Facile solid-phase synthesis of 2,3-disubstituted 6H-pyrano[2,3-f]benzimidazole-6-ones

We report herein the facile solid-phase synthesis of 2,3-disubstituted 6H-pyrano[2,3-f]benzimidazole-6-ones using 7-fluoro-4-methyl-6-nitro-2-oxo-2H-1-benzopyran-3-carboxylic acid as the scaffold. The fluorine of the resin-bound scaffold was first replaced by a primary amine. Reduction of the nitro group with tin(II) chloride afforded an o-dianilino intermediate that was treated with an aldehyde followed by the addition of 2,3-dichloro-5,6-dicyanoquinone (DDQ). 2,3-Disubstituted 6H-pyrano[2,3-f]benzimidazole-6-ones were obtained in high purity and good yield after cleavage.     

Solid-phase synthesis of 3-alkyl-2-arylamino-3,4-dihydroquinazolines

The solid-phase synthesis of 3-alkyl-2-arylamino-3,4-dihydroquinazolines using an N-Fmoc-β-amino-2-nitrobenzenepropanoic acid scaffold is described. The resin-bound scaffold was reductively alkylated with aldehydes or ketones after Fmoc deprotection, followed by reduction of the nitro group with tin(II) chloride. Subsequent cyclization of the 1,3-diamine intermediates with aryl isothiocyanates in the presence of 1,3-diisopropylcarbodiimide (DIC) afforded the desired products in high purity with moderate to good yield after trifluoroacetic acid (TFA) cleavage.     

Synthesis and Evaluation of Antioxidant Properties of 2-Substituted Quinazolin-4(3H)-ones

Quinazolinones represent an important scaffold in medicinal chemistry with diverse biological activities. Here, two series of 2-substituted quinazolin-4(3H)-ones were synthesized and evaluated for their antioxidant properties using three different methods, namely DPPH, ABTS and TEAC_CUPRAC, to obtain key information about the structure–antioxidant activity relationships of a diverse set of substituents at position 2 of the main quinazolinone scaffold. Regarding the antioxidant activity, ABTS and TEAC_CUPRAC assays were more sensitive and gave more reliable results than the DPPH assay. To obtain antioxidant activity of 2-phenylquinazolin-4(3H)-one, the presence of at least one hydroxyl group in addition to the methoxy substituent or the second hydroxyl on the phenyl ring in the ortho or para positions is required. An additional ethylene linker between quinazolinone ring and phenolic substituent, present in the second series (compounds 25a and 25b), leads to increased antioxidant activity. Furthermore, in addition to antioxidant activity, the derivatives with two hydroxyl groups in the ortho position on the phenyl ring exhibited metal-chelating properties. Our study represents a successful use of three different antioxidant activity evaluation methods to define 2-(2,3-dihydroxyphenyl)quinazolin-4(3H)-one 21e as a potent antioxidant with promising metal-chelating properties.     

Facile solid phase synthesis of 1,2-disubstituted-6-nitro-1,4-dihydroquinazolines using a tetrafunctional scaffold

Solid phase synthesis of 1, 2-disubstituted-6-nitro-1,4-dihydroquinazolines is described. The new tetrafunctional scaffold N-Alloc-3-amino-3-(2-fluoro-5-nitrophenyl)propionic acid was prepared by nitration of 3-amino-3-(2-fluorophenyl)propionic acid. The scaffold was anchored to Rink resin via its carboxylic group and treated with primary amines to displace the arylfluorine followed by cyclization with aryl isothiocyanates in the presence of DIC upon Alloc deprotection to afford 1,2-disubstituted-6-nitro-1,4-dihydroquinazolines in high yield.     

