Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis.

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.

Discovery of (S)-6-methoxy-chroman-3-carboxylic acid (4-pyridin-4-yl-phenyl)-amide as potent and isoform selective ROCK2 inhibitors.

ROCK1 and ROCK2 are highly homologous isoforms. Accumulated studies indicate that they have distinct different functions, and the development of isoform selective ROCK inhibitors will pave new roads for the treatment of various diseases. In this work, a series of amide-chroman derivatives were synthesized and biologically evaluated in order to develop potent and isoform selective ROCK2 inhibitors. Remarkably, (S)-6-methoxy-chroman-3-carboxylic acid (4-pyridin-4-yl-phenyl)-amide ((S)-7c) possessed ROCK2 inhibitory activity with an IC50 value of 3 nM and 22.7-fold isoform selectivity (vs. ROCK1). Molecular docking indicated that hydrophobic interactions were the key element for the high potency and isoform selectivity of (S)-7c. The binding free energies predicted by MM/GBSA were in good agreement with the experimental bioactivities, and the analysis of individual energy terms suggested that residue Lys105 in ROCK1 or Lys121 in ROCK2 was the key residue for the isoform selectivity of (S)-7c.

Palladium-Catalyzed Enantioselective Addition of Chiral Metal Enolates to In Situ Generated ortho-Quinone Methides.

We describe herein a conceptually novel, cooperative Brønsted acid/base catalyzed process for the conjugate addition of cyclic ß-keto esters to ortho-quinone methides both generated in situ. Upon hemiacetalization, densely functionalized chiral chromans with two adjacent quaternary and additionally a tertiary stereogenic center were obtained with very good diastereoselectivity (up to >95:5 d.r.) and typically excellent enantioselectivity (up to >99 % ee). The striking feature and key to success is the dual catalytic activation of both nucleophile and electrophile in two separate cycles with a single chiral catalyst.

Drug efficacy of novel 3-O-methoxy-4-halo disubstituted 5,7-dimethoxy chromans; evaluated via DNA gyrase inhibition, bacterial cell wall lesion and antibacterial prospective.

In this study, novel 3-O-methoxy-4-halo, disubstituted-5,7-dimethoxy chromans with bacterial cell wall degrading potentials were synthesized, characterized and evaluated as DNA gyrase inhibitors and antibacterial agents. Compounds were showed a broad spectrum of antimicrobial activity against both Gram+ve bacteria (S. aureus (MTCC 3160), C. diphtheriae (MTCC 116), S. pyogenes (MTCC 442)) and Gram-ve bacteria (E. coli (MTCC 443), P. aeruginosa (MTCC 424), K. pneumoniae (MTCC 530)). Further, a molecular docking study was carried out to get more insight into the binding mode of present study compounds to target proteins (PDB ID: 2XCT (S. aureus DNA gyrase A), PDB ID: 3G75 (S. aureus DNA gyrase B), PDB ID: 3L7L (Teichoic acid polymerase). In the results, 14 > 20 > 24 > 12 > 18 > 17 were found as the most active against almost all executed activities in this study. The predicted Lipinski's filter scores, SAR, pharmacokinetic/pharmacodynamics, and ADMET properties of these compounds envisioned the druggability prospects and the necessity of further animal model evaluations of 3-O-methoxy-4-halo disubstituted 5,7-dimethoxy chromans to establish them as an effective and future antibiotics.

Deciphering Structure-Activity Relationships in a Series of 2,2-Dimethylchromans Acting as Inhibitors of Insulin Release and Smooth Muscle Relaxants.

