Convergent multicomponent assembly of 2-acyloxymethyl thiazoles

Diverse substituted 2-acyloxymethyl thiazoles can be assembled by a new multicomponent reaction (MCR) of methyl 3-(N,N-dimethylamino)-2-isocyanoacrylate, aldehydes and thiocarboxylic acid under Lewis acid catalysis. The reaction is performed under mild conditions and is compatible with a wide range of functionalized starting materials. 13 examples are given.     

α-Amino acids as acid components in the Passerini reaction: influence of N-protection on the yield and stereoselectivity

The Passerini reaction offers an easy access to depsipeptides, when both acid and isocyanide are derived from α-amino acids. However, racemisation of isocyanides derived from α-amino acid esters severely limits their use in the Passerini reaction. In order to overcome this limitation, a study on the influence of the α-amino acid N-protecting group on the yield and diastereoisomeric ratio of the product of the Passerini reaction was performed. Six different protecting groups were tested. Their influence turns out to be crucial and is not constant when the amino acid is changed. After optimisation, the Passerini reaction products with cyclohexanone as the carbonyl component were obtained with 99% yield and >98% de.     

Novel catalysts for the enantioselective Henry reaction

Two novel catalytic systems based on the Cu^II complexes with N-(3,5-dibromo-2-hydroxybenzyl)- and N-(2-hydroxy-3-nitrobenzyl)-(S)-α,α-diphenylprolinols were developed. These systems catalyze the condensation of 4-nitrobenzaldehyde with nitromethane to produce S-nitroaldol with maximum enantiomeric excess of >90% (99% yield). The reactions of nitromethane with aliphatic aldehydes give the corresponding products in the yields above 80% and ee > 90%.     

PADAM reactions of α-aminoaldehydes: Identity of major and minor diastereomers from the Passerini reaction

The Passerini reaction was conducted using N-Boc-L-phenylalaninal and a variety of achiral or chiral acids and isocyanides to prepare a library of 27 Passerini products. Reaction diastereoselectivity varied between 1.7:1 and 2.5:1 and in most cases isolation of the individual diastereomers was possible. Passerini products were subjected to a deprotection and acyl migration sequence to give a library of peptidomimetic α-hydroxy-β-acylaminoamides in generally excellent yield. Single crystal X-ray analysis of two of the final products enabled identification of the major diastereomer as having an (R) configuration at the newly-formed stereogenic centre, which corresponds to anti-Felkin-Anh addition of the isocyanide to the aldehyde.     

A direct link between the Passerini reaction and α-lactams

α-Lactams (aziridinones) can function to replace two of the three reactants, the oxo-compound and the isonitrile, in the Passerini reaction. Four α-lactams (5a-d) were reacted with mono- and dicarboxylic acids of positive pK_a values to give 2-acyloxycarboxamides (4) and bis-2-acyloxycarboxamide products 12 and 13, respectively. The same compounds were also prepared via the Passerini reaction. Acids with a negative pK_a decarbonylate α-lactams to give immonium salts. The main path of the reaction depends on the pK_a of the acid component, the reactivity of the α-lactam, and the reaction conditions.     

Asymmetric induction in the conjugate addition of thioacetic acid to methacrylamides with chiral auxiliaries

The conjugate addition of thioacetic acid to methacrylamides with chiral C₂-symmetric trans-2,5-disubstituted pyrrolidines afforded the addition products in excellent stereoselectivities (>99% de) and good yields (80–90%). The high selectivity was attributed mainly to the steric effect of the chiral auxiliaries. The cyclic nature of the chiral auxiliaries seemed also important for both the stereoselectivity and the reaction rate. Acidic hydrolysis of the adduct containing (2R,5R)-bis(methoxymethyl)pyrrolidine gave (S)-3-mercapto-2-methylpropanoic acid, a key intermediate for captopril, in 98% ee and 96% yield. The chiral auxiliary was recovered in the demethylated form of N-Boc-(2R,3R)-bis(hydroxymethyl)pyrrolidine in 90% yield.     

A one-pot synthesis of a branched tertiary phosphine oxide from red phosphorus and 1-(tert-butyl)-4-vinylbenzene in KOH-DMSO: an unusually facile addition of P-centered nucleophiles to a weakly electrophilic double bond

Red phosphorus reacts with 1-(tert-butyl)-4-vinylbenzene in a superbase media (KOH-DMSO, 90-100 °C, 3 h) to give tris[4-(tert-butyl)phenethyl]phosphine oxide in 77% yield. Microwave activation of the reaction affords the phosphine oxide in 82% yield in 6 min.     

