Studies on the Mechanism of Action of 2-Formyl-4-pyrrolidinopyridine: Isolation and Characterization of a Reactive Intermediate

Novel Strategies for Enantio- and Site-Selective Molecular Transformations

A strategy for symmetric synthesis based on dynamic chirality of enolates (memory of chirality) has been developed. Asymmetric alkylation, conjugate addition, aldol reaction, and arylation via C-N axially chiral enolate intermediates are described. Asymmetric alkylation and conjugate addition via C-O axially chiral enolate intermediates with a half-life of racemization as short as approx. 1 s. at -78 °C have been accomplished. Organocatalysts for asymmetric acylation and site-selective acylation have been developed. Kinetic resolution of racemic alcohols via remote asymmetric induction by the catalyst is shown. Catalyst-controlled site-selective acylation of carbohydrates and its application to total synthesis of natural glycoside are described. Chemo-selective monoacylation of diols and selective acylation of secondary alcohols with reversal of inherent reactivity are also discussed. Geometry-selective acylation of tetrasubstituted alkene diols is achieved, where acylation takes place independent from the steric environments of the substrates.

Chiral Lewis acid-bonded picolinaldehyde enables enantiodivergent carbonyl catalysis in the Mannich/condensation reaction of glycine ester

A new strategy of asymmetric carbonyl catalysis via a chiral Lewis acid-bonded aldehyde has been developed for the direct Mannich/condensation cascade reaction of glycine ester with aromatic aldimines. The co-catalytic system of 2-picolinaldehyde and chiral YbIII-N,N'-dioxides was identified to be efficient under mild conditions, providing a series of trisubstituted imidazolidines in moderate to good yields with high diastereo- and enantioselectivities. Enantiodivergent synthesis was achieved via changing the sub-structures of the chiral ligands. The reaction could be carried out in a three-component version involving glycine ester, aldehydes, and anilines with equally good results. Based on control experiments, the X-ray crystal structure study and theoretical calculations, a possible dual-activation mechanism and stereo-control modes were provided to elucidate carbonyl catalysis and enantiodivergence.

Carbohydrates as efficient catalysts for the hydration of α-amino nitriles

Directed hydration of α-amino nitriles was achieved under mild conditions using simple carbohydrates as catalysts exploiting temporary intramolecularity. A broadly applicable procedure using both formaldehyde and NaOH as catalysts efficiently hydrated a variety of primary and secondary susbtrates, and allowed the hydration of enantiopure substrates to proceed without racemization. This work also provides a rare comparison of the catalytic activity of carbohydrates, and shows that the simple aldehydes at the basis of chemical evolution are efficient organocatalysts mimicking the function of hydratase enzymes. Optimal catalytic efficiency was observed with destabilized aldehydes, and with difficult substrates only simple carbohydrates such as formaldehyde and glycolaldehyde proved reliable.

o-Phthalaldehyde catalyzed hydrolysis of organophosphinic amides and other P([double bond, length as m-dash]O)-NH containing compounds

Over 50 years ago, Jencks and Gilchrist showed that formaldehyde catalyses the hydrolysis of phosphoramidate through electrophilic activation, induced by covalent attachment to its nitrogen atom. Given our interest in the use of aldehydes as catalysts, this work was revisited to identify a superior catalyst, o-phthalaldehyde, which facilitates hydrolyses of various organophosphorus compounds bearing P([double bond, length as m-dash]O)-NH subunits under mild conditions. Interestingly, chemoselective hydrolysis of the P([double bond, length as m-dash]O)-N bonds could be accomplished in the presence of P([double bond, length as m-dash]O)-OR bonds.

Organocatalysis using aldehydes: the development and improvement of catalytic hydroaminations, hydrations and hydrolyses

Aldehydes as organocatalysts? Simple aldehydes achieve difficult intermolecular reactions by exploiting temporary intramolecularity and inducing electrophilic activation.

