Convergent synthesis of 4,6-unsubstituted 5-acyl-2-aminodihydropyrimidines using Weinreb amide

A method of convergent synthesis of novel 4,6-unsubstituted 5-acyl-2-aminodihydropyrimidines 7 is developed. The synthetic intermediate of 7, 4,6-unsubstituted 2-aminodihydropyrimidines 9 having a Weinreb amide at the 5-position, is prepared by the sequential Staudinger/aza-Wittig/cyclization reactions of (E)-tert-butyl{3-azido-2-[methoxy(methyl)carbamoyl]allyl}carbamate (E)-10. The transformation of the Weinreb amide of 9 to an acyl group proceeds smoothly by a substitution reaction using aryllithiums or alkyllithiums in the presence of a catalytic amount of BF₃ etherate, affording 7 in good to high yield. The N-protecting group of 7 can be easily removed to obtain N-unsubstituted 2-amino-5-acyldihydropyrimidines 8, and the derivatives are observed as a single isomer in ¹H NMR spectroscopy. All dihydropyrimidines in this study were hitherto unavailable and difficult to synthesize by conventional methods.     

Microwave-assisted cleavage of Weinreb amide for carboxylate protection in the synthesis of a (R)-3-hydroxyalkanoic acid

A versatile route for the modular synthesis of (R)-3-hydroxyalkanoic acids, constituents of the naturally biodegradable poly(3-hydroxyalkanoate) polymers, and its application to the synthesis of (R)-3-hydroxydecanoic acid is described. Key steps include a microwave-assisted catalytic transfer hydrogenation and a facile microwave-assisted hydrolysis of an N-methoxy-N-methyl (Weinreb) amide, which enhances the practicality of this protecting group for carboxylic acids.     

Mild Amide‐Cleavage Reaction Mediated by Electrophilic Benzylation

An extremely mild method for amide‐cleavage by using the triazine‐based benzylating reagent 4‐(4,6‐diphenoxy‐1,3,5‐triazin‐2‐yl)‐4‐benzylmorpholinium trifluoromethanesulfonate (DPT‐BM), which spontaneously releases benzyl cation species when being dissolved at room temperature, has been developed. O‐Benzylation of the amide with DPT‐BM and the subsequent hydrolysis of the resulting intermediate benzyl imidate salt afford the corresponding amine and benzyl ester, which can be converted by hydrogenolysis into a carboxylic acid under neutral conditions. O‐Benzylation proceeds depending on both steric and electronic factors around the amide group. Thus, some amides have been selectively cleaved over other amides. Furthermore, intramolecular chemoselective cleavage of an amide group in the presence of an ester group was achieved. Such selective hydrolytic reactions cannot be performed with Meerwein reagents as well as under acidic or basic hydrolytic conditions.     

One-pot synthesis of Weinreb amides employing 3,3-dichloro-1,2-diphenylcyclopropene (CPI-Cl) as a chlorinating agent

The synthesis of N^α-protected amino alkyl Weinreb amides starting from the corresponding α-amino acids as well as carboxylic acids has been delineated through the in situ generation of acid chlorides using CPI-Cl as a chlorinating agent. The protocol is simple; the reaction conditions employed were mild, and compatible with all the three commonly used urethane protecting groups namely, Boc, Cbz and Fmoc groups. The resulting Weinreb amides are obtained in good yields as optically pure products.     

Cross-metathesis of allyl halides with olefins bearing an α-alkoxy amide group

We have examined whether the allyl halide cross-metathesis reaction tolerates α-alkoxy amide groups. Ruthenium-based catalysts I-III did not catalyze the cross-metathesis of allyl halides in the presence of an α-alkoxy N,N-dimethylamide group to any appreciable extent, but the reaction could tolerate either a bulky N,N-diisopropylamide or Weinreb amide group. In particular, the Grubbs-Hoveyda-Blechert 2nd generation catalyst (III) efficiently catalyzed the cross-metathesis of allyl halides with olefins bearing a Weinreb amide group.     

有机锌试剂制备Weinreb酰胺的研究

探索了带Weinreb酰胺官能团的有机锌试剂的制备及其与酰氯的反应, 得到了另一类Weinreb酰胺, 提供了一种制备Weinreb酰胺的新方法.     

Carbamoylimidazolium salts as diversification reagents: an application to the synthesis of tertiary amides from carboxylic acids

An efficient method for the preparation of tertiary amides from carbamoylimidazolium salts and carboxylic acids is described. The transformation occurs at room temperature and under relatively mild conditions. The carbamoylimidazolium salts are obtained from the reaction of secondary amines with N,N′-carbonyldiimidazole, followed by methylation with methyl iodide. The utility of this reaction was demonstrated in the formation of Weinreb amides and in a short synthesis of fused bicyclic amides. The introduction of this reaction now permits carbamoylimidazolium salts to be utilized in the formation of tertiary amides, ureas, carbamates and thiocarbamates under a single set of conditions.     

