Sudipta Pathak

Silica sulfuric acid: a reusable solid catalyst for one pot synthesis of densely substituted pyrrole-fused isocoumarins under solvent-free conditions

Summary A convenient and efficient methodology for the synthesis of densely substituted pyrrole-fused isocoumarins, which employs solid-supported silica sulfuric acid (SSA) as catalyst, has been developed. When the mixture of ninhydrin adducts of acetylacetone/ethyl acetoacetate and primary amines was heated on the solid surface of SSA under solvent-free conditions, the pyrrole-fused isocoumarins were formed in good yields. This synthetic method has several advantages such as the employment of solvent-free reaction conditions without the use of any toxic reagents and metal catalysts, the ease of product isolation, the use of a recyclable catalyst, the low cost, the easy availability of the starting materials, and the excellent yields of products.

Synthesis of 4-hydroxyindole fused isocoumarin derivatives and their fluorescence “Turn-off” sensing of Cu(II) and Fe(III) ions

A simple and efficient protocol has been developed for the synthesis of 4-hydroxyindole fused isocoumarins from easily available starting materials. Dihydroxy-indenoindoles, the cyclic hemiaminals of the condensation products of ninhydrin and enamines of 1,3-cyclohexanedione, produced indole fused isocoumarins 11-(aryl/alkyl)-8,9,10,11-tetrahydro-6-oxa-11-aza-benzo[a]fluorine-5,7-diones through an acid catalyzed intramolecular rearrangement. The above isocoumarin derivatives furnished novel 4-hydroxyindole fused isocoumarins 11-(aryl or alkyl)-7-hydroxy-11H-6-oxa-11-aza-benzo[a]fluoren-5-ones through dehydrogenation with Pd/C. The synthesized 4-hydroxyindole fused isocoumarins show fluorescence properties with good quantum yields and fluorescence “Turn-off” sensing of Cu2+ and Fe3+ ions. Importantly these molecules are found to be chemosensors only for Cu2+ ions with respect to UV-Vis spectral change and naked eye colour change in the presence and absence of UV radiation.

Facile cyclization in the synthesis of highly fused diaza cyclooctanoid compounds using retrievable nano magnetite-supported sulfonic acid catalyst

Magnetically separable Fe3O4–SO3H nano particles have been prepared and established as an efficient catalyst for the synthesis of dihydrofuran fused cyclooctanoids via the dehydration reaction of dihydroxy cyclooctanoid heterocylces. Nanomagnetite, Fe3O4–SO3H was prepared using an impregnation protocol and fully characterized by XRD, FT-IR, TEM, and FE-SEM-EDS. The catalyst was recycled for five consecutive cycles with almost unaltered catalytic activity. The dehydration step was performed keeping the acid sensitive amide linkage intact under ambient temperature and milder reaction conditions. The efficacy of the methodology lies in the excellent yield of product formation within a short span of reaction time avoiding undesirable side products due to the substrate decomposition by ring rupture and tedious purification and work-up procedure.

Monobromomalononitrile: an efficient regioselective mono brominating agent towards active methylene compounds and enamines under mild conditions

The potential of monobromomalononitrile (MBM) as a convenient source of cationic bromine in organic bromination reaction has been explored. Studies reveal that MBM can be a good substitute for N-bromosuccinimide (NBS) in various respects. Enamines and active methylene compounds bearing aromatic rings are selectively mono brominated on the vinylic and active methylene group respectively on reaction with MBM. This methodology has the advantages of easy preparation of MBM, shorter reaction time and high yields of the product formation. Moreover it provides a metal free green brominating agent which is more convenient for the pharmaceutical industry. Mono bromination reaction takes place only on active methylene groups even after addition of excess amount of MBM. Enamines containing electron withdrawing, electron donating and ortho substituted amines react smoothly affording only the vinylic mono bromo products in good yields without producing any side products.