One-pot Synthesis of 2-Aminoindole through SET Oxidative Cyclization: Concise Synthesis of Tryptanthrin and Phaitanthrin E.

An efficient domino approach for the synthesis of biologically important 2-aminoindole derivatives has been developed using CuBr2 -mediated SET oxidative cyclization as a key step. This one-pot multicomponent strategy utilizes readily available ethyl propiolate, tosyl azide, and substituted aryl amines as starting materials. The generality and scope of this mild method are demonstrated with a wide variety of substrates to furnish functionalized 2-aminoindoles in good yields. The synthetic power of this strategy is further exemplified in the concise synthesis of biologically important alkaloids, Phaitanthrin E and Tryptanthrin.

Domino Semipinacol/Iterative Aldol/Iso-Nazarov Cyclization to Triaryl-cyclopentenone: Enantioselective Synthesis of Combretastatin A-4 Analogues.

A facile domino strategy has been developed for the synthesis of a biologically active cyclopent-2-enone core containing combretastatin A-4 (CA-4) analogues from aryloxirane and aryl aldehyde. This one-pot method involves a sequence of semipinacol rearrangement, iterative aldol condensation, and iso-Nazarov cyclization reactions. The scope of this methodology is further shown in the enantioselective synthesis of 5-hydroxy-cyclopent-2-enones using the Sharpless AD catalyst. Biological studies reveal that (S)-enantiomers exhibit better inhibitory activity against cell proliferation than (R)-enantiomers.

Synthesis of 5-unsubstituted dihydropyrimidinone-4-carboxylates from deep eutectic mixtures.

A facile one-pot synthesis of 5-unsubstituted dihydropyrimidinones from ß,γ-unsaturated ketoesters in low melting ʟ-(+)-tartaric acid-N,N-dimethylurea mixtures is reported. This solvent-free method is very general and provides easy access to 5-unsubstituted dihydropyrimidinone-4-carboxylate derivatives in good yields.

Reversal of polarity by catalytic SET oxidation: synthesis of azabicyclo[m.n.0]alkanes via chemoselective reduction of amidines.

A one-pot catalytic method has been developed for the stereoselective synthesis of cyclopropane-fused cyclic amidines using CuBr2/K2S2O8 as an efficient single electron transfer (SET) oxidative system. The generality of this mild method is demonstrated with a wide variety of substrates to furnish pharmaceutically important amidines containing aza-bicyclic and novel aza-tricyclic frameworks in very good yields. A chemoselective reduction of cyclic amidines to 2-/3-azabicyclo[m.n.0]alkanes and octahydroindoles has been developed using a NaBH4/I2 reagent system. The synthetic scope of the chemoselective reduction of the amidine functionality has been exemplified in the stereoselective synthesis of an iminosugar based (±)-epiquinamide analogue.

A novel one-pot method for the stereoselective synthesis of tetrahydropyrimidinones in a low melting mixture.

A direct and metal free one-pot method has been developed for the stereoselective synthesis of tetrahydropyrimidinone derivatives from a vinyl arene and formaldehyde using a tartaric acid-dimethylurea (TA : DMU) melt as a green reaction medium. The substrate scope of this method is very general and the tetrahydropyrimidinone (THPM) derivatives are synthesized in good yields with a high degree of diastereoselectivity. In this reaction, the melt plays a triple role as the solvent, catalyst and reagent.

Silver oxide mediated novel SET oxidative cyclization: stereoselective synthesis of 3-azabicyclo[n.1.0]alkanes.

For the first time, silver oxide catalyzed SET-oxidative cyclization of an α-amidinoester tethered with an alkene, leading to a novel cyclopropane-fused cyclic amidine, has been developed. This efficient method permits easy access to the pharmaceutically important 3-azabicyclo[n.1.0]alkane framework, having a quaternary centre. The generality of SET oxidative cyclization using persulfate as a co-oxidant has been demonstrated with a wide variety of α-amidinoesters in good yields. Unusual chemoselective dialkylation of amidine with Grignard reagent and regioselective opening of the cyclopropane ring using organocopper reagent have also been developed.

A Domino Aza-Piancatelli Rearrangement/Intramolecular Diels-Alder Reaction: Stereoselective Synthesis of Octahydro-1 H-cyclopenta[ cd]isoindole.

