Ricardo L. E. Furlan

Cleavage of carboxylic esters effected by organotin oxides and hydroxides under classical heating and microwave irradiation. A comparative study

Abstract We describe some recent findings on similarities and differences in the cleavage of selected carboxylic esters from the viewpoint of reactivity effected by organotin oxides and hydroxides, under different modes of heating (oil bath or microwave irradiation). These studies complement earlier use of bis(tributyltin) oxide and provide a simple and efficient procedure of cleavage of carboxylic esters in apolar aprotic or without solvent, under mild conditions with readily available organotin reagents.

Butylstannonic acid catalyzed transesterification of carboxylic esters

Abstract Butylstannonic acid is used as catalyst for transesterification of various carboxylic esters. This method is also applicable to acetylation/deacetylation of alcohols.

Selective detachment of Boc-protected amino acids and peptides from Merrifield, PAM and Wang resins by trimethyltin hydroxide

Abstract We describe the development of a new non-acidolytic and non-nucleophilic method for the selective cleavage by trimethyltin hydroxide of amino acids and dipeptides at benzyl ester links attached to resins commonly used in peptide synthesis.

Efficient, non-acidolytic method for the selective cleavage of N-Boc amino acid and peptide phenacyl esters linked to a polystyrene resin

An efficient, non-acidolytic method for the selective cleavage of phenacyl esters of N-Boc-amino acids and -peptides linked to a polystyrene resin by (CH3)3SnOH (TMTOH) or [(n-C4H9)3Sn]2O (BBTO) is described. We highly recommend the use of trimethyltin hydroxide for the selective cleavage of carboxylic esters based on its favourable properties. The method is compatible with an N-Boc/O-Bn (benzyl ether) strategy and yields enantiomerically pure N-Boc-peptides useful for further manipulation, for segment condensations or for cyclization strategies. A mechanism for the cleavage of methyl phenylacetate in solution by TMTOH is postulated.

Multiresponse Optimisation Applied to the Development of a TLC Autography for the Detection of Tyrosinase Inhibitors.

INTRODUCTION Autographic methods are useful tools to detect bioactive compounds in complex matrixes. Experimental design and optimisation techniques were implemented for the development of an autographic assay suitable for the detection of tyrosinase inhibitors. OBJECTIVES To develop an autographic assay to detect tyrosinase inhibitors using gel entrapped enzyme, experimental design and response surface methodology (RSM) to optimise conditions with a minimum number of experiments. METHODS Gel entrapment was used for the assay and the effects of four factors on the sensitivity and the detection limit for known inhibitors of the enzyme were evaluated. The factors were: tyrosinase amount (TA), L-tyrosine amount (LTA), incubation time and incubation temperature. RESULTS The assay allowed the detection of kojic acid in an extract of Calamagrostis viridiflavescens (Poir.) Steud spiked with 0.1% w/w. CONCLUSION The developed assay is able to detect tyrosinase inhibitors present in complex matrixes in a reproducible way.

Thin Layer Chromatography-Autography-High Resolution Mass Spectrometry Analysis: Accelerating the Identification of Acetylcholinesterase Inhibitors.

INTRODUCTION The prevailing treatment for Alzheimers disease is the use of acetylcholinesterase (AChE) inhibitors. Natural extracts are the principal source of AChEs inhibitors. However, their chemical complexity demands for simple, selective and rapid assays. OBJECTIVE To develop a strategy for identification of AChE inhibitors present in mixtures employing high resolution mass spectrometry (HRMS) and thin layer chromatography (TLC)-biological staining. METHODOLOGY The strategy uses an autographic assay based on the α-naphthyl acetate - fast blue B system for the detection of AChE activity. The immobilisation of AChE in agar allowed the extraction of the compounds for analysis by HRMS. Three TLC experiments employing different solvent systems were used in parallel and the mass spectra of the compounds extracted from the inhibition halos, were compared. The analysis was performed under MatLab environment. RESULTS The strategy was used to detect the presence of physostigmine in an extract of Brassica rapa L. spiked with the inhibitor. Similarly, caffeine was straightforwardly spotted as responsible for the inhibitory properties of an extract of Ilex paraguariensis Saint-Hilaire. Comparison of the HRMS profiles lead to the facile identification of the [M+H](+) and [M+Na](+) of the compounds responsible for the inhibition. CONCLUSION The proposed methodology, coupling TLC-AChE autography-HRMS, illustrates the feasibility of assigning molecular formulas of active compounds present in complex mixtures directly from autography. The new AChE agar-immobilised assay presented a more homogenous colour and a better definition than direct spraying methods, reducing the cost of the assay and improving its sensitivity.

