species

Enantiomerically enriched alpha-amino-organolithium species, in which the lithium atom is attached to a stereogenic carbon centre, have been found to be chemically stable at room temperature in a solvent of very low polarity and undergo intramolecular carbolithiation onto an unactivated alkene. The configurational stability of the chiral organolithium species, bearing a variety of N-alkenyl substituents, was probed by studying the enantiomeric purity of the cyclization products. With N-but-3-enyl-2-lithiopyrrolidine, cyclization to the five-membered ring is more rapid than racemization and a high yield of the pyrrolizidine alkaloid (+)-pseudoheliotridane was obtained with no loss of optical purity. In contrast, with N-pent-4-enyl-2-lithiopyrrolidine, cyclization to the six-membered ring was found to occur with significant loss of optical purity. The cyclization to the six-membered ring was determined to occur with a half-life, t(1/2) approximately 90 min at 23 degrees C. The epimerization of this organolithium species in hexane/Et2O 4:1 was calculated to have a half-life, t(1/2) approximately 30 min at 23 degrees C. Enhanced levels of enantioselectivity for the formation of the indolizidine ring system were obtained using an alkene bearing a terminal phenylthio substituent. With N-[(3-phenylthio)-prop-2-enyl]-2-lithiopyrrolidine, cyclization to the four-membered ring occurs with poor enantioselectivity at low temperature in THF but is highly enantioselective at room temperature in a solvent of very low polarity.

Diastereoselective tandem 6-exo carbolithiation intramolecular ring opening in (-)-8-aminomenthol-derived perhydrobenzoxazines. A new synthesis of enantiopure 4-substituted tetrahydro isoquinolines and 2-azabenzonorbornanes

Aryllithiums prepared by bromine-lithium interchange in chiral 2-(o-bromophenyl)-substituted perhydro-1,3-benzoxazines participate in the intramolecular 6-exo carbolithiation reaction with unactivated double bonds attached to the nitrogen substituent of the heterocycle. When the reaction time is extended or no TMEDA is used, the cyclized lithium intermediates react intramolecularly with the N,O-acetal system leading to 2-azabenzonorbornane derivatives. The reactions are highly stereoselective and constitute a high-yielding synthesis of enantiopure 4-substituted tetrahydroisoquinolines or 7-substituted 2-azabenzonorbornanes.