For the first time, an efficient one-pot method for the construction of an angularly fused 5-6-5 aza-tricyclic framework has been developed in a highly stereoselective manner. This domino reaction is a novel combination of aza-Piancatelli rearrangement and intramolecular Diels-Alder reaction, which readily furnishes hexahydro-2a,5-epoxy-cyclopenta[ cd]isoindole adducts, bearing six contiguous stereogenic centers in very good yields. The BBr3-mediated cleavage of the oxa-bridged adduct results in the formation of octahydro-1 H-cyclopenta[ cd]isoindole, an aza-tricyclic BCE core of a gracilamine alkaloid.

One-Pot Synthesis of Structurally Diverse Iminosugar-Based Hybrid Molecules.

A one-pot iminium-ion-based strategy has been developed for the synthesis of structurally novel iminosugar-based hybrid molecules. Iminium ion derived from l-rhamnose lactol-mesylate reacted with electron-rich aromatic systems in an inter/intra molecular fashion to furnish pyrrolidine-based iminosugar C-aryl glycosides with a high degree of stereoselectivity. Iminium ion also reacted readily with active methylene compounds such as 4-hydroxycoumarin, 4-hydroxyquinolinone, and lawsone to provide iminosugar C-coumarin/quinolinone/naphthoquinonyl glycosides in very good yields. Azomethine ylide generated from an iminium ion derivative underwent dipolar cycloaddition reaction with 1,4-quinones to furnish novel isopyrrolonaphtho/anthroquinon-based iminosugar-hybrids. The preliminary cytotoxic activities of some of the synthesized iminosugar-hybrids have been assayed against various human cancer cell lines and some of the hybrid molecules exhibited promising anticancer activities.

Iminosugar C-Nitromethyl Glycoside: Stereoselective Synthesis of Isoxazoline and Isoxazole-Fused Bicyclic Iminosugars.

A simple and efficient method for the stereoselective synthesis of isoxazoline/isoxazole-fused iminosugar derivatives has been developed using intramolecular nitrile oxide cycloaddition (INOC) as a key step. Iminosugar C-nitromethyl glycosides, derived from simple carbohydrates, served as excellent nitrile oxide precursors in 1,3-dipolar cycloaddition reactions. N-Alkenyl iminosugar C-nitromethyl glycosides afforded novel isoxazoline-fused indolizidine-, pyrrolizidine-, and quinolizidine-based iminosugars in excellent yields with a high degree of stereoselectivity, whereas N-alkynyl iminosugar C-nitromethyl glycosides furnished the corresponding isoxazole containing tricyclic iminosugars in very good yields.

One-Pot Synthesis of Cyclopropane-Fused Cyclic Amidines: An Oxidative Carbanion Cyclization.

A novel and efficient one-pot method has been developed for the synthesis of cyclopropane-fused bicyclic amidines on the basis of a CuBr2 -mediated oxidative cyclization of carbanions. The usefulness of this unique multicomponent strategy has been demonstrated by the use of a wide variety of substrates to furnish novel cyclopropane-containing amidines with a quaternary center in very good yields. This ketenimine-based approach provides straightforward access to biologically active and pharmaceutically important 3-azabicyclo[n.1.0]alkane frameworks under mild conditions. The synthetic power of this methodology is exemplified in the concise synthesis of the pharmaceutically important antidepressant drug candidate GSK1360707 and key intermediates for the synthesis of amitifadine, bicifadine, and narlaprevir.

Iminosugar C-Nitromethyl Glycosides and Divergent Synthesis of Bicyclic Iminosugars.

An efficient one-pot method for the stereoselective synthesis of novel iminosugar C-nitromethyl glycosides is described. This new class of iminosugar glycosides has versatile nitromethyl functionality whose utility was further demonstrated in the single-step synthesis of bicyclic iminosugars. Under reagent-free conditions, the N-allyl-C-nitromethyl glycosides resulted in intramolecular cyclization to iminosugar-oximes, whereas under SET oxidation, they furnished cyclopropane-fused iminosugars. The N-propargyl-C-nitromethyl glycosides underwent an unprecedented ketenimine-acrylamidine-Michael addition cascade reaction to give bicyclic amidines.

