Synthesis of and tautomerism in 3-acyltetramic acids

The synthesis of 3-acyltetramic acids, the substructure of bioactive natural products, via O-acylation of tetramic acids with carboxylic acids followed by acyl migration, has been investigated. This acylation sequence is mediated by N,N'-dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) and is very sensitive to the nature of the nitrogen substituent (R(1)), the nature of the carboxylic acid (R(2)CO(2)H), and the amount of DMAP. Acylation of N-acyl tetramic acids with an alkyl carboxylic acid using 1.3 equiv of DMAP (with 1.1 equiv of DCC) unexpectedly gave the 3-acyltetramic acid directly as a result of acyl migration induced by excess amounts of DMAP. On the other hand, N-unsubstituted, N-alkyl, and N-acyl tetramic acids with alkyl and aromatic carboxylic acids gave the O-acyl tetramic acids by using only 0.1 equiv of DMAP (with 1.1 equiv of DCC); these could be further rearranged to the acyl product by treatment with excess DMAP. The tautomeric equilibrium of these 3-acyltetramic acids in solution was found to strongly depend on the nitrogen substituent group (R(1)) rather than the 3-acyl group.     

Polymer-supported 1,3-oxazolium-5-olates: synthesis of 1,2,4-triazoles

A traceless synthesis of 3,5-disubstituted 1,2,4-triazoles has been developed on polymeric supports. The synthetic process utilizes immobilized mesoionic 1,3-oxazolium-5-olates (munchnones) as key intermediates in the 1,3-dipolar cycloaddition reaction. The initial step in the synthesis involves reductive alkylation of phenylglycine methyl esters with Ameba resin. The resulting immobilized amino acid esters were subsequently acylated with a variety of carboxylic acid chlorides and subjected to hydrolysis with 15% KOH to yield the polymer-bound carboxylic acids. Finally, the cycloaddition between diethyl diazocarboxylate or 4-phenyl-4H-1,2,4-triazoline-3,5-dione and the polymer-bound munchnones generated from the corresponding carboxylic acids afforded the polymer-bound 3,5-disubstituted 1,2,4-triazoles. Cleavage from the polymeric support using trifluoroacetic acid gave the desired 3,5-disubstituted 1,2,4-triazoles with excellent yield and high purity.     

Protocols for amide high-speed analoging. Preparation of novel, small molecule cathepsin D inhibitors

Procedures for solution phase parallel synthesis of new, small molecule cathepsin D inhibitors by the coupling of acyl chlorides, sulfonyl chlorides and carboxylic acids with nitrogen nucleophiles have been established using polymer-bound reagents to facilitate the chemistry and/or to efficiently scavenge the unreacted starting materials and reaction by-products. Silver(I) benzoate was used to facilitate the coupling of acyl chlorides with less reactive anilines. Compound 4 was identified as a sub-micromolar cathepsin D inhibitor (IC₅₀=320 nM).     

Novel and efficient method for esterification, amidation between carboxylic acids and equimolar amounts of alcohols, and amines utilizing Me2NSO2Cl and N,N-dimethylamines; its application to the synthesis of coumaperine, a natural chemopreventive dieneamide

Various carboxylic esters or amides were prepared in good to excellent yield between carboxylic acids and equimolar amounts of alcohols or amines under very mild conditions (0-45°C; within 3 h) using dimethylsulfamoyl chloride (Me₂NSO₂Cl; 1) combined with N,N-dimethylamines (Me₂NR: 2a; R=Me, 2b; R=Bu). The choice of the sulfamoyl chloride and the amine is crucial for the reaction; that is, sterically uncrowded amines accelerated the present esterification and amidation. This agent had some advantages over methanesulfonyl chloride (3)/amines as for the atom-economy, avoidance of side reactions, and had very high chemoselectivity toward the carboxyl group vs the hydroxyl group; the experiment was performed by the addition of 1 to the mixture of carboxylic acids and alcohols. Application of this method to the synthesis of coumaperine, a chemopreventive natural product, was performed using the present amidation as a key step.     

Solid-phase synthesis of pyrazolopyridines from polymer-bound alkyne and azomethine imines

Study was made of the 1,3-dipolar cycloaddition of polymer-bound alkynes to azomethine imines generated in situ from N-aminopyridine iodides. Aromatization of the cycloadducts gives polymer-bound pyrazolopyridines that can be released from the resin as carboxylic acids with trifluoroacetic acid or as methyl esters with sodium methoxide.     