Dithioacetal Exchange: A New Reversible Reaction for Dynamic Combinatorial Chemistry

Reversibility of dithioacetal bond formation is reported under acidic mild conditions. Its utility for dynamic combinatorial chemistry was explored by combining it with orthogonal disulfide exchange. In such a setup, thiols are positioned at the intersection of both chemistries, constituting a connecting node between temporally separated networks.

A reversed-phase compatible thin-layer chromatography autography for the detection of acetylcholinesterase inhibitors

A dual readout autographic assay to detect acetylcholinesterase inhibitors present in complex matrices adsorbed on reversed-phase or normal-phase thin-layer chromatography plates is described. Enzyme gel entrapment with an amphiphilic copolymer was used for assay development. The effects of substrate and enzyme concentrations, pH, incubation time, and incubation temperature on the sensitivity and the detection limit of the assay were evaluated. Experimental design and response surface methodology were used to optimize conditions with a minimum number of experiments. The assay allowed the detection of 0.01% w/w of physostigmine in both a spiked Sonchus oleraceus L. extract chromatographed on normal phase and a spiked Pimenta racemosa (Mill.) J.W. Moore leaf essential oil chromatographed on reversed phase. Finally, the reversed-phase thin-layer chromatography assay was applied to reveal the presence of an inhibitor in the Cymbopogon citratus (DC.) Stapf essential oil. The developed assay is able to detect acetylcholinesterase inhibitors present in complex matrixes that were chromatographed in normal phase or reversed-phase thin-layer chromatography. The detection limit for physostigmine on both normal and reversed phase was of 1×10(-4) μg. The results can be read by a change in color and/or a change in fluorescence.

Chemical diversification of essential oils, evaluation of complex mixtures and identification of a xanthine oxidase inhibitor

A set of chemically engineered essential oils has been generated through chemical diversification by reaction with bromine. The impact of the reaction over the chemical composition of the mixtures was qualitatively demonstrated through GC-MS and utilizing multivariate analysis of 1H NMR and GC-MS. Most of the components of the essential oils are transformed by the reaction expanding the chemical diversity of the mixtures. Biological changes between essential oils and brominated essential oils were demonstrated through image analysis of xanthine oxidase autography profiles. The highest biological activity increase was obtained for the Foeniculum vulgare Mill essential oil. Coupling of xanthine oxidase autography with the BIOMSID strategy allowed the identification of the molecular formula of the active compound. Bioguided fractionation of the mixture led to the isolation of (RS)-2-bromo-1-(4-methoxyphenyl) propan-1-one for being responsible for the observed bioactivity. This xanthine oxidase inhibitor could have been formed from the inactive natural component anethole. The inhibitory potency of this semisynthetic compound was in the same order of magnitude as allopurinol, the most used inhibitor.

Recent Applications of Organotin Oxides/Hydroxides and Alkylstannonic Acids in Organic Synthesis

We describe the application of trimethyltin hydroxide to the solid phase cleavage of the benzyl ester linkage between amino acids/dipeptides and resins. Butylstannonic acid is used as catalyst for transesterification of various carboxylic esters. This method is also applicable to acetylation/deacetylation of alcohols.