Open-chain remote stereocontrol using allylgermanes

Alk-2-enyl(trialkyl- and -triaryl)germanes are transmetallated by tin(IV) halides at −78 °C with 15 min being sufficient for the transmetallation to proceed effectively to completion. Alk-2-enyl(trialkyl)germanes are transmetallated by both tin(IV) chloride and tin(IV) bromide under these conditions, but the corresponding triphenylgermanes are only transmetallated using tin(IV) chloride. These observations led to procedures for reactions of 5-substituted-pent-2-enyl- and -hex-2-enyl-germanes with aldehydes that proceed with useful levels of 1,5- and 1,6-stereocontrol in favour of the (Z)-1,5-anti- and (Z)-1,6-syn-alkenols, respectively. Slightly improved stereoselectivities were obtained using (Z)-alk-2-enylgermanes. The reaction of (Z)-6-hydroxyhept-2-enyl(triethyl)germane with benzaldehyde when promoted using tin(IV) bromide gave a 3-ethenyl-6-methyl-2-phenyltetrahydropyran rather than the expected homoallylic alcohol.     

One-pot synthesis of polycyclic heterocyclic compounds by condensation of 1-carbamoylmethyl-2,3,3-trimethyl-3H-indolium salts with pyridine-2, 3, and 4- and quinoline-4-carboxaldehydes

A one-pot synthesis of new polycyclic heterocyclic compounds was carried out via the condensation of 1-carbamoylmethyl-2,3,3-trimethyl-3H-indolium chloride with pyridine- and quinolinecarboxaldehydes. The heating of the aforementioned 3H-indolium salts with 1 eq. of pyridine-2, 3, and 4- or quinoline-4-carboxaldehyde in ethanol in the presence of piperidine as a catalyst provided 9a-[2-(pyridyl)ethenyl]- or 9a-[2-(quinolyl)ethenyl]-9,9a-dihydro-1H-imidazo[1,2-a]indol-2(3H)-one derivatives as the main products. However, reaction outcome was dramatically different for the analogous reactions in acetic acid. In this case, the heating of the chloride with 2 eq. of pyridine-2-carboxaldehyde afforded derivatives of 9a-[3-(pyridin-2-yl)indolizin-2-yl]-9,9a-dihydro-1H-imidazo[1,2-a]indol-2(3H)-one as the major product, while the use of 2 eq. of pyridine-3 and 4- or quinoline-4-carboxaldehyde led to the formation 2-heteroaryl-1-heteroarylmethyl-9H-pyrrolo[1,2-a]indole-3-carboxamides. Plausible pathways for the cyclization reactions are discussed. The structural assignments were based on ¹H, ¹³C and ¹⁵N NMR spectroscopy, HRMS and single-crystal X-ray diffraction data.     

Asymmetric synthesis and biological evaluation of 3-nitro-2H-chromenes as potential antibacterial agents

A one-pot organocatalytic domino Henry/Michael/dehydration sequence of various salicylaldehyde derivatives with nitromethane as a dual-nucleophile is disclosed. This straightforward strategy assembles the optically active (R)-2-alkyl 3-nitro-2H-chromenes with good to excellent enantioselectivities (up to 93% ee) and a broad substrate scope. Preliminary in vitro antibacterial evaluation revealed most of the chiral 3-nitro-2H-chromene derivatives exhibit antibacterial activities against four Gram-positive bacteria. Compound 3p with two bromine substituents on the core scaffold was shown to be the best antibacterial agent in the series against S. aureus, B. subtilis and Bacillus cereus with minimal bactericidal concentration (MBC) values of 4?μg/mL, and against Staphylococcus epidermidis with a MBC value of 8?μg/mL.     

1,5-Stereocontrol in reactions of 2,4-disubstituted alk-2-enylstannanes with aldehydes; an approach to the stereoselective synthesis of branched triols

2-(tert-Butyldimethylsilyloxymethyl)-4-(methoxymethoxy)pent-2-enyl(tributyl)stannane is transmetallated by tin(IV) chloride stereoselectively to give a pent-1-en-3-yltin trichloride that reacts with aldehydes with excellent (E)-1,5-syn-stereocontrol, e.g. (3E)-1,5-syn-3-(tert-butyldimethylsilyloxymethyl)-5-(methoxymethoxy)-1-phenylhex-3-en-1-ol was the dominant product with benzaldehyde. The products from these reactions were taken through to more complex 2-substituted alk-2-enyl(tributyl)stannanes but only very low yields of the expected products were obtained from tin(IV) chloride mediated reactions of these stannanes and aldehydes. Nevertheless a stereoselective synthesis of 2-substituted 4-[(E)-2-alkoxypropylidene]tetrahydrofurans was developed.     

