Design for carbon–carbon bond forming reactions under ambient conditions

The carbon–carbon (C–C) bond forms the ‘backbone’ of nearly every organic molecule, and lies at the heart of the chemical sciences! Let us explore designing of carbon–carbon frameworks at ambient conditions.

Coumarin-3-carboxylic acid derivatives as potentiators and inhibitors of recombinant and native N-methyl-D-aspartate receptors

N-Methyl-d-aspartate receptors (NMDARs) are known to be involved in a range of neurological and neurodegenerative disorders and consequently the development of compounds that modulate the function of these receptors has been the subject of intense interest. We have recently reported that 6-bromocoumarin-3-carboxylic acid (UBP608) is a negative allosteric modulator with weak selectivity for GluN2A-containing NMDARs. In the present study, a series of commercially available and newly synthesized coumarin derivatives have been evaluated in a structure-activity relationship (SAR) study as modulators of recombinant NMDAR activity. The main conclusions from this SAR study were that substituents as large as iodo were accommodated at the 6-position and that 6,8-dibromo or 6,8-diiodo substitution of the coumarin ring enhanced the inhibitory activity at NMDARs. These coumarin derivatives are therefore excellent starting points for the development of more potent and GluN2 subunit selective inhibitors, which may have application in the treatment of a range of neurological disorders such as neuropathic pain, epilepsy and depression. Surprisingly, 4-methyl substitution of UBP608 to give UBP714, led to conversion of the inhibitory activity of UBP608 into potentiating activity at recombinant GluN1/GluN2 receptors. UBP714 also enhanced NMDAR mediated field EPSPs in the CA1 region of the hippocampus. UBP714 is therefore a novel template for the development of potent and subunit selective NMDAR potentiators that may have therapeutic applicability in the treatment of patients with cognitive deficits or schizophrenia.

Asymmetric Total Synthesis of (−)-Linderol A

The first asymmetric total synthesis of (-)-Linderol A, a potent inhibitor of melanin biosynthesis of cultured B-16 melanoma cells, has been achieved via two key reactions: a diastereoselective [2+2] photocycloaddition of a coumarin-3-carboxylate bearing a chiral auxiliary with 3-methyl-1-butene and a subsequent stereoconvergent transformation of the photoadducts with use of dimethylsulfoxonium methylide to afford a tetrahydrodibenzofuran derivative.

N-Acylureido functionality as acceptor substituent in solvatochromic fluorescence probes: detection of carboxylic acids, alcohols, and fluoride ions

N,N'-Dicyclohexyl-N-(6'-methoxyanthryl-2-carbonyl)urea (1a) exhibits a strong solvatochromism with respect to its fluorescence properties. The emission maxima of 1a correlate well with the anion-stabilizing properties of the solvent. This feature allows the detection of analytes with high acceptor numbers, such as alcohols and carboxylic acids, and the detection of analytes such as fluoride ions which form strong hydrogen bonds with the amido hydrogen atom.

Synthesis of N epsilon-(7-diethylaminocoumarin-3-carboxyl)- and N epsilon-(7-methoxycoumarin-3-carboxyl)-L-Fmoc lysine as tools for protease cleavage detection by fluorescence

Two coumarin-labelled lysines were conveniently prepared as a fluorescence resonance energy transfer (FRET) pair for peptide cleavage detection. 7-Methoxy and 7-diethylamino coumarin-3-carboxylic acids were synthesized according to a modification of known procedures. Labelling at lysine was achieved in solution via the active N-hydroxysuccinimide ester of the carboxylic acid coumarin derivatives to give the target compounds in good yield. Subsequently, these modified amino acids were used in solid phase peptide synthesis (SPPS), and their potential utility in an extracellular matrix metalloprotease (MMP-1) activity measurement via FRET and/or quenching studies was demonstrated.

Novel stereoconvergent transformation of 1,2a-disubstituted 1,2,2a,8b-tetrahydro-3H-benzo[b]cyclobuta[d]pyran-3-ones to 1,3-disubstituted 1,2,4a,9b-tetrahydrodibenzofuran-4-ols and its application to the second-generation synthesis of (+/-)-linderol A

1,2a-Disubstituted 1,2,2a,8b-tetrahydro-3H-benzo[b]cyclobuta[d]pyran-3-ones bearing an electron-withdrawing group at the 2a-position were treated with two equivalents of dimethylsulfoxonium methylide to give r-1,t-4a,t-9b-1,3-disubstituted 1,2,4a,9b-tetrahydrodibenzofuran-4-ols stereoconvergently regardless of the stereochemistry of the 1-position on the benzocyclobutapyran ring. This methodology was applied to the second-generation synthesis of (+/-)-linderol A, a melanin biosynthesis inhibitory natural product.

Design, Synthesis, Physicochemical Properties, and Evaluation of Novel Iron Chelators with Fluorescent Sensors

The synthesis of a range of novel 3-hydroxypyridin-4-ones and 3-hydroxypyran-4-ones linked with different coumarin substituents is described. These compounds have been developed in order to provide a series of molecular probes for the quantification of intracellular labile iron pools. An evaluation of the effect of iron(III) on fluorescence intensity was undertaken. Chelation of iron(III) causes quenching of fluorescence. The relationship between iron(III) concentration and the extent of fluorescence quenching indicates that the metal is chelated in a complex with a metal-to-ligand stoichiometry of 1:3. The fluorescence of hydroxypyridinone compounds was found to be more efficiently quenched by iron(III) than were the hydroxypyranones. The metal-to-ligand stoichiometry at which maximum quenching is observed was found to depend on the site at which coumarin is attached. The efficiency of fluorescence quenching by iron(III) is markedly influenced by solvent polarity and pH. The permeability of two representative fluorescent chelators across human erythrocyte ghost membranes was investigated. The rate of permeability for a series of probes was found to be related to the corresponding ClogP values.

Inhibition of monoamine oxidases by coumarin-3-acyl derivatives: biological activity and computational study

A series of coumarin-3-acyl derivatives have been synthesized and investigated for the ability to inhibit selectively monoamine oxidases. The coumarin-3-carboxylic acids, 2a-e, proved to be reversible and selective inhibitors of the MAO-B isoform, displaying pIC(50) values of particular interest: 2a shows pIC(50) 7.76 and a selectivity index (pS.I.) 2.94 and 2b shows pIC(50) 7.72 and a pS.I. of 2.80. The coumarin-3-acyl chlorides 3a-e showed high pIC(50) values against both MAO-A and MAO-B isoforms, 3d being the highest against MAO-B with a pIC(50) value of 8.00. In order to rationalize the activity/selectivity results, molecular descriptors were generated. Further insight about enzyme-inhibitor interaction was obtained by docking experiments with the MAO-B isoform.