Letters in Organic Chemistry - Volume 11, Issue 8, 2014

L-proline is found to be an efficient catalyst for the Knoevenagel condensation of 2-chloroquinoline-3- carboxaldehyde 1a-c with an active methylene compound i.e., 2,4-thazolidinedione 2 in IPA affording novel substituted olefins 3a-c. The latter products reacted with N-substituted-3-phenylpiperazine 4a-c in the presence of KF in DMF to afford the corresponding 5-{[2-(2-phenylpiperazin-1-yl)quinolin]methylene}-2,4-dione derivatives 6a-i. Alternatively, 6ai were also synthesized from another reaction sequence 1 → 5 → 6. The structures of the synthesized compounds have been established on the basis of spectral and analytical data.

3-Substituted coumarins were synthesized very efficiently, using Knoevenagel method from salicylaldehydes 1 and active methylene compounds 2 under green conditions. The effect of catalyst and solvent on this condensation was studied. L-proline was found to be the best catalyst and triethanolamine the best reaction medium for this reaction.

The synthesis of a series of 1,2,4-triazole Schiff bases and Mannich bases incorporating an indole moiety is described. The triazole Schiff bases were synthesized from 4-amino-3-mercapto-5-[(1H-indol-3-yl)methyl]-1,2,4-triazole on treatment with a series of arylaldehyde in presence of (+)-tartaric acid as an acidic catalyst. The triazole Schiff bases are further condensed with piperidine and formaldehyde to yield the corresponding series of Mannich bases. The structures of Schiff bases and Mannich bases were established by IR, NMR and mass spectral data. All the synthesized compounds were screened for their antibacterial and free radical scavenging activities. Schiff base 2d comprising of dichloro substitution exhibited promising antibacterial activity against Bacillus subtilis spizizenni, Bacillus cereus and Staphylococcus aureus at MIC 7.81 μg/ml. Mannich bases demonstrated weak free radical scavenging activity when compared to their Schiff base counterparts.

Ultrasound and microwave assisted expedient synthesis of potential antibacterial compounds, 2-(anilino)-5- hydroxy-1,4-naphthoquinone derivatives 3a-c and 5-hydroxybenzo[f]indole-4,9-dione derivatives 8a-c and 10- hydroxybenzo[b]carbazole-6,11-dione derivatives 9a-c have been developed. For the preparation of 3a-c derivatives the methods Room Temperature Synthesis (RTS), Conventional Heating Synthesis (CHS) and Ultrasound Assisted Synthesis (UAS) were performed. In addition, the Conventional Synthesis (CS) and Microwave Assisted Synthesis (MAS) were used for 8a-c and 9a-c derivatives. UAS and MAS showed the best results and are considered green alternatives of synthesis. In general, the yields obtained were good to excellent (58 to 93%). In addition, antibacterial activity against five bacterial strains was tested, showing bacteriostatic activity at lower concentrations and greater bactericidal against Gram negative strains. The compounds carrying chlorine atoms at 2 and 4 positions on the phenyl ring were the most active. The results obtained indicate that the 1,4-naphthoquinone derivatives presented here have promising use as antibacterial agents. A reaction mechanism is also proposed.

Seven coumarin derivatives were screened for their inhibitory effect on laccase utilizing ABTS and L-DOPA as substrates. 2-[(4-Methyl-2-oxo-2H-1-benzopyran-7-yl)oxy]acetic acid (E)-2-[(dihydroxyphenyl)methylene]hydrazides were proven to be stronger inhibitors than corresponding thiosemicarbazides when ABTS [2,2'-azinobis(3- ethylbenzothiazoline-6-sulfonic acid)] was used as a substrate. In kinetic experiments mixed type inhibition was determined for 2-[(4-methyl-2-oxo-2H-1-benzopyran-7-yl)oxy]acetic acid (E)-2-[(2,5-dihydroxyphenyl)methylene]hydrazide and (E)-2-[(3,4-dihydroxyphenyl)methylene]hydrazide. In reaction of L-DOPA (L-3,4-dihydroxyphenylalanine) oxidation catalyzed by laccase none of the tested compounds has shown inhibitory effect.