Solid-phase synthesis of sn-1,2- and sn-2,3-diacylglycerols via ring-opening of the glycidyl-bound resin

A general method developed for the parallel solid-phase synthesis of sn-1,2- and sn-2,3-diacyglycerol derivatives. The technique relies on the use of carboxylic acid-promoted epoxide ring-opening reactions of the glycidyl-bound resin 3. The polymer-bound monoacylglycerol 5, formed in this manner, is transformed to the respective polymer-bound diacylglycerols 7 and 9 by reaction of the free alcohol moiety with a second carboxylic acid under conditions that lead to retention or inversion of C-2 stereochemistry. The sequence is completed by BCl3-promoted cleavage of 7 and 9 to form the sn-1,2- and sn-2,3-diacylglycerols, respectively.     

Solid-phase synthesis of 1,2,3-triazoles via 1,3-dipolar cycloaddition

The solid-phase synthesis of 1,2,3-triazoles via 1,3-dipolar cycloaddition of polymer-bound azides to various alkynes is reported. Polymer-bound azides were synthesized from polymer-bound halides and sodium azide and reacted with alkynes to produce polymer-bound 1,2,3-triazoles. Cleavage of the triazoles was performed with trifluoroacetic acid. A traceless synthesis of 1,2,3-triazoles was developed using 2-methoxy-substituted resin (polymer-bound 4-hydroxy-2-methoxybenzyl alcohol). In addition, a synthesis of 4-hydroxybenzyl-substituted 1,2,3-triazoles from the bromo-Wang resin (4-(bromomethyl)phenoxymethyl polystyrene) was achieved.     

A Novel and Efficient Approach for the Combinatorial Synthesis of Structurally Diverse Pyrimidines on solid support

We describe a versatile novel approach for the synthesis of 2, 4, 6-trisubstituted pyrimidines on solid support. Thus, polymer-boun J thiouronium salt 2 reacted in high yield in a cyclocondensation reaction with the acetylenic ketones 3 to form, after tert-butyl-ester cleavage, the polymer-bound carboxylic acids 4 , which were cleaved by oxidation with 3-chloroperbenzoic acid and pyrrolidine to form the 2-pyrrolidinylpyrimidine-4-carboxylic acids 6a-c in high yields and purities without further purification (Scheme 1). Alternatively, acid 4a was subjected to an Ugi four-component condensation which gave the polymer-bound Ugi products 9a-e in good yields (Scheme 2). Multidirectional cleavage reaction of sulfone 8a with different nucleophiles resulted in the clean formation of pyrimidine-4-carboxamides 10–13 (Scheme 3). This strategy combines efficiently solid-phase chemistry with a multicomponent reaction and a multidirectional cleavage step to form highly diverse pyrimidines in a parallel array.     

Salen-based chiral fluorescence polymer sensor for enantioselective recognition of α-hydroxyl carboxylic acids

(R,R)-Salen-based chiral polymer P-1 was synthesized by the polymerization of 5,5'-((2,5-dibutoxy-1,4-phenylene)bis(ethyne-2,1-diyl))bis(2-hydroxy-3-(piperidin-1-ylmethyl) benzaldehyde (M-1) with (1R,2R)-cyclohexane-1,2-diamine (M-2) via nucleophilic addition- elimination reaction, and (R,R)-salan-based polymer P-2 could be obtained by the reduction reaction of P-1 with NaBH(4). (R,R)-Salen-based chiral polymer P-1 can exhibit greater fluorescence enhancement response toward (l)-α-hydroxyl carboxylic acids, and the value of enantiomeric fluorescence difference ratio (ef) can reach as high as 8.41 for mandelic acid and 6.55 for lactic acid. On the contrary, (R,R)-salan-based chiral polymer P-2 shows obvious fluorescence quenching response toward α-hydroxyl carboxylic acids. Most importantly, (R,R)-salen-based polymer P-1 can display bright blue fluorescence color change in the presence of (l)-α-hydroxyl carboxylic acids under a commercially available UV lamp, which can be clearly observed by the naked eyes.     

A Preferred Synthesis of 1,2,4‐Oxadiazoles

Abstract

An efficient and high‐yielding one‐pot synthesis of 1,2,4‐oxadiazoles from carboxylic acids and amidoximes is described. Activation of the carboxylic acid using hydroxybenzotriazole (HOBt) and EDC/HCl followed by reaction with an amidoxime generates an oxime ester. Without isolation, the oxime ester is dehydrated to give the oxadiazole ring.     