Parallel solid-phase synthesis of 2-arylamino-6H-pyrano[2,3-f]benzimidazole-6-ones

The parallel solid-phase synthesis of a hydrophilic psoralen analogue, 2-arylamino-6H-pyrano[2,3-f]benzimidazole-6-ones (2-arylaminoimidazocoumarins), has been developed. The resin-bound 7-fluoro-4-methyl-6-nitro-2-oxo-2H-1-benzopyran-3-carboxylic acid underwent aromatic substitution with primary amines, followed by reduction of the nitro group with tin(II) chloride. The cyclization of the o-dianilino intermediates was accomplished with aryl isothiocyanates in the presence of 1,3-diisopropylcarbodiimide (DIC). The final products were released from the resin with trifluoroacetic acid (TFA) and obtained in high purity and good isolated yield. The 2-arylaminoimidazocoumarins exhibit interesting spectral properties.     

Synthesis of trans-4-aryl-3-(3-chloropropyl)azetidin-2-ones and their transformation into trans- and cis-2-arylpiperidine-3-carboxylates

Treatment of arylmethylideneamines with 5-chloropentanoyl chloride in benzene in the presence of 2,6-lutidine afforded novel trans-4-aryl-3-(3-chloropropyl)azetidin-2-ones in good yields. The latter 3-(3-chloropropyl)-β-lactams were transformed selectively into trans-methyl 1-alkyl-2-arylpiperidine-3-carboxylates in high yields and purity upon subsequent treatment with hydrogen chloride in methanol and triethylamine in dichloromethane. These trans-1-alkyl-2-arylpiperidine-3-carboxylates were easily converted into either their cis-isomers upon treatment with hydrazine monohydrate in methanol, or into the corresponding piperidine-1,3-dicarboxylates by reaction with alkyl chloroformates in benzene. Finally, 3-(3-chloropropyl)-1-(4-methoxybenzyl)-4-phenylazetidin-2-one was transformed into the corresponding trans-1-tert-butoxycarbonyl-3-(4-methoxybenzylcarbamoyl)piperidine via a three-step sequence in a good overall yield.     

Selective synthesis of some imidazopyridine-fused chromones

A fused heterocyclic scaffold combining the imidazo[1,2-a]pyridine with a substituted chromone was synthesized in a one-pot procedure. The reaction proceeds by intramolecular cyclization of 1-(2-imino-2H-chromen-3-yl)pyridinium chloride in ethanol and in the presence of DABCO. A detailed study of the experimental conditions allowed a clear understanding of the reaction pathway.     

Preparation of 4,5 disubstituted-2H-1,2,3-triazoles from (Z)-2,3-diaryl substituted acrylonitriles

2H-1,2,3-Triazoles (2) were synthesized by [3+2] cycloaddition of (Z)-2,3-diaryl substituted acrylonitriles (1) with sodium azide and ammonium chloride in DMF/water. This method represents a facile and efficient reaction procedure for the synthesis of 4,5-diaryl-2H-1,2,3-triazoles in modest to good yields.     

Stereoselective synthesis of an ansa-zirconocene in a spirane scaffold

Methodology is described for the preparation of a rigid C₂-bridged zirconocene catalyst system where the C₂-bridge is embedded in a spirane scaffold. The ligand was prepared from 3,4-dihydro-2H-fluoren-1(9H)-one. Initially, the oxo group was converted into a spirocyclobutanone function. Further conversion to a fulvene was followed by regio- and stereoselective saturation to provide the ligand cis-2-(cyclopentadienyl)-1′,2′,3′,4′-tetrahydro-9H-spiro[cyclobutane-1,1′-fluorene]. The ligand was dilithiated and reacted with zirconium tetrachloride to provide the corresponding (η⁵-cyclopentadienyl)-spiro[cyclobutane-1,1′-(η⁵-fluorenyl)]zirconium dichloride complex. The structure of the zirconocene dichloride has been established by crystal X-ray analysis.     