Diarylureas as allosteric modulators of the cannabinoid CB1 receptor: structure-activity relationship studies on 1-(4-chlorophenyl)-3-{3-[6-(pyrrolidin-1-yl)pyridin-2-yl]phenyl}urea (PSNCBAM-1)

The recent discovery of allosteric modulators of the CB1 receptor including PSNCBAM-1 (4) has generated significant interest in CB1 receptor allosteric modulation. Here in the first SAR study on 4, we have designed and synthesized a series of analogs focusing on modifications at two positions. Pharmacological evaluation in calcium mobilization and binding assays revealed the importance of alkyl substitution at the 2-aminopyridine moiety and electron deficient aromatic groups at the 4-chlorophenyl position for activity at the CB1 receptor, resulting in several analogs with comparable potency to 4. These compounds increased the specific binding of [(3)H]CP55,940, in agreement with previous reports. Importantly, 4 and two analogs dose-dependently reduced the Emax of the agonist curve in the CB1 calcium mobilization assays, confirming their negative allosteric modulator characteristics. Given the side effects associated with CB1 receptor orthosteric antagonists, negative allosteric modulators provide an alternative approach to modulate the pharmacologically important CB1 receptor.

Asymmetric desymmetrization of meso-diols by C(2)-symmetric chiral 4-pyrrolidinopyridines

In this work we developed C₂-symmetric chiral nucleophilic catalysts which possess a pyrrolidinopyridine framework as a catalytic site. Some of these organocatalysts effectively promoted asymmetric desymmetrization of meso-diols via enantioselective acylation.

Catalysis through temporary intramolecularity: mechanistic investigations on aldehyde-catalyzed Cope-type hydroamination lead to the discovery of a more efficient tethering catalyst

Mechanistic investigations on the aldehyde-catalyzed intermolecular hydroamination of allylic amines using N-alkylhydroxylamines are presented. Under the reaction conditions, the presence of a specific aldehyde catalyst allows formation of a mixed aminal intermediate, which permits intramolecular Cope-type hydroamination. The reaction was determined to be first-order in both the aldehyde catalyst (α-benzyloxyacetaldehyde) and the allylic amine. However, the reaction displays an inverse order behavior in benzylhydroxylamine, which reveals a significant off-cycle pathway and highlights the importance of an aldehyde catalyst that promotes a reversible aminal formation. Kinetic isotope effect experiments suggest that hydroamination is the rate-limiting step of this catalytic cycle. Overall, these results enabled the elaboration of a more accurate catalytic cycle and led to the development of a more efficient catalytic system for alkene hydroamination. The use of 5-10 mol % of paraformaldehyde proved more effective than the use of 20 mol % of α-benzyloxyacetaldehyde, leading to high yields of intermolecular hydroamination products within 24 h at 30 °C.

A catalytic tethering strategy: simple aldehydes catalyze intermolecular alkene hydroaminations

Herein we describe a catalytic tethering strategy in which simple aldehyde precatalysts enable, through temporary intramolecularity, room-temperature intermolecular hydroamination reactivity and the synthesis of vicinal diamines. The catalyst allows the formation of a mixed aminal from an allylic amine and a hydroxylamine, resulting in a facile intramolecular hydroamination event. The promising enantioselectivities obtained with a chiral aldehyde also highlight the potential of this catalytic tethering approach in asymmetric catalysis and demonstrate that efficient enantioinduction relying only on temporary intramolecularity is possible.

Scaffolding catalysts: highly enantioselective desymmetrization reactions

Ex-changing places: a highly enantioselective desymmetrization of 1,2-diols has been developed in which the catalyst utilizes reversible covalent bonding to the substrate to achieve both high selectivity and rate acceleration (see scheme, PMP=pentalmethylpiperidine, TBS=tert-butyldimethylsilyl). Induced intramolecularity is responsible for the enhanced rate, thus allowing the reaction to be performed at room temperature.

O-nucleophilic amino alcohol acyl-transfer catalysts: the effect of acidity of the hydroxyl group on the activity of the catalyst

[reaction: see text]. Amino alcohol-derived acyl-transfer catalysts are shown to operate by an O-nucleophilic mechanism, and catalysts bearing electron-withdrawing groups in proximity to the hydroxyl group are found to be more active. This is attributed to an increase in the acidity of the hydroxyl group of the catalyst.

