Regio- and stereoselective synthesis of 1,4-dihydropyridines by way of an intramolecular interaction of a thiocarbonyl or carbonyl with a pyridinium nucleus

Regiodivergent Asymmetric Pyridinium Additions: Mechanistic Insight and Synthetic Applications

A practical protocol for the first regiodivergent asymmetric addition of aryl and alkenyl organometallic reagents to substituted N-alkyl pyridinium heterocycles is described. The couplings proceed with high regiochemical and stereochemical selectivities, and provide access to chiral 1,2,3- and 1,3,4-trisubstituted dihydropyridine products, controlled by judicious choice of nitrogen activating agent. To this end, a correlation was found between the parameterized size of the activating group and the C2/C4 regioselectivity in the couplings. The utility of the described chemistry was demonstrated in two concise asymmetric syntheses of (+)-N-methylaspidospermidine and (-)-paroxetine.

Azine Activation via Silylium Catalysis

Practical, efficient, and general methods for the diversification of N-heterocycles have been a recurrent goal in chemical synthesis due to the ubiquitous influence of these motifs within bioactive frameworks. Here, we describe a direct, catalytic, and selective functionalization of azines via silylium activation. Our catalyst design enables mild conditions and a remarkable functional group tolerance in a one-pot setup.

Tetraalkylammonium Salts as Hydrogen-Bonding Catalysts

Although the hydrogen-bonding ability of the α hydrogen atoms on tetraalkylammonium salts is often discussed with respect to phase-transfer catalysts, catalysis that utilizes the hydrogen-bond-donor properties of tetraalkylammonium salts remains unknown. Herein, we demonstrate hydrogen-bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich-type reactions. The structure and the hydrogen-bonding ability of the new ammonium salts were investigated by X-ray diffraction analysis and NMR titration studies.

Anion-binding catalysis by electron-deficient pyridinium cations

A new activation principle in organocatalysis is presented: halide binding through Coulombic interactions. This mode of catalysis was realized by using 3,5-di(carbomethoxy)pyridinium ions that carry an additional electron-withdrawing substituent on the nitrogen atom, for example, pentafluorobenzyl or cyanomethyl. For the N-pentafluorobenzyl derivative, Coulombic interaction with the pyridinium moiety is complemented in the solid state by anion-π interactions with the perfluorophenyl ring. Bromide and chloride are bound by these cations in a 1:1 stoichiometry. Catalysis of the C-C coupling between 1-chloroisochroman (and related electrophiles) with silyl ketene acetals occurs at -78 °C and at low catalyst loading (2 mol%).

Intramolecular cation-pi interaction in organic synthesis

Controlling molecular conformation is a significantly important issue in a wide variety of organic reactions because the ground state structure is significantly responsible for the transition one. As observed in enzymes and proteins, the cation-pi interaction plays a key role in the formation of the tertiary structure and the biochemical processes. Therefore, the cation-pi interaction would be a promising conformation-controlling tool not only in large molecules, but also in small molecules due to its stronger interaction force. This article describes the utility of the intramolecular cation-pi interaction in various organic syntheses with evidence for the existence of the cation-pi interactions.

Regio- and Stereoselective Addition of Allylmetal Reagents to Pyridinium−π and Quinolinium−π Complexes

[reaction: see text] Regio- and stereoselective allylation of pyridinium and quinolinium salts was performed by the addition of allylindium and allyltributyltin reagents toward intermediary cation-pi complexes. The reaction with allylindium and allyltributyltin reagents afforded a 1,2-adduct, whereas the addition of a prenylindium reagent gave a 1,4-adduct with good regio- and stereoselectivities. X-ray structural analysis, 1H NMR studies, and DFT calculations elucidated the intermediary cation-pi complex formation with face-to-face orientation.

New Class of Pyridine Catalyst Having a Conformation Switch System: Asymmetric Acylation of Various sec-Alcohols

We have developed a new class of pyridine catalyst for asymmetric acylation of sec-alcohols having a conformation switch system in which interconversion between self-complexation and uncomplexation is induced by acylation and deacylation steps, respectively. Kinetic resolution of various sec-alcohols is performed by the asymmetric acylation with isobutyric anhydride using 0.05 to 0.5 mol % catalyst 1a with s values of up to 30. In addition, dl-diols are also resolved in a similar manner in good selectivity. Moreover, asymmetric desymmetrization of meso-1,X-diols (X = 2-6) are achieved in the presence of 0.5-5 mol % catalyst 1a. A working model for the reaction mechanism is proposed on the basis of the (1)H NMR measurements, X-ray structural analyses, and AM1 and DFT calculations, where the conformation switch system governed by an intramolecular cation-pi interaction between a pyridinium ring and a thiocarbonyl group would play a key role to attain both good selectivity and high catalytic activity.