Novel preparation of N′-arylthiocarbamoyl-N,N-dialkylamidines and their synthetic utilities

Treatment of 5-(arylimino)-4-(dialkylamino)-5H-1,2,3-dithiazoles (2) with NaOH in aqueous EtOH at room temperature gave N′-arylthiocarbamoyl-N,N-dialkylamidines (3) in good to excellent yields. The reaction of 3 with sulfur monochloride, thiophosgene, thionyl chloride, sulfuryl chloride, N-phenylimidoyl dichloride, and phthaloyl chloride in CH₂Cl₂ gave 2, 5-(arylimino)-4-(dialkylamino)-Δ³-thiazoline-2-thiones (5), 5-(arylimino)-4-(dialkylamino)-5H-2-oxo-1,2,3-dithiazoles (6), 5-(arylimino)-4-(dialkylamino)-5H-2,2-dioxo-1,2,3-dithiazoles (7), 5-(arylimino)-4-(dialkylamino)-2-(phenylimino)-Δ³-thiazolines (8), and 3-(arylimino)-4-(dialkylamino)-2,5-benzothiazocine-1,6-diones (10) as major products, respectively.     

Diverse synthesis of pyrimido[1,2-a]benzimidazoles and imidazo[2,1-b]benzothiazoles via CuI-catalyzed decarboxylic multicomponent reactions of heterocyclic azoles,aldehydes and alkynecarboxylic acids

We have developed a straightforward approach to diverse synthesis of 2,3-, 2,4-disubstituted pyrimido [1,2-a]benzimidazoles, 2,4,10-trisubstituted 2,10-dihydropyrimido [1,2-a]benzimidazoles and 2,3-disubstituted imidazo [2,1-b]benzothiazoles via multicomponent reactions (MCRs) of heterocyclic azoles, aldehydes with easily storable and handling alkynecarboxylic acids. In the presence of a catalytic amount of CuI and K₂CO₃, the pyrimido [1,2-a]benzimidazole or imidazo [2,1-b]benzothiazole scaffold could be rapidly constructed through a 6-endo-dig or 5-exo-dig cyclization, respectively. The preliminary mechanistic study suggested that the formation of 2,3- disubstituted pyrimido [1,2-a]benzimidazoles, which completes the assembly of the scaffold and its C-3 position functionalization in one pot, undergoes a novel cascade process involving a decarboxylation, A [3] coupling, 6-endo-dig cyclization, nucleophilic addition and dehydration.     

Synthesis of a β-strand mimetic based on a pyridine scaffold

A synthetic route to a 2,4-disubstituted pyridine as a potential β-strand mimetic has been developed and applied in the synthesis of a tripeptidomimetic of Leu-Gly-Gly. The pyridine scaffold replaces the central glycine, and is substituted with analogues of leucine and glycine in positions 4 and 2, respectively. 2-Fluoro-4-iodopyridine was chosen as the functionalized scaffold and was substituted with protected leucinal in position 4 via a Grignard exchange reaction using iso-propyl magnesium chloride. The glycine moiety was introduced in position 2 via a nucleophilic aromatic substitution reaction (S_NAr) facilitated by microwave irradiation. The synthetic sequence involved 12 steps with an overall yield of 7%.     