New mannose-derived ketones as organocatalysts for enantioselective dioxirane-mediated epoxidation of arylalkenes. Part 3: Chiral ketones from sugars

New d-arabino-hexopyranosid-3-uloses were synthesized by a simple method from mannopyranoside derivatives. The common skeleton possesses a tunable alkoxy group as steric sensor on carbon 2 of the sugar. The new ketones were employed in the dioxirane-mediated epoxidation of a range of trans- and trisubstituted arylalkenes giving enantiomeric excesses from low to good (30-90%). The effect of the size of the steric sensor on the enantioselectivity was also studied. The least bulky group (methoxy group) enhanced the stereoselectivity (up to 90% ee toward triphenylethylene).     

A practical, multigram synthesis of the 2-(2-(4-alkoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide (DPA) class of high affinity translocator protein (TSPO) ligands

A practical, multigram approach to the 2-(2-(4-alkoxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide (DPA) class of ligands targeting the translocator protein (TSPO) is described. This synthetic route offers several improvements over all previously described sequences, including the isolation of intermediates without resort to chromatography. The common precursor to the DPA class of high affinity TSPO ligands, N,N-diethyl-2-(2-(4-hydroxyphenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide, was produced in 40% yield over six steps, and was cleanly alkylated to give multigram quantities of several DPA analogues in 90-96% yield after recrystallization.     

A new type of organocatalyst for highly stereoselective Michael addition of ketones to nitroolefins on water

(S)-2-((Naphthalen-2-ylsulfonyl)methyl)pyrrolidine, prepared in three steps from (S)-N-Boc-2-[((4-toluenesulfonyl)oxy)methyl]pyrrolidine in 62% overall yield, was used as a new type of organocatalyst bearing a pyrrolidine and a sulfone moiety. It shows very high catalytic activity toward the direct asymmetric Michael reaction of cyclohexanone and nitroolefins. All the corresponding adducts can be furnished in 90-99% yields and with up to 98% ee and over 99:1 dr on water in the presence of this catalyst (15 mol %) without any additive.     

Double nucleophilic reaction of amines to the imidazole nucleus and selective synthesis of 5-aminoimidazoles

Reaction of 2-(1-chloro-2,2-dimethylpropyl)-1-methyl-1H-imidazole with an excess of N,N-dimethylamine at room temperature gave an abnormal adduct, trans-4,5-bis(dimethylamino)-1-methyl-2,2-dimethylpropyl-2-imidazoline, which was derived from a serial, double nucleophilic addition into the imidazole nucleus in 74% yield together with a normal S_N product, 1-methyl-2-(1-dimethylamino-2,2-dimethylpropyl)-1H-imidazole in 15% yield. The former was easily converted to 1-methyl-5-(dimethylamino)-2-(2,2-dimethylpropyl)-1H-imidazole by only reflux in toluene in 90% yield. The scope, mechanism and limitation of these reactions are discussed.     

Enantioselective synthesis of (S)-timolol via kinetic resolution of terminal epoxides and dihydroxylation of allylamines

An efficient enantioselective synthesis of (S)-timolol has been described using chiral Co-salen-catalyzed kinetic resolution of less expensive (±)-epichlorohydrin with 3-hydroxy-4-(N-morpholino)-1,2,5-thiadiazole in good overall yield (55%) and excellent enantioselectivity (98%). Synthesis of (S)-timolol has also been achieved using hydrolytic kinetic resolution as well as asymmetric dihydroxylation routes in 90% ee and 56% ee, respectively.     

Lipase-catalysed selective monoacylation of 1,n-diols with vinyl acetate

A simple enzymatic methodology for the selective monoacetylation of 1,n-diols (n=5,8) using vinyl acetate is reported. Monoacetylation excesses of 81-87% at 74-90% 1,n-diol conversions were obtained in toluene and diisopropyl ether using Thermomyces lanuginosus lipase (TLL) immobilised on polypropylene powder as catalyst.     

Preparation of molecularly imprinted polymers with thiourea group for phosphate

Molecularly imprinted polymers selective for phosphate were prepared with the two types of functional monomers, 1-allyl-2-thiourea and N-methyl-N′-(4-vinylphenyl)-thiourea, and the binding abilities of the polymers were evaluated. Phenylphosphonic acid or diphenyl phosphate were used as the template molecules and the imprinted polymers prepared with 1-allyl-2-thiourea as functional monomer showed high binding ability to phosphate in aqueous media and nearly 90% of phosphate could be recovered. Also, the imprinted polymer prepared with N-methyl-N′-(4-vinylphenyl)-thiourea as functional monomer had a high binding ability and specific interaction with phosphate in acetonitrile solution and over 90% of phosphate-derivatives could be recovered selectively.     

