Reactivity of imidazolidin-4-one derivatives of primaquine: implications for prodrug design

In contrast to peptide-based imidazolidin-4-ones, those synthesized from N-(α-aminoacyl) derivatives of the antimalarial drug, primaquine and ketones are unexpectedly stable in pH 7.4 at 37 °C. The kinetics of hydrolysis of primaquine-based imidazolidin-4-ones were investigated in the pH range 0.3-13.5 at 60 °C. The hydrolysis to the parent α-aminoacylprimaquine is characterized by sigmoidal-shaped pH-rate profiles, reflecting the spontaneous decomposition of both unionized and protonated (at N-1) forms of the imidazolidin-4-one. The kinetically determined pK_a values are ca. 3.6-4.0, i.e., 4 pK_a units lower than those of amino acid amides, thus implying that hydrolysis of imidazolidin-4-ones at pH 7.4 involves the unionized form. Reactivity of this form decreases with the steric crowding of the amino acid α-substituent. In contrast, the rate constant for the spontaneous decomposition of the unionized form increases sharply for imidazolidin-4-ones derived from cyclic ketones, an observation that can be explained by the I-strain (internal strain) effect. These results are consistent with a mechanism of hydrolysis involving an S_N1-type unimolecular cleavage of the imidazolidin-4-one C2-N3 bond with departure of an amide-leaving group. The mechanism for the decomposition of the protonated imidazolidin-4-one is likely to involve an amide-carbonyl oxygen protonated species, followed by the C2-N3 bond scission, as supported by computational studies. The results herein presented suggest that imidazolidin-4-ones derived from simple N-alkyl α-aminoamides are too stable and therefore, may be useful as slow drug release prodrugs.     

Synthesis of saturated 1,4-benzodiazepines via Pd-catalyzed carboamination reactions

A new synthesis of 1,4-benzodiazepines and 1,4-benzodiazepin-5-ones is reported. The Pd-catalyzed coupling of N-allyl-2-aminobenzylamine derivatives with aryl bromides affords the heterocyclic products in good yield, and substrates bearing allylic methyl groups are transformed to cis-2,3-disubstituted products with >20:1 dr.     

Stereoselective synthesis of N-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides

The stereoselective synthesis of N-acyl- and N-Boc-protected pyrrolidines via Pd-catalyzed reactions of γ-(N-acylamino) alkenes and γ-(N-Boc-amino) alkenes with aryl bromides is described. These reactions effect formation of two bonds in a single operation and proceed with generally high levels of diastereoselectivity. In contrast to previously described reactions of γ-(N-arylamino) alkenes, these transformations proceed in high yield and high regioselectivity with both electron-rich and electron-deficient aryl bromides as well as vinyl bromide substrates.     

Novel route of the reaction of trifluoromethyl-containing N-methyl(4-ethoxyphenyl)imidazolidin-2-ones with urea

The reactions of perfluorobiacetyl with N-(4-ethoxyphenyl)- and N-methylurea afforded cis- and trans-isomers of 1-methyl-4,5-dihydroxy-4,5-bis(trifluoromethyl)imidazolidin-2-one and 1-(4-ethoxyphenyl)-4,5-dihydroxy-4,5-bis(trifluoromethyl)imidazolidin-2-one in a yield of ～60—75%. N-Alkyl(aryl)bis(trifluoromethyl)imidazooxazoles were obtained as unexpected products in the reaction of 1-alkyl(aryl)-4,5-dihydroxy-4,5-bis(trifluoromethyl)imidazolidin-2-ones with urea in dimethylacetamide. The reaction is accompanied by the rearrangement of imidazolidin-2-ones to N-alkyl(aryl)-5,5-bis(trifluoromethyl)hydantoins with CF₃ group migration from position 5 to position 4 of the starting heterocycle. A similar rearrangement is observed on boiling of the studied imidazolidin-2-ones in dimethylacetamide. The molecular structures of 3-(4-ethoxyphenyl)-5,5-bis(trifluoromethyl)imidazolidine-2,4-dione and 6-(4-ethoxyphenyl)-3a,6a-bis(trifluoromethyl)tetrahydroimidazo[4,5-d]oxazole-2,5-dione were studied by X-ray diffraction analysis.     