Synthesis and characterization of phosphorescent cyclometalated platinum complexes

The synthesis, electrochemistry, and photophysics of a series of square planar Pt(II) complexes are reported. The complexes have the general structure C(wedge)NPt(O(wedge)O),where C(wedge)N is a monoanionic cyclometalating ligand (e.g., 2-phenylpyridyl, 2-(2'-thienyl)pyridyl, 2-(4,6-difluorophenyl)pyridyl, etc.) and O(wedge)O is a beta-diketonato ligand. Reaction of K(2)PtCl(4) with a HC(wedge)N ligand precursor forms the chloride-bridged dimer, C(wedge)NPt(mu-Cl)(2)PtC(wedge)N, which is cleaved with beta-diketones such as acetyl acetone (acacH) and dipivaloylmethane (dpmH) to give the corresponding monomeric C(wedge)NPt(O(wedge)O) complex. The thpyPt(dpm) (thpy = 2-(2'-thienyl)pyridyl) complex has been characterized using X-ray crystallography. The bond lengths and angles for this complex are similar to those of related cyclometalated Pt complexes. There are two independent molecular dimers in the asymmetric unit, with intermolecular spacings of 3.45 and 3.56 A, consistent with moderate pi-pi interactions and no evident Pt-Pt interactions. Most of the C(wedge)NPt(O(wedge)O) complexes display a single reversible reduction wave between -1.9 and -2.6 V (vs Cp(2)Fe/Cp(2)Fe(+)), assigned to largely C(wedge)N ligand based reduction, and an irreversible oxidation, assigned to predominantly Pt based oxidation. DFT calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes. The HOMO levels are a mixture of Pt and ligand orbitals, while the LUMO is predominantly C(wedge)N ligand based. The emission characteristics of these complexes are governed by the nature of the organometallic cyclometalating ligand allowing the emission to be tuned throughout the visible spectrum. Twenty-three different C(wedge)N ligands have been examined, which gave emission lambda(max) values ranging from 456 to 600 nm. Well-resolved vibronic fine structure is observed in all of the emission spectra (room temperature and 77 K). Strong spin-orbit coupling of the platinum atom allows for the formally forbidden mixing of the (1)MLCT with the (3)MCLT and (3)pi-pi states. This mixing leads to high emission quantum efficiencies (0.02-0.25) and lifetimes on the order of microseconds for the platinum complexes.

First direct C-2-Lithiation of 4-DMAP. Convenient access to reactive functional derivatives and ligands

The first direct alpha-lithiation of 4-DMAP has been performed via reaction with the BuLi-Me(2)N(CH(2))(2)OLi (BuLi-LiDMAE) reagent. This new methodology avoids the use of a activation-lithiation-regeneration sequence or halogen-metal exchange classically employed. New useful DMAP-containing synthons and polyheterocycles have been efficiently prepared.

Enhanced selectivities for the hydroxyl-directed methanolysis of esters using the 2-acyl-4-aminopyridine class of acyl transfer catalysts: ketones as binding sites

In this paper we describe the preparation of a series of 2-acyl-4-aminopyridines, and their use as catalysts for the hydroxyl-directed methanolysis of alpha-hydroxy esters in preference to alpha-methoxy esters. Hydroxyl-direction with these catalysts, which contain ketones at the 2-position of the pyridine, is achieved by reversible addition of the alcohol of the hydroxy ester to the ketone to provide the corresponding hemiketal. Their activity is compared to that of the previously described catalyst 2-formyl-4-pyrrolidinopyridine (FPP), which contains an aldehyde at the 2-position of the pyridine. The catalysts which contain ketones at the 2-position range in reactivity from 10 times slower to slightly faster than FPP, and certain of these are much more selective for the methanolysis of hydroxy esters than FPP. This increase in selectivity is ascribed to a decrease in the rate of the nondirected methanolysis reaction with the ketone-derived catalysts. The evidence suggests that the nondirected reaction does not proceed by an intermolecular general base mechanism, but rather via a nucleophilic catalysis mechanism in which the hydroxyl group of the hemiacetal formed upon addition of methanol to the aldehyde of FPP acts as the nucleophile. Since the hydroxyl group derived from a hemiketal is more hindered and less nucleophilic than that derived from a hemiacetal, the nondirected reaction is much slower for the catalysts containing ketones as binding sites.