SciCombinator

Discover the most talked about and latest scientific content & concepts.

Journal: The Journal of organic chemistry

31

While the use of triphenylphosphine as a reductant is common in organic synthesis, the resulting triphenylphosphine oxide (TPPO) waste can be difficult to separate from the reaction product. While a number of strategies to precipitate TPPO are available, none have been reported to work in more polar solvents. We report here that mixing ZnCl2 with TPPO precipitates a TPPO-Zn complex in high yield in several common polar organic solvents. The solvent compatibility of this procedure and the reliability of the precipitation in the presence of polar functional groups were examined to show the utility and limitations of this method.

Concepts: Oxygen, Chemical reaction, Ethanol, Solubility, Benzene, Ether, Acetic acid, Solvent

28

A simple and efficient Ru-catalyzed conjugate addition reaction of arylboronic acids to alpha,beta-unsaturated ketones under neutral conditions without any additional ligands has been developed. This Ru(II)-catalytic system fulfilled both the inhibition of the beta-hydride elimination in the catalytic cycle, and minimized the protonolysis of arylboronic acids.

Concepts: Catalysis, Organic reaction, Alkene, Elimination reaction, Addition reactions, Nucleophilic conjugate addition, Addition reaction, Cyclohexenone

28

Efforts to substitute the cyclopropane ring in a series of aryl cyclopropylnitriles led to the discovery of an operationally simple one-pot method for Knoevenagel condensation and subsequent Corey-Chaykovsky cyclopropanation giving diastereomerically pure products as a racemic mixture of enantiomers. Method development and results for variably substituted aryl acetonitriles and aldehydes in the reaction are reported. A concise synthesis of (+/-)-bicifadine in two steps is provided to demonstrate the utility of the method.

Concepts: Stereochemistry, Enantiomer, Functional groups, Aldol condensation, Racemic mixture

28

Novel enantiopure pseudopeptide models containing a central -(beta-lactam)-(Aa)- scaffold characterized by the combined presence of an alpha-alkyl-alpha-amino-beta-lactam (i+1) residue and a alpha-substituted (i+2) amino acid have been readily synthesized from alpha-alkyl serines. The conformational analysis of such beta-lactam pseudopeptides conducted in CDCl3 and DMSOd6 solutions using 1D and 2D-NMR techniques revealed an equilibrium between beta-II turn and gamma-turn conformers, which was ultimately modulated by the relative configuration of the -(beta-lactam)-(Aa)- residues. Long range chiral effects on the alpha-lactam pseudopeptide conformers were also found when two (i) and (i+3) chiral residues were attached to the termini of a central -(beta-lactam)-(Aib)- segment. In such mimetics, heterochiral (i) and (i+3) residues reinforced a beta-II turn conformer, whereas homochiral corner residues stabilized an overlapped beta-II/ beta-I double turn motif. No beta-hairpin nucleation was observed in any instance. In good agreement with the conformers found in solution, beta-turned and open structures were also characterized by X-ray crystallography. Relative stabilities of the different conformers were estimated computationally at a B3LYP/6-31++G** calculation level and, finally, a conformation equilibrium model based on steric inter-residual interactions around the -(beta-lactam)-(i+2)- segment was proposed to account for the observed chiral effects.

Concepts: Amino acid, Stereochemistry, Steric effects, Conformational isomerism, Isomer, Folding, Cyclohexane conformation

28

We developed a new conjugated cruciform flourophore (XF) featuring imine groups. The condensation of an XF containing aldehyde functionalities and selected primary amines leads to several XF- imine derivatives. Upon addition of Cu2+ or Zn2+ ions to solutions of the imine XFs in different solvents a red shifted emission is detected, resulting in an altering emission color. The imine here acts as a metallo-reactive fluorophor.

Concepts: Amino acid, Amine, Functional group, Alkene, Carbonyl, Amide, Imine, Alkylimino-de-oxo-bisubstitution

28

Regioselective alkylation of gamma-cyclodextrin with allyl or propargyl bromide, using optimized reaction conditions, followed by peracetylation of remaining hydroxyl groups and separation of isomers resulted in the set of peracetylated 2(I)-O-, 3(I)-O- and 6(I)-O-alkylated cyclodextrins in up to 19% yields. Ozonolyzis or oxidative cleaveage of peracetylated allyl derivatives resulted in a complete set of peracetylated 2(I)-O-, 3(I)-O- and 6(I)-O- formylmethyl or carboxymethyl derivatives. All these derivatives are useful precursors for further preparation of regioselectively monosubstituted derivatives of gamma-cyclodextrin.

Concepts: Alcohol, Chemical reaction, Functional groups, Hydroxyl, Chemical engineering, Cyclodextrin

28

2-Methoxymethylpyrrolidine best performed, among several other proline derivatives, to control the enantioselective [3+3] annulation of beta-(hetero)aryl-alpha nitro-alpha,beta-enals with commercial 2,2-dimethyl-1,3-dioxan-5-one, a procedure which renders highly-oxygenated nitrocyclohexanes endowed with five new stereocenters. Use of this reaction allowed the development of a total synthesis of the antitumoral natural product (+) pancratistatin; it also converted our previous racemic route to tetrodotoxin into an enantioselective one.

Concepts: Chemical reaction, Total synthesis, Paclitaxel total synthesis, English-language films, Chemical synthesis, Enantiomer, Paclitaxel, Racemic mixture

28

The photocycloaddition reaction of benzene with ethylene has been studied at the CASSCF level, including the characterization of an extended conical intersection seam. We show that the chemical selectivity is, in part, controlled by this extended conical intersection seam and that the shape of the conical intersection seam can be understood in terms of simple VB arguments. Further, the shape and energetics of the asynchronous segment of the conical intersection seam suggest that 1,2 (ortho) and 1,3 (meta) will be the preferred chemical products with similar weight. The 1,4 (para) point on the conical intersection is higher in energy and corresponds to a local maximum on the seam. VB analysis shows that the pairs of VB structures along this asynchronous seam are the same and thus the shape will be determined mainly by steric effects. Synchronous structures on the seam are higher in energy and belong to a different branch of the seam separated by a saddle point on the seam. On S(1) we have documented three mechanistic pathways corresponding to transition states (with low barriers) between the reactants and the conical intersection seam: a mixed asynchronous/synchronous [1,2] ortho path, an asynchronous [1,3] meta path, and a synchronous [1,3] meta path.

Concepts: Chemical reaction, Transition state, Metaphysics, Second derivative test, Critical point, Maxima and minima, Steric effects, Separated sets

28

We present a general model for understanding the stereochemical course of intramolecular Michael reactions. We show that the addition of beta-ketoester enolates to alpha,beta-unsaturated esters and imides bearing adjacent stereocenters X and Y leads to high levels of asymmetric induction. Reinforcing and non-reinforcing (X-Y, syn, anti) stereochemical relationships are present. On the basis of synthetic, spectroscopic, and computational studies, we propose that the outcomes of these reactions may be rationalized by a dipole-minimized chair transition state model.

Concepts: Magnetic field, Molecule, Transition state, Stereochemistry, Computational chemistry, Logic, Asymmetric induction, Asymmetric synthesis

28

We report that aryl phthalate esters are fast and robust self-immolative linkers. This linker is easy to conjugate and releases output phenols within seconds upon cleaving a fluoride-sensitive mask to yield a benign phthalic acid byproduct, making these linkers potentially useful as fluoride sensors and promising for use in biological and materials applications.

Concepts: Alcohol, Functional groups, Phthalate, Phthalic acid, Linkers, Linker, Phthalic anhydride