A practical catalytic reductive amination of carboxylic acids

We report reductive alkylation reactions of amines using carboxylic acids as nominal electrophiles. The two-step reaction exploits the dual reactivity of phenylsilane and involves a silane-mediated amidation followed by a Zn(OAc)₂-catalyzed amide reduction. The reaction is applicable to a wide range of amines and carboxylic acids and has been demonstrated on a large scale (305 mmol of amine). The rate differential between the reduction of tertiary and secondary amide intermediates is exemplified in a convergent synthesis of the antiretroviral medicine maraviroc. Mechanistic studies demonstrate that a residual 0.5 equivalents of carboxylic acid from the amidation step is responsible for the generation of silane reductants with augmented reactivity, which allow secondary amides, previously unreactive in zinc/phenylsilane systems, to be reduced.

We report reductive alkylation reactions of amines using carboxylic acids as nominal electrophiles.     

Efficient synthesis of the 6,6-spiroacetal of spirofungin A

The 6,6-spiroacetal segment of spirofungin A, an antifungal antibiotic isolated from Streptomyces violaceusniger Tü 4113, was efficiently prepared via the coupling reaction of the Weinreb amide and the alkyne which are readily available from the common intermediate. The synthesis includes the unsymmetrization through a stereoinversion at the C11 position and the transacetalization as the key steps.     

Aluminum dodecatungstophosphate (AlPW12O40) as a non-hygroscopic Lewis acid catalyst for the efficient Friedel-Crafts acylation of aromatic compounds under solvent-less conditions

Stable and non-hygroscopic aluminum dodecatungstophosphate (AlPW₁₂O₄₀), which is prepared easily from cheap and commercially available compounds was found to be an effective catalyst for Friedel-Crafts acylation reactions using carboxylic acids, acetic anhydride and benzoyl chloride in the absence of solvent under mild reaction conditions.     

Synthesis of Some Benzotriazole-Substituted Perimidines

2-substituted perimidines 3(a–g) were prepared from an acid-catalyzed reaction of 1,8-diaminonaphthalene with carboxylic acids using microwave irradiation. Addition of these perimidines to 1-chloromethyl benzotriazole in the presence of the sodium amide under reflux conditions gave benzotriazole-substituted perimidines 5(a–f). Yields of these products following recrystallization from water were of the order of 60–65%. IR and ¹HNMR spectra and elemental analysis were used for identification of these compounds.     

Facile amidation of esters with aromatic amines promoted by lanthanide tris (amide) complexes

The development of catalysts capable of catalyzing amidation of esters with amines to construct amides under mild conditions is of great importance. Compared to aliphatic amines, the direct catalytic amidation of esters with less nucleophilic aromatic amines is rather difficult. Employing simple lanthanide tris (amide) complexes Ln[N (SiMe₃)₂]₃(μ-Cl)Li (THF)₃ as the catalysts, it was found a broad range of aromatic amines and esters were efficiently converted into various amides in good yields under mild conditions. A plausible mechanism for this transformation was experimentally supported as starting from an amide exchange reaction between the lanthanide tris (amide) complex and the substrate amine.     

Preparation of Amides of Carboxylic Acid Herbicides for Gas Chromatographic Analysis

Abstract

Amides of 14 carboxylic acid herbicides were prepared by reacting the free acid with the amine in toluene for 1 hr at 80[ddot]C in the presence of PCl₃ or P₂I₄. The acids include phenoxyacetic acids, arylacetic acids, and benzoic acids. Aniline, o-toluidine, 3,5-bis(trifluoromethyl)aniline, piperidine, and tetrahydroquinoline were the amine components. Excess of reagents and by-products of the reaction were removed by partitioning into aqueous acid and base. Retention times relative to 2,4-D anilide on 1% OV-22 and FSOT RSL-150 columns are listed for the anilides and should be useful for confirmation purposes. The anilides of 2,4-D, silvex and 2,4,5-T were obtained in better than 90% yield.     

A Photochemical Protocol for the Synthesis of Weinreb and Morpholine Amides from Carboxylic Acids

In recent years, the development of light-mediated methods for synthetic applications has attracted high interest, because such methods constitute alternative approaches for the production of valuable chemical products. We study herein a photochemical protocol for the synthesis of Weinreb amides and morpholine amides from carboxylic acids. Various carboxylic acids were directly coupled to N,O-dimethylhydroxylamine, upon irradiation with either LED 370 nm or sunlight in the presence of 4-dimethylaminopyridine and bromotrichloromethane, providing Weinreb amides in moderate to high yields. Similarly, morpholine amides were obtained in satisfactory to high yields under sunlight or LED 370 nm irradiation. Thus, the versatile building blocks Weinreb amides and morpholine amides may be efficiently synthesized directly from carboxylic acids by light-mediated approaches, without the need of coupling reagents or pre-activation of carboxylic acids.     