Regiospecific additions of hydrazoic acid and benzylamine to 1-(arylsulfonyl)bicyclo[1.1.0]Butanes. Application to the synthesis of cis and trans 2,7-methanoglutamic acids

Addition of hydrazoic acid or benzylamine to 1-(arylsulfonyl) bicyclobutanes introduces the nitrogen nucleophlle at position 3 of the derived cyclobutane, even when a carboxyl derivative is present at this position as a second activating group. Precursors of α-amino cyclobutane carboxylic acids may thus be obtained and these can be further transformed to the title diacids via carbonylation α to the sulfone and reduction.     

Investigation of the effects of various parameters on the synthesis of oligopeptides in aqueous solution

Oligomerization efficiency of amino acids in aqueous solution has been compared under different conditions (temperature, activating agent, etc.) using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 1,1^′-carbonyldiimidazole (CDI) as coupling agents. Glycine (H₂N-CH₂-COOH) and α-alanine (H₂N-CH(CH₃)-COOH) were chosen as α-amino acids and β-alanine (H₂N-CH₂-CH₂-COOH) as the β-amino acid. The coupling reaction between EDC and glycine was shown to occur but does not go to completion either at ambient temperature or at 70 °C. The presence of a carboxylic activating agent such as N-hydroxysuccinimide improves the EDC-mediated coupling reaction, and the amino acid structure (α- or β-) was shown to have an influence on the oligomerization efficiency, with β-alanine polymerisation being more efficient. These findings are explained by reference to the reaction mechanism.     

Total selective synthesis of enantiopure O1-acyl-3-aminoalkane-1,2-diols by ring opening of aminoepoxides with carboxylic acids

[reaction: see text] Synthesis of (2R,3S)- or (2S,3S)-O1-acyl-3-aminoalkane-1,2-diols by ring opening of enantiopure (2R,1'S)- or (2S,1'S)-2-(1-aminoalkyl)epoxides 1 or 2, with carboxylic acids in the presence of BF3 x Et2O and chlorotrimethylsilane, is described. The conversion takes place with total selectivity and in good yield. In addition, (2R,3S)-O,O-diacyl-3-aminoalkane-1,2-diols 3 were also prepared from reaction of (2R,1'S)-2-(1-aminoalkyl)epoxides 1 with carboxylic acids under the same reaction conditions and without chlorotrimethylsilane. Mechanisms to explain both transformations are proposed.     

A straightforward, one-pot protocol for the preparation of libraries of 2-oxazolines

A practical protocol for the parallel synthesis of 2-oxazolines using polymer-supported reagents is described. Polymer-supported Mukaiyama reagent is used to couple a carboxylic acid with an amino alcohol, giving a beta-hydroxyamide, which is then cyclized in situ using either polymer-supported sulfonyl chloride resin or polymer-bound 2-fluoropyridinium triflate. Both 2,4-disubstituted and 2,4,5-trisubstituted 2-oxazolines are obtained in high yields and excellent purities after a simple resin filtration and solvent evaporation routine.     

6-oxy-(acetyl piperazine) fluorescein as a new fluorescent labeling reagent for free fatty acids in serum using high-performance liquid chromatography

A new fluorescein-based fluorescent derivatizating reagent, 6-oxy-(acetyl piperazine) fluorescein (APF), has been designed, synthesized and developed for carboxylic acid labeling. It was used as a pre-column derivatizing reagent for the determination of seven free fatty acids (lauric acid, myristic acid, arachidonic acid, linoleic acid, palmitic acid, oleic acid, and stearic acid) with high-performance liquid chromatography (HPLC). The derivatization reaction of APF with seven fatty acids was completed at 60 degrees C for 1 h using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) as the condensing reagent. On a C18 column, the derivatives of APF with seven free fatty acids could be separated completely in 22 min using a mobile phase of methanol-water (88:12, v/v) containing 7 mmol L(-1) pH 6.5 Na2HPO4-H3Cit3 buffer with fluorescence detection at lambdaex/lambdaem=467/512 nm. The detection limits could reach 0.1-6.4 nmol L(-1) (signal-to-noise=3). This reagent was applied to the determination of the free fatty acids in human serum samples with satisfying recovery efficiencies varying from 93 to 105%.     