A new titanate/(+)-(1R,2S)-cis-1-amino-2-indanol system for the asymmetric synthesis of (S)-tenatoprazole

Tenatoprazole, a substituted imidazopyridinyl derivative, is an irreversible proton pump inhibitor (PPI), which is used for the prevention and treatment of gastric acid-related diseases. A new highly efficient asymmetric oxidation using cumene hydroperoxide (CHP) as the oxidant in the presence of titanium tetraisopropoxide (Ti(OiPr)₄) and (+)-(1R,2S)-cis-1-amino-2-indanol, in a polar aprotic solvent at 0-20 °C, has been developed to prepare tenatoprazole with an enantiomeric excess of >99%, a chemoselectivity of >90% and a chemical yield of >90% from the corresponding sulfide. This procedure was successfully implemented on scales ranging from 100 mg to multiple kilograms. Detailed studies of the parameters controlling purity and yield for this reaction are presented.     

Phosphine-free Pd-salen complexes as efficient and inexpensive catalysts for Heck and Suzuki reactions under aerobic conditions

Phosphine-free palladium-salen complexes, N,N′-bis(salicylidene)-ethylenediamino-palladium and N,N′-bis(salicylidene)-1,2-phenylenediamino palladium, are found to be highly active catalysts for the Heck olefination of aryl iodides and Suzuki reaction of aryl iodides and bromides giving excellent yields (70-90%) of products under aerobic conditions, in short reaction times (10-60 min).     

Separation of secondary alcohols via enzymatic kinetic resolution using fatty esters as reusable acylating agents

A two consecutive step procedure for the resolution-separation of secondary alcohols employing ethyl tetradecanoate in the presence of lipase allowed the enzymatic kinetic resolution of two target molecules, 1-phenylethanol and 6-methylhept-5-en-2-ol. (S)-1-Phenylethanol was isolated in a yield of 47% with an ee of 94% and (R)-1-phenylethanol in a yield of 51% with an ee of 95%. (S)-6-Methylhept-5-en-2-ol was isolated in a yield 47% and an ee of 87% and (R)-6-methylhept-5-en-2-ol in a yield 49% and an ee of 90%.     

Sol–gel synthesis,characterization and water vapor adsorption properties of 1,1′-(1,6-hexanediyl)-bis(imidazolium)dichloride-silica hybrid material

1,1′-(1,6-hexanediyl)-bis(imidazolium)dichloride-silica hybrid material was prepared in 90 % yield by ammonia catalyzed sol–gel condensation of 1,1′-(1,6-hexanediyl)-bis[3-(3-triethoxysilylpropyl)-imidazolium]dichloride in aqueous ethanol. This novel ionic liquid-silica hybrid material can adsorp 27 % w/w water vapor in 90 min at room temperature and atmospheric pressure. This is a higher water adsorption capacity and rate than commercial desiccant Drierite which showed only 16 % w/w of water vapor adsorption and required 10 h to reach the maximum adsorption under identical conditions. The new adsorbent could be reused for five cycles without any significant loss in activity after regeneration by heating at 110 °C, for 24 h.     

Synthesis of oxazolidinylphosphine chalcogenides from aminoethyl vinyl ethers

N-(2-Vinyloxyethyl)phosphinothioamides and -phosphinoselenoamides prepared by oxidative cross-coupling of 2-vinyloxyethylamine with secondary phosphine chalcogenides undergo thermal (75–100 °C) cyclization into the corresponding 3-(diorganylchalcogenophosphoryl)-2-methyl-1,3-oxazolidines in 80–90% yields.     

An efficient multicomponent stereoselective synthesis of 1,2,4-trisubstituted 1,3-thiazetidines

The first one-pot three-component coupling reaction of O,O-diethyl hydrogen phosphorodithioate, aldehydes, and aldimines affording 1,2,4-trisubstituted 1,3-thiazetidines is reported. The product is obtained in moderate to high yields (45-94%) and has excellent diastereoselectivity (90-96%) in favour of the cis isomer. Shorter reaction times, ambient temperature, operational simplicity, and high yields are the salient features of the present procedure.