Facile synthesis of regio-isomeric naphthofurans and benzodifurans

Naphtho[1,2-b]furans 1a-f, naphtho[2,1-b]furans 2a-f, benzo[1,2-b:5,4-b′]difurans 3a-b, benzo[1,2-b:4,5-b′]difurans 4a-b, and benzo[1,2-b:4,3-b′]difurans 5a-b were synthesized by base-catalyzed cyclization reaction of the corresponding o-alkoxybenzoylarene derivatives. The o-alkoxybenzoylarenes were obtained from the etherification reaction of the o-hydroxybenzoylarenes, which were prepared either by the reaction of methoxyarenes with benzoyl chloride in the presence of aluminum chloride or by photo-Fries rearrangement of aryl benzoates.     

Heck-like coupling and Pictet-Spengler reaction for the synthesis of benzothieno[3,2-c]quinolines

A series of 6-arylbenzothieno[3,2-c]quinolines were synthesised in three steps from benzo[b]thiophene. After a selective Heck-type coupling of benzo[b]thiophene with different o-nitroaryl bromides to obtain 2-(o-nitroaryl)benzo[b]thiophenes, corresponding 2-(benzo[b]thiophen-2-yl)anilines were involved in a Pictet-Spengler reaction to form the quinoline cycle.     

Bargellini condensation of coumarins. Expeditious route to o-carboxyvinylphenoxyisobutyric acids and application to the synthesis of sesquiterpenes helianane, heliannuol A and heliannuol C

The direct Bargellini condensation of coumarins involving reaction with chloroform and acetone in the presence of aqueous sodium hydroxide furnished o-carboxyvinylphenoxyisobutyric acids in good yields. The utility of this new useful protocol was demonstrated by the transformation of the three diesters 9b, 9f and 9g to the sesquiterpenes helianane 4, heliannuol A 2 and heliannuol C 3, respectively.     

Preparation of enantiomerically pure (1S,2S)-1-aminocyclopropanephosphonic acid from methylcyclopropanone acetal via spirophosphonate intermediates

An easy and efficient one-pot reaction from readily available methylcyclopropanone acetal (2S)-4b gave the spirophosphonates 8a–b with excellent diastereoselectivity. These phosphonates, after catalytic hydrogenolysis and hydrolysis, furnished the enantiomerically pure (1S,2S)-1-amino-2-methylcyclopropanephosphonic acid 3b (analogue of (1R,2S)-allo-norcoronamic acid).     

Solvation controlling reaction paths and gel-formation in imide derivatives

1,8-Naphthalic anhydride condenses with 4-nitro-1,2-diaminobenzene in acetic acid to give 11-nitrobenzo[d,e]benzo[2,1-a]isoquinoline-1,3-dione (1), whereas the same reaction carried out in DMF gives 2-(2-amino-4-nitrophenyl)-benzo[d,e]isoquinoline-1,3-dione (2). The condensation reaction of 1,8-naphthalic anhydride with 1-amino 3,5-benzene dicarboxylic acid leads to corresponding imide, which forms a gel in a mixed solvent such as water in DMSO. A similar compound 5-(1,3-dioxo-1,3-dihydroisoindol-2-yl)-isophthalic acid derived from phthalic anhydride forms gel in a mixed solvent of DMSO and water, whereas a crystalline solvate of DMSO with 2 could be obtained upon crystallization from DMSO. The crystal structure of this solvate is determined and its structure is compared with 2-(1,3-dioxo-1,3-dihydro-isoindol-2-yl)-benzoic acid. The latter does not form a gel in the mixed solvent and adopts an intermolecular hydrogen bonded structure.     

A facile domino protocol for the regioselective synthesis and discovery of novel 2-amino-5-arylthieno-[2,3-b]thiophenes as antimycobacterial agents

A series of novel 2-amino-5-arylthieno[2,3-b]thiophenes has been synthesized regioselectively from the reaction of 5-aryldihydro-3(2H)-thiophenones with ethyl cyanoacetate/malononitrile and sulfur powder in the presence of morpholine under thermal as well as microwave irradiation conditions. This transformation presumably occurs via domino Gewald reaction-dehydrogenation sequence. The 2-amino-5-arylthieno[2,3-b]thiophenes were evaluated for their in vitro activity against Mycobacterium tuberculosis H37Rv (MTB) and multi-drug resistant M. tuberculosis (MDR-TB). Among 12 compounds screened, ethyl 2-amino-5-(1-naphthyl)thieno[2,3-b]thiophene-3-carboxylate was found to be the most active compound with MIC of 1.1 μM against MTB and MDR-TB.     