4,6-Disubstituted 2,2-dimethylchromans are reported as pharmacologically active compounds that mainly target the ATP-sensitive potassium channels. The present study is an attempt to characterize the impact of the nature of substituents introduced at the 4- and 6-positions of 2,2-dimethylchromans on their capacities to inhibit insulin release from pancreatic ß-cells or to relax vascular smooth muscle cells, both biological responses that are supposed to reflect interaction with specific ion channels. From the core structure 4-amino-2,2-dimethylchroman, a progressive increase in the steric hindrance of the chemical functionalities introduced at the 4-position (amino, formamido, acetamido, arylureido/thioureido) and at the 6-position (amino, formamido, acetamido, alkoxycarbonylamino) led to a progressive magnification of the inhibitory effect on the insulin release process and, to a lesser extent, of the vasorelaxant activity. Moreover, the dextrorotatory enantiomer of 2,2-dimethylchroman compound 29 was more potent than its levorotatory counterpart for inhibiting the insulin secretory process. Additional pharmacological investigations suggested, however, that the myorelaxant activity of 11 and 15 resulted from a direct Ca2+ entry blockade.

3-Phenylalkyl-2H-chromenes and -chromans as novel rhinovirus infection inhibitors.

Following our studies on structure-activity relationships of anti-rhinovirus chromene and chroman derivatives, we designed and synthesized new series of 3-phenylalkyl-2H-chromenes and -chromans bearing differently sized, aliphatic linker chains between the two cycles. The cytotoxicity and the antiviral activity of the new compounds on human rhinovirus (HRV) serotype 1B and 14 infection were evaluated in HeLa cell cultures. Most of the tested compounds interfered with HRV1B multiplication in the micromolar or submicromolar concentrations while HRV14 was less susceptible. 3-[3-(4-Chlorophenyl)propyl]chroman (9c) was selected for preliminary mechanism of action studies due to its potent activity against both serotypes (IC50 of 0.48µM and 1.36µM towards HRV1B and 14, respectively) coupled with high selectivity (SI=206.18 and 73.26, respectively). Results of time of addition/removal studies suggest that 9c, similarly to related derivatives, behaves as a capsid binder interfering with some early events of the HRV1B infectious cycle.

Enantioselective Reactions of 2-Sulfonylalkyl Phenols with Allenic Esters: Dynamic Kinetic Resolution and [4+2] Cycloaddition Involving ortho-Quinone Methide Intermediates.

We report herein a dynamic kinetic resolution (DKR) involving ortho-quinone methide (o-QM) intermediates. In the presence of Et3 N and the cinchonine-derived nucleophilic catalyst D, the DKR of 2-sulfonylalkyl phenols with allenic esters afforded chiral benzylic sulfones in 57-79 % yield with good to excellent enantioselectivity (85-95 % ee). Furthermore, with 2-(tosylmethyl)sesamols or 2-(tosylmethyl)naphthols, from which stable o-QM substrates can be generated, a formal [4+2] cycloaddition delivered 4-aryl- or alkyl-substituted chromans with excellent enantioselectivity (88-97 % ee).

Organocatalytic, Enantioselective Synthesis of Cyclohexadienone Containing Hindered Spirocyclic Ethers through an Oxidative Dearomatization/Oxa-Michael Addition Sequence.

An unprecedented enantioselective oxa-Michael reaction of α-tertiary alcohols using cinchona-alkaloid-based chiral bifunctional squaramide catalysts is reported. An oxidative dearomatization of phenol followed by an enantioselective oxa-Michael addition sequence provided a broad array of chiral sterically hindered tetrahydrofurans and tetrahydropyrans attached to a cyclohexadienone moiety in spiro fashion. In general, good yields and excellent enantioselectivities (up to 99 %) were observed. The chiral oxo-cycles obtained have easily been transformed into chromans without disturbing the enantioselectivity.

Organocatalytic Domino Oxa-Michael/1,6-Addition Reactions: Asymmetric Synthesis of Chromans Bearing Oxindole Scaffolds.

An asymmetric organocatalytic domino oxa-Michael/1,6-addition reaction of ortho-hydroxyphenyl-substituted para-quinone methides and isatin-derived enoates has been developed. In the presence of 5 mol % of a bifunctional thiourea organocatalyst, this scalable domino reaction affords 4-phenyl-substituted chromans bearing spiro-connected oxindole scaffolds and three adjacent stereogenic centers in good to excellent yields (up to 98 %) and with very high stereoselectivities (up to >20:1 d.r., >99 % ee).

Larvicidal and Adulticidal Activity of Chroman and Chromene Analogues against Susceptible and Permethrin-Resistant Mosquito Strains.