Amino Acid Based Epoxy-Poly(Ester Amide)s—A New Class of Functional Biodegradable Polymers: Synthesis and Chemical Transformations

A new class of biodegradable functional polymers – epoxy-poly(ester amide)s (EPEAs), was synthesized for the first time on the basis of naturally occurring α-amino acids, fatty diols and cis- and trans-epoxysuccinic acids. The syntheses were conducted via both solution Active Polycondensation and Interfacial Polycondensation using, accordingly, either activated di-p-nitrophenyl esters or dichlorides of cis- and trans-epoxysuccinic acids as bis-electrophilic monomers. Di-p-toluenesulfonic acid salts of bis-(L-phenylalanine)-1,6-hexylene and bis-(L-leucine)-1,6-hexylene diesters were used in both cases as bis-nucleophilic counter-partners. High-molecular-weight polymers (M_w up to 67.000) with desirable material properties were obtained via solution polycondensation using di-p-nitrophenyl-trans-epoxy succinate. The EPEAs were chemically modified further under mild conditions: oxirane groups along the backbone were reacted with both nucleophilic and electrophilic reagents, and also subjected to thermal and chemical curing. The macromolecular transformations of EPEAs substantially broaden material properties and, hence, the potential to apply amino acid-based biodegradable poly(ester amide)s as absorbable drug carriers and surgical devices.     

Ruthenium‐Catalyzed Reductive Arylation of N‐(2‐Pyridinyl)amides with Isopropanol and Arylboronate Esters

A new three‐component reductive arylation of amides with stable reactants (iPrOH and arylboronate esters), making use of a 2‐pyridinyl (Py) directing group, is described. The N‐Py‐amide substrates are readily prepared from carboxylic acids and PyNH₂, and the resulting N‐Py‐1‐arylalkanamine reaction products are easily transformed into the corresponding chlorides by substitution of the HN‐Py group with HCl. The 1‐aryl‐1‐chloroalkane products allow substitution and cross‐coupling reactions. Therefore, a general protocol for the transformation of carboxylic acids into a variety of functionalities is obtained. The Py‐NH₂ by‐product can be recycled.     

Lanthanide Amide-catalyzed Aza-Henry Reaction of N-Tosyl Imines with Nitroalkanes

In the presence of 10 mol% lanthanide amide [(Me₃Si)₂N]₃Ln(µ‐Cl)Li(THF)₃, the aza‐Henry reaction of N‐tosyl imines with nitroalkanes (1:5 molar ratio) could be performed in good yields. The lanthanide amide‐catalyzed aza‐Henry reaction has the features of mild reaction conditions, tolerance of a variety of aromatic aldehyde‐derived imines and nitroalkanes, short time and good chemical yields. A catalytic mechanism for the reaction was also proposed.     

Isocyanide based multicomponent reactions of oxazolidines and related systems

N-Alkyloxazolidines react in a multicomponent reaction with carboxylic acids and isocyanides to give N-acyloxyethylamino acid amides. The previously reported reaction conditions were improved using a design of experiments approach (DoE). Under the optimised conditions, good yields of the N-acyloxyethylamino acid amide products are obtained both via a three- or four-component approach from N-alkylethanolamines, aldehydes/ketones, isocyanides and carboxylic acids. The reaction of oxazolidines without a nitrogen substituent was found to give either the expected Ugi products or the N-acyloxyethylamino acid amides depending on the choice of reaction conditions. Optimised reaction conditions were also developed for the ring-expansion of oxazolidines to morpholin-2-ones via reaction with an isocyanide followed by hydrolysis. The mechanistic pathway of the multicomponent reaction was briefly investigated using an ¹⁸O labelling experiment. The carboxylic acid component can be replaced by a range of other acidic nucleophiles including thiobenzoic acid, thiophenol or 5-phenyltetrazole, which are incorporated via an alternative pathway. These latter reactions can also be applied to 2-aminotetrahydrofurans, 2-aminotetrahydropyrans or 4-hydroxybut-2-one, further extending the structural diversity of the multicomponent reaction products.     

Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

The metal-free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO(OCCl₃)₂) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP-catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP-mediated H₂-activation.     

Lipase-catalyzed enantioselective reaction of amines with carboxylic acids under reduced pressure in non-solvent system and in ionic liquids

Lipase-catalyzed enantioselective acylation of 1-phenylethylamine and 2-phenyl-1-propylamine was performed by reacting the amines with carboxylic acids in a non-solvent system or in ionic liquids as reaction media. The reaction equilibrium was shifted toward amide synthesis by the removal of formed water under reduced pressure.