SnCl4-mediated oxidative biaryl coupling reaction of 1-naphthol and subsequent ring closure of 2,2′-binaphthol to the dinaphthofuran framework

A simple method for the direct synthesis of 2,2′-binaphthols 2 and dinaphtho[1,2-b;2′,1′-d]furans 3 under mild conditions was developed, utilizing a biaryl coupling reaction via electron donor-acceptor complexes of 1-naphthols with SnCl₄. Heating of the complex in a sealed tube for (18-24 h) afforded the corresponding o-o coupled product 2 in excellent yield. Prolonged reaction (56-65 h) under the same conditions afforded 3 in high yield in one step. We also found that in the case of α-naphthol without substituents other than a hydroxyl group at the C-1 position, regioselective o-o coupling reaction proceeded. The products 2a, 2b and 2g should be useful as synthetic intermediates for naturally occurring 3,3′-bijuglone, 3,3′-biplumbagin and elliptinone.     

Development of an unexpected reaction pathway for the synthesis of 1,2,4-trisubstituted pyrrolo[1,2-a]quinoxalines through palladium-catalyzed cascade reactions

1,2,4-trisubstituted pyrrolo[1,2-a]quinoxalines are synthesized through the multi-component reaction of 3-substituted 2-chloroquinoxalines, propargyl bromide, and excess secondary amines in the presence of a palladium copper catalytic system. This one-pot process provides an unexpected synthesis of new trisubstituted pyrrolo[1,2-a]quinoxalines by the introduction of two amine substituents onto the fused pyrrole rings in a single reaction procedure. The compounds formed are fully characterized by the analytical spectral data and X-ray analysis. A number of synthesized pyrrolo[1,2-a]quinoxaline derivatives are also screened against the three bacterial strains Micrococcus luteus, Pseudomonas aeruginos, and Bacillus subtilis. According to the results obtained, compounds 3b, 3c, and 3e are active against M. luteus, compounds 3b and 3e are active against Ps. Aeruginos, and only compound 3f is active against all the three bacterial strains.     

Biphenylenes-xxxi: Condensation of benzocyclobutene-1,2-dione with aliphatic and heterocyclic 1,2-diamines and the synthesis of cis-2-cyano-3-(2'-cyanovinyl)-1,4-diaz

As possible routes to 1,4-diazabiphenylene and its 2,3-disubstituted derivatives we have studied the condensation of benzocyclobutene-1,2-dione (BBD) with various 1,2-diamines. Instead of giving the 1,4-diazabiphenylene ring system, BBD reacted with ethylenediamine, diaminomaleonitrile, 4,5-diaminopyrimidine, 2-aminopyridine, also 2,3- and 3,4-diaminopyridine to give, respectively, 2-o-carboxyphenylimidazolidinium acetate 4, 3,4-dicyano-2,5-dihydro[2,5]benzodiazocine-1,6-dione 10, 4-amino-5a,9b-dihydro-5-,9b-dihydroxybenzo [3',4']cyclobuta[1',2'-4,5]imidazo[1,2-c]pyrimidine 14, 5a,9b-dihydro-5a,9b-dihydroxybenzo[3',4']cyclobuta[1',2'-4,5] imidazo[1,2-a]pyridine 17, the 4-amino derivative 16 of the latter, and the zwitter ion 18 of 4-amino-3(2-carboxy-benzylideneamino) However, BBD reacted with 4,5-diaminobenzotriazole to give the expected 1,2,3,6,11-penta-aza-1-H-indeno [4,5-b]biphenylene 20, which, on amination followed by oxidation, gave a very low yield of cis-2-cyano-3-(2'-cyanovinyl)-1,4-diazabiphenylene 3. In model experiments, 7,8-diphenylfurazano [3,4-f]quinox-aline 28 was reduced to 2,3-diamino-5,6-diphenyl quinoxaline 29, which on oxidation, gave a mixture of cis- and trans-2-cyano-3-(2'-cyanovinyl)5,6-diphenylpyrazine, 30 and 31. The pentacyclic compounds, 1,3,6,II-tetra-aza-2-oxa-2H-indeno [4,5-b]biphenylene 23 and 1,3,5,10-tetra-aza-1-H-indeno[5,6-b] biphenylene 25, were formed from BBD and the appropriate 1,2-diamines but the 5-membered heterocyclic rings could not be cleaved by reduction and hydrolysis respectively) to give tetracyclic diamines which might have undergone oxidation to give derivatives of 1,4-diazabiphenylene. Compounds 14, 16, 20, 23, 25 and 28 are derivatives of new heterocyclic systems.     