A new class of readily available and conformationally rigid phosphino-oxazoline ligands for asymmetric catalysis

A new class of conformationally rigid phosphino-oxazoline ligands 3 were synthesized via an efficient ortho-substitution of phenyl glycinol as the key step. Divergent synthetic routes for easy ligand modulation, as well as a procedure suitable for scale-up synthesis, were established. The catalytic potential of ligands 3 has been demonstrated in the highly enantioselective Ir-catalyzed hydrogenation of alkenes and Pd-catalyzed allylic substitution and intermolecular Heck reactions.     

Electrophilic Substitution of Hydrogen in Betulin and Diacetylbetulin

Betulin and diacetylbetulin, which can be regarded as sterically hindered alkenes, reacted with N-chloro-, N-bromo-, and N-iodosuccinimides to give products of allylic and vinylic substitution in quantitative overall yield. The contribution of allylic substitution increases in the series Cl < Br < I. Quantum chemical simulation of the reactions of diacetylbetulin with N-halosuccinimides showed that, regardless of the electrophile power, all reactions involve open-chain carbocationic intermediates. The direction of deprotonation of the latter with formation of allylic or vinylic substitution products is determined by preferential orientation of the vacant orbital and C–Hlg bond.     

Ring closures from eneearbamoyl azides. Syntheses of tetrahydro-3-indazolinones and 4-imidazolin-2-ones

Reaction of phosgene with cyclohexylidene amines gives good yields of (1-eyclohexen-l-yl)-carbamoyl chlorides ( 1 ). Compound 1 can be converted to the corresponding eneearbamoyl azide ( 2 ), which on pyrolysis gives an improved synthesis of 1-substituted-4,5,6,7-tetrahydro-3-indazolinones ( 7 ). When 1 is substituted by an allylic rather than alkyl or aryl group, the major products are 4-imidazolin-2-ones ( 8 ) accompanied by only minor amounts of 7 . The thermolysis reaction has been extended to N-allylcarbamoyl azides in general, thus providing a new and facile synthesis for 1,4-disubstituted 4-imidazolin-2-ones (9). A tentative mechanism is advanced, involving intermediate azide addition to the allylic double bond.     

N,N-Disubstituted propargylamines as tools in the sequential 1,3-dipolar cycloaddition/arylation processes to the formation of polyheterocyclic systems

Starting from N,N-disubstituted propargylamines, through a one-pot sequential 1,3-dipolar cycloaddition/Pd-catalyzed arylation, polyheterocyclic systems were obtained in only one step. The outcome of the cycloadditions, performed with 1,3-dipoles nitriloxide and azide, was totally regioselective. The subsequent Pd-catalyzed arylation achieved using ligand-free conditions involved the unsubstituted carbon of the heterocyclic ring.     

Stereoselective synthesis of cis- or trans-3,5-disubstituted pyrazolidines via Pd-catalyzed carboamination reactions: use of allylic strain to control product stereochemistry through N-substituent manipulation

The stereoselective synthesis of either trans- or cis-3,5-disubstituted pyrazolidines is accomplished via Pd-catalyzed carboamination reactions of unsaturated hydrazine derivatives. The products are obtained in good yield with up to >20:1 diastereoselectivity. Stereocontrol is achieved by modulating the degree of allylic strain in the transition state for syn-aminopalladation through a simple modification of the substrate N(2)-substituent. The pyrazolidine products can be further transformed to 3,5-disubstituted pyrazolines via deprotection/oxidation, or to substituted 1,3-diamines via N-N bond cleavage.     