Synthesis, characterization, and biological evaluation of new oxime-phosphazenes

Hexachlorocylotriphosphazene (1) was reacted with 4-hydroxy-3-methoxybenzaldehyde to give hexakis[(4-formyl-2-methoxy)phenoxy]cyclotriphosphazene (2). Hexakis[(4-(hydroxyimino)2-methoxy)phenoxy]cyclotriphosphazene (3) was synthesized by reaction of 2 with hydroxlamine hydrochloride in pyridine. Compound 3 was reacted with benzyl chloride, acetyl chloride, allyl bromide, benzoyl chloride, propanoyl chloride, 4-methoxybenzoyl chloride, 2-chlorobenzoyl chloride, chloroacetyl chloride, methyl iodide, and thiophene-2-carbonyl chloride. From these reactions, full or partially substituted compounds were obtained, usually in high yields. Pure or defined products could not be obtained from reaction of 3 with methacryloyl chloride and O-acetylsalicyloyl chloride. The structures of the compounds were determined by elemental analysis, and IR, ¹H, ¹³C, and ³¹P NMR spectroscopy. The synthesized compounds were screened for in-vitro antimicrobial activity against two Gram-positive bacteria (Staphylococcus aureus and Enterococcus faecalis), two gram-negative bacteria (Escherichia coli and Klebsiella pneumonia), and fungal strains (Aspergillus niger, and Candida albicans) by the agar well diffusion method. Few compounds had significant activity against both Gram-positive and Gram-negative bacteria. None of the compounds had antifungal activity except compounds 7 and 9, which had moderate activity.     

Stereochemical aspects of the chemistry of 2-[trialkyl(aryl)silyloxy]alkyl-4-alkoxyalk-2-enylstannanes

2-tert-Butyldimethylsilyloxymethyl-4-(methoxymethoxy)pent-2-enyl(tributyl)stannane, prepared predominantly as the (Z)-isomer, is transmetallated by tin(IV) chloride to generate an allyltin trichloride which reacts with aldehydes with excellent stereocontrol in favour of (E)-1,5-syn-3-tert-butyldimethylsilyloxymethyl-5-(methoxymethoxy)alk-3-en-1-ols. These were taken through to 3-[(E)-2-(methoxymethoxy)-propylidenyl]-5-alkyl(aryl)tetrahydrofurans and used to prepare more complex 4-(methoxymethoxy)-pent-2-enylstannanes.     

Enantiospecific synthesis, separation and olfactory evaluation of all diastereomers of a homologue of the sandalwood odorant Polysantol

The four stereoisomers of (5E)-4,4-dimethyl-6-(2′,2′,3′-trimethylcyclopent-3′-en-1′-yl)-hex-5-en-3-ol, a homologue of the valuable sandalwood-type odorant Polysantol^®, were enantiospecifically synthesized from (+)- and (−)-α-pinene, through (−)- and (+)-campholenic aldehyde, by aldol condensation with 3-pentanone, deconjugative α-methylation and reduction. The mixtures of epimeric alcohols obtained after reduction were separated by means of derivatization with (−)-(1S)-camphanic chloride. The enantiomerically pure final products were evaluated organoleptically.     

Selective reduction of tertiary alkyl,benzyl, and allyl halides to hydrocarbons using lithium 9,9-di-n-butyl-9-borabicyclo[3.3.1]nonanate

The title 9-borabicyclo[3.3.1]nonane(9-BBN) ate complex (1) brings about selective removal of tertiary alkyl, benzyl and allyl halides to give the corresponding hydrocarbons in excellent yields without concomitant attack on secondary, primary and aryl derivatives. The reduction of cis- and trans - 4 - t - butyl - 1 - methylcyclohexyl chlorides (2) with 1 gives 4 - t - butyl - 1 - methylcyclohexanes (3) with partial inversion of configuration in cyclohexane, while that in benzene gives thermodynamically stable trans-3 predominantly. The reactions of 1,1 - dimethyl - 5 - hexenyl chloride (4) and 1,7,7 - trimethylbicyclo[2.2.1]hept - 2 - yl chloride (8) with 1 proceed with the rearrangements characteristic to a carbonium ion intermediate. The reduction of 1 - ethyl - 1 - methylpentyl chloride with 1 follows a second-order rate equation.