Fluorine-containing heterocycles: Part III. Synthesis of some new furo[2,3-b]-, pyrazolo[3,4-b]- and thieno[2,3-b]pyridines with anticipated biological activities

3-Cyano-6-(2-thienyl)-4-trifluoromethylpyridine-2(1H)-one (1) and its thiono analog 2 were prepared by the reaction of (2-thenoyl)-ω,ω,ω-trifluoroacetone with cyanoacetamide or cyanothioacetamide, respectively. Interaction of compound 1 with ethyl chloroacetate or chloroacetamide led to the regioselective formation of O-alkylated pyridines 3 and 10. The latter compounds underwent some successive reactions to furnish the promising furopyridines (4 and 7–9) and pyrazolopyridines (12–15). The reaction of 2 with chloroacetamides or chloroacetonitrile furnished 2-functionalized 3-amino-6-(2-thienyl)-4-trifluoromethyl-thieno[2,3-b]pyridines (16a, b) which were used as key intermediates in the synthesis of the title thienopyridines. Structures of the newly synthesized compounds were established on the basis of their elemental and spectral (IR, ¹H NMR and mass) analyses.     

3-Amino- and 3-acylamido-2-phosphonopyridines: synthesis by Pd-catalyzed P-C coupling, structure and conversion to pyrido[b]-anellated PC-N heterocycles

Palladium catalyzed cross-coupling of 3-amino- and 3-acylamido-2-bromopyridines 1a-f with triethyl phosphite allowed the synthesis of 3-amino- and 3-acylamido pyridine-2-phosphonic acid diethyl esters 2a-f, whereas nickel catalysts, although providing access to related anilido-2-phosphonates, proved inactive. Reduction of the aminophosphonate 2a with LiAlH₄ afforded 3-amino-2-phosphinopyridine (3a), which was cyclocondensed with dimethylformamide dimethyl acetal (DMFA) via phosphaalkene intermediates 4a to the novel pyrido[b]-anellated 1,3-azaphosphole 5a. Reaction of amidophosphonates 2b-f with LiAlH₄ did not result in the expected reductive cyclization, as shown by closely related anilido-2-phosphonates, but led to product mixtures containing N-secondary 3-amino-2-phosphinopyridines 3b-f as the main or major component. The conversion of 3b,d,e with DMFA to 5b,d,e provides first examples of N-substituted pyrido[b]-anellated azaphospholes. Structures were confirmed by multinuclear NMR and X-ray crystallography (for 2c, 3b).     

Synthesis of 3-substituted-4-hydroxyquinoline N-oxides from the Baylis-Hillman adducts of o-nitrobenzaldehydes

The reaction of the Baylis-Hillman adducts 1b-f derived from o-nitrobenzaldehydes in trifluoroacetic acid in the presence of triflic acid (0.2 equiv.) afforded 3-substituted-4-hydroxyquinoline N-oxides 2b-e and 2a in good to moderate yields. The reaction mechanism was evidenced by the experiment with 1f, the Baylis-Hillman adduct of 2-nitrobenzaldehyde N-tosylimine, as the one involving N-hydroxyisoxazoline as the key intermediate.     

Formation and amination of dibenz[b,f,-1]azapen-talene dianion: Attempted synthesis of dibenz[b,f,-1]azapentalene

Dibenz[b, f,-1]azapentalene dianion (3) was allowed to react with N,N diethyl - O - mesitylenesulfonylhydroxylamine (4) in order to obtain N,N - diethyl - 5,10 - dihydroindeno[1,2 - b] - indol - 10 - amine (11). The available data indicated that electron transfer processes were involved in the formation of 11. Attempts to obtain dibenz[b, f,-1]azapentalene (1) and 11 by Hofman elimination or by the reaction of 11 with trifluoroacetic acid were unsuccessful.     

Synthesis of 3-hetaryl-4,5,6,7-tetrahydrobenzo[b]thiophenes

A novel efficient synthesis of 3-hetaryl-4,5,6,7-tetrahydrobenzo[b]thiophenes was achieved by the cyclocondensation of ethyl 2-amino-4,5,6,7-tetrahydrobenzo[6]thiophene-3-carboxylate with selected o-substituted aromatic amines or with alifatic, aromatic and heterocyclic acid hydrazides in the presence of polyphosphoric acid.     