A convenient parallel synthesis of low molecular weight hydroxamic acids using polymer-supported 1-hydroxybenzotriazole

A convenient two-step procedure for the parallel synthesis of hydroxamic acids from carboxylic acids and hydroxylamine in good to high yields is reported. It involves the formation of a polymer-bound HOBt active ester and subsequent reaction with O-protected or free hydroxylamine. The hydroxamates are isolated with high purities by simple evaporation of volatile solvents. The use of free hydroxylamine leads to increased yields while maintaining high purities. Recycling of the spent resin to produce the same or a different hydroxamic acid has been achieved by a three-step protocol which is easily amenable to automation and cost-economical. The method presented here is well suited to the preparation of the title compounds and can be used effectively to synthesise large molecules containing a hydroxamic acid group.     

A new route to trans-2,6-disubstituted piperidine-related alkaloids using a novel C2-symmetric 2,6-diallylpiperidine carboxylic acid methyl ester

A novel C2-symmetric 2,6-diallylpiperidine carboxylic acid methyl ester 1 was prepared by the double asymmetric allylboration of glutaldehyde followed by an aminocyclization and carbamation. On the basis of desymmetrization of 1 using iodocarbamation, one allyl group of 1 was protected and monofunctionalizations of the resulting oxazolidinone 11 were performed. The reaction of the N-methoxycarbonyl piperidine 25 employing decarbamation reagent (n-PrSLi or TMSI) as a key step gave oxazolidinone 26 or 17 including an intramolecular ring formation, which was transformed in a few steps into (-)-porantheridine (2) and (-)-2-epi-porantheridine (3), respectively. In addition, the expedient synthesis of (+)-epi-dihydropinidine (4), (2R,6R)-trans-solenopsin A (5), and precoccinelline (6), starting from 11 is described.     

A new protecting group and linker for uridine ureido nitrogen

(2,6-Dichloro-4-methoxyphenyl) (2,4,6-trichlorophenyl) methoxymethyl chloride [1, monomethoxydiphenylmethoxylmethyl chloroide (MDPM-Cl)] shows a significant relative stability and 1 reacts with uridine ureido nitrogen in the presence of DBU to form the corresponding protected uridine 8 in 95% yield. The MDPM-protected uridines are stable to a wide variety of conditions utilized for the synthesis of analogs of capuramycin and muraymycins. Significantly, the MDPM protecting group can conveniently be deprotected by using 30% TFA in CH(2)Cl(2). In addition, polymer-bound MDPM-Cl 23 is useful for immobilization of uridine derivatives.     

Derivatisation for liquid chromatography/electrospray mass spectrometry: synthesis of pyridinium compounds and their amine and carboxylic acid derivatives

A simple method has been developed for the pre-column derivatisation of low molecular weight primary and secondary amines and carboxylic acids using quaternary nitrogen compounds to enhance their detection by liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS). The synthesis of seven novel quaternary nitrogen reagents is described. The derivatives are designed to be relatively small molecules to avoid some of the steric hindrance problems that may be associated with larger derivatisation reagents. The compounds have amine and carboxylic acid functional groups with which to derivatise carboxylic acids and amines, respectively. Two of the compounds contain a bromine atom in order to assess the advantages of a bromine isotope pattern in the mass spectra. This acts as a simple marker for derivatisation and enables data processing by cluster analysis.Activation of the carboxylic acid group was achieved by the use of either 1-chloro-4-methylpyridinium iodide (CMPI) or the more reactive 1-fluoro-4-methylpyridinium p-toluenesulphonate (FMP).1 Using both of these active reagents, the degree of nucleophilic substitution was investigated for the derivatisation of a variety of small molecules. Whilst giving some increase in the ESI-MS response for the derivatised compounds, the FMP itself acted as a derivatising reagent in a competing reaction. In the light of this finding, FMP was reacted with the test compounds separately and gave positive results as a derivatising reagent. Detection of the 'pre-charged' derivatives of amines and carboxylic acids by LC/ESI-MS was investigated with respect to their ESI response and chromatography.     

Ammonium salts from polymer-bound N-hydroxysuccinimide as solid-supported reagents for EDC-mediated amidations

New ammonium and alkylammonium salts derived from a polymeric N-hydroxysuccinimide (P-HOSu) have been prepared and used for the amidation of carboxylic acids and amino acids mediated by 1-ethyl-3-(3′-dimethylamino-propyl)carbodiimide hydrochloride (EDC). These polymer-supported ammonium salts afforded the corresponding amides in good yield, without detectable α-racemization and with easy recovery of the P-HOSu after the amidation reaction, being especially suitable for the amidation of Fmoc-protected amino acids.