Pd-catalyzed C-N coupling of primary (het)arylamines with 5-substituted 3-chloro-4H-1,2,6-thiadiazin-4-ones

The Buchwald-Hartwig Pd-catalyzed C-N coupling reaction of 5-substituted 3-chloro-4H-1,2,6-thiadiazin-4-ones with primary (het)arylamines is described, affording twenty new 3,5-disubstituted 4H-1,2,6-thiadiazin-4-ones in 56–99% yield. The protocol enables the mild preparation of difficult to access 3,5-dianilino-1,2,6-4H-thiadiazin-4-ones. The reaction scope and limitations are discussed.     

Concise and stereoselective synthesis of 2,5- and 2,4-disubstituted thiazole amino acid subunits for synthesizing thiazole-containing peptides

A concise, highly stereoselective synthesis of 2,4- and 2,5-disubstituted thiazole amino acids was developed. These are important building blocks for various biologically active thiazole-containing natural peptides and their regioisomeric analogues. The fundamental reactions are diastereoselective addition of (4- or 5-bromothiazol-2-yl)lithium to N-tert-butanesulfinyl imine with subsequent Pd-catalyzed phenoxycarbonylation.     

Metal complexes-catalyzed hydrolysis and alcoholysis of organic substrates and their application to kinetic resolution

Metal complexes-catalyzed hydrolysis and alcoholysis of organic substrates such as alkenyl esters, alkenyl ethers, and azlactones (oxazol-5(4H)-ones) are described. These reactions were applied for kinetic resolution of chiral compounds and high selectivities were achieved with vinyl ethers of 2-substituted cyclohexanols, 1,1′-bi-2-naphthols, 1,1′-bi-2-phenols, and 4,4-disubstituted azlactones. Oxidative carbon–carbon bond cleavage reactions, which were found in the course of the study of asymmetric hydrolysis were also described.     

Amino acids as selective acylating agents: regioselective N-acylation of imidazolidin-4-one derivatives of the antimalarial drug primaquine

The acylation of bioactive primaquine-based imidazolidin-4-ones was studied using N^α-Boc-protected glycine as acylating agent. Two synthesis routes, eight different coupling methods and seven distinct solvents were compared. Mild carbodiimide-based couplings on high dielectric constant solvents such as DMF or MeCN increased acylation yields, whereas alcohols inhibited carbodiimide-mediated acylations to take place. Achievement of the synthetic goals was limited by the size of the imidazolidin-4-one ring substituents R¹, R² and R³, but resort to MW-assisted synthesis allowed overcoming such obstacle, though with very modest reaction yields. All reactions involving a Boc-protected amino acid were regioselective, independent of reaction conditions employed. In contrast, regioselective acetylation of the imidazolidin-4-ones could only be achieved by resort to very mild coupling procedures.     

Palladium-catalyzed alkene carboamination reactions for the synthesis of substituted piperazines

A strategy for the stereoselective preparation of enantiomerically enriched cis-2,6-disubstituted piperazines from amino acid precursors is described. The target compounds are generated in 95-99% ee with good to excellent levels of diastereoselectivity (usually 14:1 to >20:1) using Pd-catalyzed carboamination reactions between aryl or alkenyl halides and substituted ethylenediamine derivatives to form the heterocyclic rings. The synthesis requires only 4-5 steps from commercially available amino acids, and allows for the modular construction of piperazines bearing different substituents at N¹, N⁴, C², and C⁶. The use of this strategy for the construction of 2,3-disubstituted piperazines, fused bicyclic piperazines, and tetrahydroquinoxalines is also reported. In addition, the mechanism of the key carboamination reactions is discussed, and new models that predict and explain the stereochemical outcome of these transformations are presented.     