Mosquitoes play a major role as vectors that transmit parasitic and viral diseases worldwide, especially in tropical and subtropical countries. Mosquito borne diseases not only affect humans but they also affect livestock in many parts of the world. They carry diseases that are lethal to dogs and horses. Dog heartworm disease (Dirofilaria immitis) is a parasitic disease spread through mosquitoes. This disease is not limited to dogs, but it can affect other animals and humans as well. Eastern equine encephalitis (EEE) and West Nile virus (WNV) are also mosquito borne diseases that affect the central nervous system of horses and cause severe complications and death. Emergence of resistance among mosquitoes to current pesticides has increased the importance of the search for alternate compounds that are effective and environmentally benign with diverse modes of actions than those that are commercially available. Aedes aegypti mosquitoes are the primary vector for transmission of Zika viral fever, yellow fever, dengue fever, and chikungunya. Mosquito control is currently the best strategy to prevent mosquito borne diseases. There are numerous approaches for control of potentially dangerous mosquito populations. These approaches include the use of adulticides (insecticides), larvicides, and, to a limited extent, the use of repellents. Our previous studies have shown the mosquito repellent activity of chromenes. In the present study, we demonstrate larvicidal and adulticidal activity of chroman and chromene analogues against a permethrin susceptible laboratory strain as well as activity against a permethrin-resistant strain of Aedes aegypti.

Synthesis of Chiral 1,4-Benzodioxanes and Chromans by Enantioselective Palladium-Catalyzed Alkene Aryloxyarylation Reactions.

A highly enantioselective alkene aryloxyarylation led to the high-yielding formation of a series of 1,4-benzodioxanes, 1,4-benzooxazines, and chromans containing quaternary stereocenters with excellent enantioselectivity. The sterically bulky and conformationally well defined chiral monophosphorus ligand L4 or L5 was responsible for the high reactivity and enantioselectivity of these transformations. The application of this method to the synthesis of the chiral chroman backbone of α-tocopherol was demonstrated.

Gold-catalyzed asymmetric allylic substitution of free alcohols: an enantioselective approach to chiral chromans with quaternary stereocenters for the synthesis of vitamin E and analogues.

The enantioselective synthesis of α- and γ-tocopherol (the most biologically active members of vitamin E family) and analogues has been accomplished employing a new enantioselective gold catalyzed intramolecular allylic alkylation reaction followed by an olefin cross-metathesis as key steps. The methodology proved to be applicable to different olefins highlighting its potential for the synthesis of diverse libraries.

Pyranoxanthones: Synthesis, growth inhibitory activity on human tumor cell lines and determination of their lipophilicity in two membrane models.

The benzopyran and dihydrobenzopyran moieties can be considered as "privileged motifs" in drug discovery being good platforms for the search of new bioactive compounds. These moieties are commonly found fused to the xanthonic scaffold belonging to the biologically important family of the generally designated prenylated xanthones. Several pyranoxanthones have shown promising antitumor activity and since most of them are from natural origin, the biosynthetic pathway only allows a particular pattern of substitution which limits their structural diversity and renders any broad structure-activity study hard to be established. Accordingly, with the aim of rationalizing the importance of the fused ring orientation and oxygenation pattern in pyranoxanthones, this study describes the synthesis of 14 new pyranoxanthones and evaluation of their cell growth inhibitory activity in four human tumor cell lines as well as their lipophilicity in two membrane models. This systematic approach allowed establishing structure-activity and structure-lipophilicity relationships for the obtained compounds in combination with 6 previously described compounds. From this work an angular pyranoxanthone scaffold emerged as particularly promising, presenting a potent cell growth inhibitory activity and suitable drug-like lipophilicity.

Gold(I)-catalysed one-pot synthesis of chromans using allylic alcohols and phenols.

A gold(I)-catalysed reaction of allylic alcohols and phenols produces chromans regioselectively via a one-pot Friedel-Crafts allylation/intramolecular hydroalkoxylation sequence. The reaction is mild, practical and tolerant of a wide variety of substituents on the phenol.