Scutellarin (1) [5,6-dihydroxy-2-(4-hydroxyphenyl)-4-oxo-4H-1-benzopyran-7-yl β-D-glucopyranosiduronic acid] is very effective in the clinical treatment of cerebral infarction and coronary heart disease in China. Pharmacokinetic studies showed that scutellarin (1) is readily metabolized to scutellarein (2) [5,6,7-trihydroxy-2-(4-hydroxyphenyl)-4H-1- benzopyran-4-one] in the intestine by β-glucuronide enzyme prior to absorption. In order to improve the biological activity of scutellarin (1), our group has previously synthesized many scutellarin derivatives based on their in vivo metabolic mechanism. The results showed that morpholine ring substituted at C-7 or C-8 position induced better antioxidant activity, water solubility and anticoagulant activity compared to scutellarin (1). In this paper, an efficient synthetic method for the construction of 5,6,7-trihydroxy-2-[4-[2-(4-morpholinyl)ethoxy]phenyl]-4H-1-benzopyran-4-one (5) is reported. This synthetic route will facilitate the synthesis of scutellarein derivatives containing an amine side chain at the C- 4' position.

A simple and green method is reported for the completely chemoselective synthesis of some new dicoumarol derivatives {3,3'-(2-aryl-2-oxoethylidene)bis[4-hydroxycoumarin]s} from the reaction of 4-hydroxycoumarin with arylglyoxals in a molar ratio of 2:1 respectively. The reactions efficiently promoted by titanium(IV) oxide nanoparticles as a heterogeneous catalyst via an on water process. Catalyst loading is very low and it shows recyclability.

An efficient and eco-friendly synthesis of flavones, promoted by naturally occurring acids, via cyclodehydration of 1-(2-hydroxyphenyl)-3-aryl-1,3-propanediones using conventional and microwave heating under solvent-free condition is described.

In this contribution we describe a series of pyranylidene derivatives bearing bromine atoms in various positions. Some of these bromo derivatives have been converted in carboxylic acid by halogen-metal exchange reaction using gaseous CO2 as electrophile. Selected compounds have been studied by UV-visible absorption and cyclic voltametry to study the influence of the substituent on the pyranylidene core.

An improved synthetic route of (Z)-2-(5-amino-1,2,4-thiadiazol-3-yl)-2-(ethoxyimino)acetic acid (2a), an important intermediate for the preparation of Ceftaroline, from N-(3-aminoisoxazol-5-yl)acetamide (8) in total yield of 47% has been developed. The key intermediate (Z)-N-(3-(2-acetamido-1-(ethoxyimino)-2-oxoethyl)-1,2,4-thiadiazol-5- yl)pivalamide (12a) was easily isolated from (Z/E)-N-(3-(2-acetamido-1-(ethoxyimino)-2-oxoethyl)-1,2,4 -thiadiazol-5- yl)pivalamide. The title compound 2a was obtained from the hydrolysis of 12a with a yield of 83%, and its structure was comfirmed by X-ray single crystal diffraction analysis. Moreover, (E)- and (Z)-N-(3-(2-acetamido-1-(ethoxyimino)-2 - oxoethyl)-1,2,4-thiadiazol-5-yl)pivalamide were prepared respectively from the etherification of oxime hydroxyl group under different conditions. This improved procedure has many appealing attributes such as convenient separation, good yields, and easy access to large scale.

A series of phenylhydrazone derivatives was synthesized from arenediazonium tetrafluoroborates and active methylene compounds under high-speed ball milling. The reaction occurred in the absence of the solvent and products were obtained in good yield within short reaction times (no more than 30 min).

(S)-N-Boc-3-Hydroxyadamantylglycine is a key intermediate of saxagliptin. It was synthesized from 1- adamantanecarboxylic acid to afford 1-acetyladamantane (2), which was converted into 2-(1-adamantyl)-2-oxoacetic acid (3) through oxidation, and then into 1-adamantyl-2-hydroxyimino acetic acid (4) followed by treatment of hydroxylamine hydrochloride, then 4 was reducted and Boc-protected to give N-Boc-adamantylglycine (5), which was oxidized with KMnO4 and treated with a chiral base. Utilizing this route, (S)-N-Boc-3-Hydroxyadamantylglycine was prepared. This work is to elaborate a convenient route to synthesize (S)-N-Boc-3-Hydroxyadamantylglycine and provide a new idea for the synthesis of saxagliptin.