Regioselectivity in Iron-Catalyzed [2+2+1] Cycloadditions: A DFT Investigation of Substituent Effects in 1,4-Diazabutadienes

The transition-metal-catalyzed [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, in which the imine-carbon atoms are part of an oxazine ring system, with ethylene and carbon monoxide leads to the regioselective formation of pyrrolidinone derivatives. To explain this regioselectivity, the transition states and intermediates of the rate-determining step of the catalysis are determined by high-level DFT calculations. The experimentally observed regioselectivity is consistent with the lower activation energy of the addition of ethylene towards the carbon atom next to the oxazine oxygen atom. Furthermore, the activation barrier of a conceivable back reaction is higher for the intermediates with the experimentally observed regioselectivity. These thermodynamic and kinetic arguments at first sight appear to be confirmed by the calculated NPA charges in the transition states, which reveal that the differences in these charges are greatest for those transition states that lead to the formation of the energetically favored transition structures. Nevertheless, calculations of analogous transition structures and reaction products starting from 1,4-diazabutadienes with a 2-fluoro, 2-hydroxo or 2-amino substituent revealed that the regioselectivity is not determined by the electronegativity of the heteroatom and thus by the differences in the NPA charges or the resulting Coulombic interactions in the transition structures. The main reason for the observed regioselectivities is the pi-donor ability of the substituent to contribute to a delocalized pi system incorporating the adjacent imine moiety. The increasing pi-donor capability results in decreased reactivity of this moiety and increases the (relative) reactivity of the second imine group. This effect can even overcompensate for strong intramolecular Coulombic attractions in the transition structures.

Cation-pi interactions of a thiocarbonyl group and a carbonyl group with a pyridinium nucleus

Attractive interactions between a thiocarbonyl group and a pyridinium nucleus, and between a carbonyl group and a pyridinium nucleus have been proven by (1)H and (13)C NMR studies, UV-vis spectral analyses, and X-ray crystallographic analyses of nicotinic amides 1 and 3, and pyridinium salts 2 and 4. Comparison of the Deltadelta values, which are the differences in the chemical shifts with reference compounds 5 or 6, showed that the absolute Deltadelta values of 2 and 4 are much larger than those of 1 and 3. In the UV-vis spectra, the n-->pi absorption of the C=S group of 2a exhibited a significant blue shift in CHCl(3). X-ray crystallographic analysis of 1-4 clearly showed that the C=S group of 2a and the C=O group of 4 are very close to the pyridinium moiety compared to the case of 1 and 3. In addition, the X-ray crystal packing structure of 2a showed the C=S group is sandwiched between two pyridinium rings. These experimental results strongly suggested the existence of attractive (C=S)...Py(+) and (C=O)...Py(+) interactions in solution and in crystal. The optimized geometries of 1 and 2 calculated at the HF/6-311G level are in good agreement with their X-ray geometries. MP2/6-311G calculations for the model systems of pyridinium salts 2 and 4 predicted that the electrostatic and induction energies are the major source of the attractive interactions. Since the larger contribution of electrostatic and induction interactions are characteristic features of cation-pi interactions, the (C=S)...Py(+) and (C=O)...Py(+) interactions would be classified as a cation-pi interaction.

Removal of Thiazolidinethione Auxiliaries with Benzyl Alcohol Mediated by DMAP

In the presence of DMAP, a range of N-acylthiazaolidinethiones carrying different substituents were smoothly converted into the corresponding benzyl esters.

Addition of ester enolates to N-alkyl-2-fluoropyridinium salts: total synthesis of (+/-)-20-deoxycamptothecin and (+)-camptothecin

Several 4-substituted dihydropyridones or 2-pyridones have been prepared by nucleophilic addition of alpha-(methylsulfanyl)ester enolates to N-alkyl-2-fluoropyridinium salts, followed by acid hydrolysis or oxidation with concomitant hydrolysis, of the intermediate 2-fluoro-1,4-dihydropyridine adducts, respectively. Addition of the enolate derived from isopropyl alpha-(methylsulfanyl)butyrate to N-(quinolylmethyl)-2-fluoropyridinium triflate 21 followed by DDQ treatment gave pyridone 29, from which (+/-)-20-deoxycamptothecin (31), a known precursor of camptothecin, was synthesized by a radical cyclization-desulfurization, with subsequent elaboration of the lactone E ring by chemoselective reduction. A similar sequence starting from the enolate of a chiral 2-hydroxybutyric acid derivative (33) provides access to natural (+)-camptothecin (37).

Face-selective addition to a cation-pi complex of a pyridinium salt: synthesis of chiral 1,4-dihydropyridines

Selective shielding of one side of the pyridinium face by way of intramolecular cation-pi complex formation enabled nucleophiles to attack only from the non-shielded side, and consequently, chiral 1,4-dihydropyridines were produced stereoselectively with up to 99% de. The structure of the cation-pi complex was elucidated by 1H NMR studies and X-ray analysis.