Regioselective synthetic approach for key precursors of 6-arylbenzo[c]phenanthridin-10-ol derivatives: a useful compound for selective chromogenic recognition of fluoride.

A regioselective synthetic strategy for 6-aryl-8,9-dihydrobenzo[c]phenanthridine-10(7H)-ones (4) is accomplished using a one-pot four-component reaction by fine-tuning the reaction temperature. DMSO is excellently used as a reactant-cum-solvent to introduce a carbonyl functionality regioselectively at the C-10 position of the benzophenanthridine backbone, via an MCR, which is unknown yet. The elegant features of this strategy are the formation of two CC, one CN, and one CO bonds in a single step, without using a base and an activator for the oxygenation process. Then, a few compounds (4) are easily aromatised to achieve 6-arylbenzo[c]phenanthridin-10-ol derivatives (7) using I2/DMSO at 100 °C. Nay, a dangling hydroxyl group in 4s, 4u, 4x, and 4z helped them to be employed as promising 'naked eye' colorimetric chemosensors for fluoride with limits of detection of 0.65, 0.60, 0.34, and 2.2 ppm, respectively. Moreover, the reversibility of the chemosensors makes them suitable for combinatorial INHIBIT logic gate formulation. The compounds have also been employed for solid-state F- detection via the spot TLC test.

DMSO-assisted environmentally benign synthesis of benzo[c]-chromeno[4,3,2-gh]phenanthridines by remote oxidative hetero cross-coupling cyclization and aromatization reaction.

An unprecedented metal-free and catalyst-free synthesis of benzo[c]chromeno[4,3,2-gh]phenanthridine derivatives, a class of 1,6-diheterophenalenoid heterocycle, is reported for the first time. The oxidative cross-coupling reaction for the remote cyclization is achieved through the in situ generated o-quinone methide intermediate followed by an electrocyclic ring closure reaction. The aromatization of the cyclohexane ring is achieved by sequential H shift, hydroxylation, and elimination reaction. DMSO-assisted concomitant cyclization and aromatization reactions are also disclosed for the first time.

Reactivity switch-over of 4-hydroxydithiocoumarins under various conditions and their application in organic synthesis.

4-Hydroxydithiocoumarin is a valuable organic precursor to architect important heterocycles. The three different reactive nucleophilic sites in 4-hydroxydithiocoumarins display intriguing regioselectivity in their reaction towards various electrophiles. Previously, 4-hydroxydithiocoumarins have been used in the synthesis of heterocycles using Claisen and thio-Claisen reactions. Hetero-Diels-Alder reactions involving 4-hydroxydithiocoumarins have proven to be very convenient in assembling complex molecular organic frameworks from readily available feedstocks. Development of multicomponent reactions employing 4-hydroxydithiocoumarins has given rise to several important reaction protocols to access polycyclic compounds. Recently, this moiety has been used in forging some unusual S-S, S-N and S-O bonds under oxidative conditions which further explores its hidden reactivity in organic synthesis. Besides that, hydrothiolation of various alkynes and alkenes using 4-hydroxydithiocoumarins has led to the synthesis of some potential lead molecules. This mini-review provides an account of the reactivity pattern of 4-hydroxydithiocoumarins and their strategic applications in various reactions for the synthesis of several heterocycles and other important organic syntheses.

Synthesis of vinyl sulfides and thioethers via a hydrothiolation reaction of 4-hydroxydithiocoumarins and arylacetylenes/styrenes.

The synthesis of vinyl sulfides (3a-m) and thioethers (7a-k), exclusive Markovnikov products, is reported by a copper(I) iodide catalyzed regioselective hydrothiolation reaction of terminal alkynes/alkenes and 4-hydroxydithiocoumarins. However, anti-Markovnikov hydrothiolation products (5a-f) were obtained in the case of 2-ethynylpyridine, with exclusive Z selectivity in good yields. The important aspects of this protocol are the absence of expensive metal complexes and additives to act as ligands, mild reaction conditions, high regioselectivity, good yields, and shorter reaction times.