Microwave assisted synthesis of novel imidazo [2,1-b]thiazole derivative attached to quinoxalinones

3-(6-Phenylimidazo[2,1-b]thiazol-5-yl)quinoxalin-2(1H)-ones (qunoxalinone) (6a-q) have been synthesized by the reaction of ethyl 2-oxo-2-(6-phenylimidazo[2,1-b]thiazol-5-yl)acetates (4a-e) with suitably substituted o-phenylenediamines (5a-f) under microwave heating. The ethyl 2-oxo-2-(6-phenylimidazo[2,1-b]thiazol-5-yl)acetates (4a-e) were prepared by the reaction of 6-phenylimidazo[2,1-b]thiazoles (3a-e) with ethyl chlorooxoacetate in refluxing 1,4-dioxane whereas the thiazoles (3a-e) were synthesized by the reaction of 2-bromo-1-phenylethanones (2a-e) with thiazol-2-amine in refluxing acetone.     

Synthesis of substituted 3-trifluoromethylbenzo[b]thiophenes

A straightforward method for the synthesis of 5/6-substituted 3-trifluoromethylbenzo[b]thiophenes and their precursor o-fluorinated trifluoro acetophenones is reported.     

A new synthesis of benzo[b]thiophenes utilizing an interrupted Pummerer reaction

A convenient synthesis of 3-arylbenzo[b]thiophenes utilizing an interrupted Pummerer reaction of 2-(1-arylvinyl)phenyl ethyl sulfoxides is described. Thus, treatment of these sulfoxides, which were readily prepared from 2-sulfanylphenyl ketones or 2-fluoro-5-methoxybenzaldehyde, with acetic anhydride at 100 °C afforded 3-arylbenzo[b]thiophenes in reasonable yields.     

New pentacyclic ring systems: intramolecular cyclization of o,o′-disubstituted bibenzothiazoles

Efficient methods for the preparation of isomeric o,o′-diaminobibenzothiazoles (8a and 11a) and o,o′-diamino-2,2′-dimethylbibenzothiazoles (8b and 11b), potentially valuable building blocks for construction of hitherto unknown dithiazolo annulated pentacyclic heterocycles, have been developed. The dithiazolo annulated benzo[c]cinnolines 9a, 9b, and 12a were prepared from the corresponding diamines by oxidation with PhI(OAc)₂ in good yield. The dithiazolo annulated carbazoles 13 and 14 were efficiently prepared from the corresponding diamines by thermal cyclization in H₃PO₄. The unusual course of reduction and product formation of o,o′-dinitrosubstituted bibenzothiazoles 6a and 6b with SnCl₂ under acidic conditions was rationalized by DFT quantum-mechanical calculations. It was suggested that cyclic products are formed from dinitroso derivatives and open-shell species immediately following on a reduction path.     

Enantiomeric synthesis of carbocyclic d-4′-C-methylribonucleosides as potential antiviral agents

An efficient synthetic approach for the preparation of enantiomerically pure carbocyclic d-4′-C-methylribonucleosides 3a–f is reported. The key intermediate, d-2,3-O-cyclohexylidene-4-methylcyclopentenone 8, was prepared starting from d-ribose in eight steps via an oxidative rearrangement. Conjugate addition of a catalytic vinylcopper(I) reagent to the α,β-unsaturated ketone 8 yielded cyclopentyl alcohol 10, which bears a quaternary stereogenic carbon at the C4-position. The cyclopentyl alcohol 10 was subsequently coupled with 6-chloropurine or 2-amino-6-chloropurine via an S_N2 reaction, followed by a series of functional group transformations and deprotections to furnish purine ribonucleosides 3a–c. Pyrimidine bases were constructed on cyclopentylamine 29 using a linear approach, which furnished the pyrimidine nucleosides 3d–f.     

Substituted coumarin amidines: useful building blocks for the preparation of [1]benzopyrano[4,3-b]pyridin-5-one and [1]benzopyrano[4,3-d]pyrimidin-5-one derivatives

The synthesis of [1]benzopyrano[4,3-b]pyridin-5-ones 4a-f and 4g-j starting from 3-formylcoumarin and 3-cyanocoumarin N-functionalized amidines 3a-f and 3g-j, respectively, was reported. The ring-closure reaction mechanism, under basic or acidic media, was proposed. Furthermore, the reaction of 3-formylamidines 3a,c-f with ammonium acetate gave good yields of 2-substituted [1]benzopyrano[4,3-d]pyrimidin-5-ones 7.