N-Heterocyclic carbene–gold(I)-catalyzed carboheterofunctionalization of alkenes with arylboronic acids

A new approach to the synthesis of pyrrolidine, tetrahydrofuran, and imidazolidin-2-one via N-heterocyclic carbene–gold(I)-catalyzed intramolecular amino- or oxyarylation reactions from a wide variety of alkene substrates such as N-allyl amides, alcohols, carboxylic acids, and ureas in the presence of Selectfluor under mild conditions has been developed.     

Retrospect of Se and Te Chemistry

Retrospect of organoselenium and tellurium chemistry for these 30 years is described focusing on our novel findings in this field: (1) telluroxide elimination leading to alkenes and allylic compounds, (2) Pd-catalyzed or –mediated carbodetelluration for a new C–C bond formation, (3) synthesis of chiral diferrocenyl dichalcogenides and their use as chiral auxiliaries, (4) asymmetric selenoxide elimination for making optically active allenes and alkenes, (5) meta chloroperbenzoic acid (MCPBA) oxidation of organic selenides and tellurides leading to a substitution of a PhSe or PhTe moiety, as well as (6) preparation of chalcogen-bridged diruthenium complexes and their catalytic use for propargylic substitution reactions.     

Peracid dependent stereoselectivity and functional group contribution to the stereocontrol of epoxidation of (E)-alkene dipeptide isosteres

Twelve Boc-protected phenylalanyl-phenylalanine and phenylalanyl-glycine trans-vinyl isosteres were epoxidised with magnesium monoperoxyphtalate hexahydrate (MMPP) and trifluoroperacetic acid, and the results have been compared with those from earlier studies on epoxidations with m-CPBA. The alkenes were synthesised in high yields with high E/Z-selectivities using either the Julia or Schlosser reactions. The formation of threo isomers was favoured in all epoxidation reactions except with CF₃CO₃H on substrates containing two allylic/homoallylic functional groups directing the peracid to opposite faces of the alkene. The switch to erythro selectivity observed with CF₃CO₃H is suggested to emanate from coordination to the allylic ester functionalities via hydrogen bond donation from the peracid. The other peracid reagents seem to be preferentially coordinated to the allylic carbamate function. The contribution of individual functional groups to the stereopreference was also investigated.     

Simple pyridylmethylamines: efficient and robust N,N-ligands for Suzuki-Miyaura coupling reactions

A series of pyridylmethylamines have been synthesized in one step from commercially available starting material and identified as effective ligands for Pd-catalyzed carbon-carbon bond formation through Suzuki-Miyaura coupling reaction. The N,N-pyridylmethylamine-Pd catalytic systems appeared as an interesting and robust compromise between catalytic efficiency, substrate compatibility, and practical aspects.     

Synthesis of bridgedhead azolo[3,2-a]pyrimidines and imidazo[2,1-b]thiazines through ring transformation of 2H-pyran-2-ones

Suitably functionalized 3-carbomethoxy/cyano-2H-pyran-2-ones are excellent synthons for the synthesis of arenes and heteroarenes of therapeutic importance. The compounds 6-aryl-3-cyano-4-methylsulfanyl-2H-pyran-2-ones have been transformed into bridgedhead azolopyrimidines and imidazothiazines through thermal and base-induced ring transformation reactions with aminoazoles and imidazolidin-2-thiones, respectively.     

Single step synthesis of 2,3-dialkyl-6-nitro-quinazolin-4(3H)-imines and 3,5-dialkyl-9-nitro-imidazo[1,2-c]quinazolin-2(3H)-ones

A single step synthesis of 2,3-dialkyl-6-nitro-quinazolin-4(3H)-imines and 3,5-dialkyl-9-nitro-imidazo-[1,2-c]-quinazolin-2(3H)-ones from simple carbonyl compounds, primary amines or amino acid methyl esters and 2-azido-5-nitro-benzonitrile was developed. Key intermediates were N,N′-disubstituted amidines obtained by rearrangement of 4,5-dihydrotriazoles; the new heterocyclic rings were formed by spontaneous intramolecular reaction of the amino and cyano groups which are present in the intermediates.