Synthesis of biologically active fused 1,4-oxathiin derivatives from 4-hydroxydithiocoumarins, arylacetylenes and dimethyl sulfoxide by Cu(i)-catalyzed C-H functionalization and cross-dehydrogenative C-S coupling reactions.

The hitherto unreported 2-aryl-10H-thiochromeno[3,2-b][1,4]oxathiin-10-one derivatives are obtained in a single pot from 4-hydroxydithiocoumarins, arylacetylenes and dimethyl sulfoxide in the presence of 10 mol% CuI and K2CO3 in an oil bath at 70 °C. The novelties of the present protocol are (i) selective C-H functionalization at the C-3 position of 4-hydroxydithiocoumarin, (ii) regioselective hydrothiolation with arylacetylenes and (iii) concomitant cyclisation. The major advantages are mild reaction conditions, broad substrate scope and good yield. Among the synthesized compounds, the following five compounds 3aa, 3bd, 3ec, 3fa, and 3fd showed anticancer activity against a human breast cancer cell line (MCF-7) and a cervical cancer cell line (HeLa).

Newly synthesized 3-sulfenylindole derivatives from 4-hydroxydithiocoumarin using an oxidative cross dehydrogenative coupling reaction (OCDCR): potential lead molecules for antiproliferative activity.

An expedient and efficient synthetic method was developed for the oxidative cross dehydrogenative coupling reaction between 4-hydroxydithiocoumarin and indole at the C-3 position regio-selectively using a combination of 10 mol% molecular iodine and TBHP in the presence of 10 mol% CuBr2 as an additive at room temperature. Mild reaction conditions, good yields and a broad substrate scope are some of the salient features of the present protocol. Additionally, the synthesized 3-sulfenylindoles derived from 4-hydroxydithiocoumarin were converted into biologically active sulfone derivatives. Interestingly, some of the compounds exhibit anti-cell proliferative activity on breast cancer (MCF-7) cells due to reactive oxygen species (ROS) mediated cell damage.

Reaction behaviour of arylamines with nitroalkenes in the presence of bismuth(iii) triflate: an easy access to 2,3-dialkylquinolines.

We report the reaction behaviour of arylamines with nitroalkenes in the presence of bismuth(iii) triflate (10 mol%) and diacetoxyiodobenzene (10 mol%). We obtained 2,3-dialkylquinoline derivatives instead of the expected 3-alkylindole derivatives. The present reaction is an alternative approach for the synthesis of 2,3-dialkylquinoline derivatives under milder conditions. Furthermore, we establish the mechanistic pathway by theoretical calculations using Gaussian 09 software [B3LYP/6-311+G(d,p)], which shows that the conventional aza-Michael reaction is preferred over Michael addition. Aliphatic nitroalkenes behave in a different manner than aromatic nitroalkenes. An aza-Michael adduct gives rise to an imine by the elimination of water which may tautomerize to the corresponding enamine. The resulting imine and enamine intermediates react together to afford the desired quinoline derivatives. This protocol has the advantages of consecutive formation of one C-N and two C-C bonds, high regioselectivity, broad substrate-scope and good yields.

Iodine monobromide catalysed regioselective synthesis of 3-arylquinolines from α-aminoacetophenones and trans-ß-nitrostyrenes.

A simple and efficient method for regioselective synthesis of 3-arylquinolines is described from α-aminoacetophenones and trans-ß-nitrostyrenes using 20 mol% iodine monobromide as a catalyst in acetonitrile solvent at 80 °C. The present method involves tandem reaction of α-aminoacetophenones and trans-ß-nitrostyrenes, formation of two new C-C bonds and cleavage of one C-C bond in a single step. The salient features of the protocol are metal- and oxidant-free reaction conditions, broad substrate scope, and good yields.