A Short and Efficient Synthesis of Iminosugar 2-Acyl Indolizidine.

A facile and convergent approach has been developed for the stereoselective construction of biologically important polyhydroxylated 2-acyl indolizidine framework using aza-Cope rearrangement-Mannich cyclization as a key step. The generality of this methodology is demonstrated with various lactol-tosylates derived from carbohydrates. The presented method provides an easy access to indolizidine- and tetrahydroindolizine-based iminosugar derivatives in good yields.

Benzylidene Acetal Protecting Group as Carboxylic Acid Surrogate: Synthesis of Functionalized Uronic Acids and Sugar Amino Acids.

Direct oxidation of the 4,6-O-benzylidene acetal protecting group to C-6 carboxylic acid has been developed that provides an easy access to a wide range of biologically important and synthetically challenging uronic acid and sugar amino acid derivatives in good yields. The RuCl3 -NaIO4 -mediated oxidative cleavage method eliminates protection and deprotection steps and the reaction takes place under mild conditions. The dual role of the benzylidene acetal, as a protecting group and source of carboxylic acid, was exploited in the efficient synthesis of six-carbon sialic acid analogues and disaccharides bearing uronic acids, including glycosaminoglycan analogues.

One-Pot Synthesis of Hydrophobically Modified Iminosugar C-Alkynylglycosides: Facile Synthesis of Polyhydroxy Tetrahydroindolizines.

A mild and efficient one-pot method has been developed for the stereoselective synthesis of structurally diverse novel iminosugar C-alkynylglycosides. The generality of this methodology has been demonstrated with a wide variety of amines and copper acetylides. This one-pot method has been exploited in the synthesis of new class of DNA cross-linking agents, polyhydroxy 1-vinyl-tetrahydroindolizine derivatives.

A chemoselective oxidation of monosubstituted ethylene glycol: facile synthesis of optically active α-hydroxy acids.

A mild and efficient method for the synthesis of optically active α-hydroxy acids through chemoselective oxidation of monosubstituted ethylene glycols using the TEMPO-NaOCl reagent system is described. It is evident from our studies that the solvent, pH and reaction temperature are very crucial for the success of this oxidation. The versatility of this method has been demonstrated with a variety of aliphatic, aromatic and carbohydrate substrates bearing various functional groups.

A diversity oriented one-pot synthesis of novel iminosugar C-glycosides.

A mild and highly efficient one-pot method has been developed for the stereoselective synthesis of structurally diverse novel iminosugar C-aryl glycosides. The generality of this methodology is demonstrated with a wide variety of aryl nucleophiles and amines. The synthetic potential of this methodology is further shown in the domino synthesis of iminosugar based hybrid molecules.

Synthesis of substituted hydantoins in low melting mixtures.

A novel domino synthesis of 1,3,5-trisubstituted hydantoin derivatives has been developed in low melting L-(+)-tartaric acid-DMU melt mixtures. The functionalized hydantoins are obtained in good yields from ß,γ-unsaturated ketoacids and urea under environmentally benign and simple reaction conditions.

A mechanistic study on the Hooker oxidation: synthesis of novel indane carboxylic acid derivatives from lapachol.

The Hooker oxidation is one of the most intriguing transformations wherein lapachol (1) is readily converted to norlapachol (2) in very good yield. This one-pot reaction involves a very intricate mechanism in which the alkyl side chain of lapachol is shortened by one carbon unit. Previous studies have unequivocally established the involvement of an indane carboxylic acid derivative 3, as a key intermediate (Hooker intermediate), and its simultaneous conversion to norlapachol (2) via the oxidative cleavage of vicinol diol and subsequent intramolecular aldol reaction of the resulting keto acid. However, the formation of the key Hooker intermediate 3 from lapachol (1) remains ambiguous. The present study has thrown some light on the formation of the key intermediate 3 from lapachol (1) via benzilic acid rearrangement of the corresponding labile o-diquinone intermediate 8 derived from lapachol. The involvement of o-diquinone intermediate 8 in the Hooker oxidation has been further established by trapping of this labile intermediate as the corresponding phenazine derivative 9. The involvement of benzilic acid rearrangement as a key step in the Hooker oxidation is further shown with a variety of o-quinones prepared from lapachol (1).

Fischer indole synthesis in low melting mixtures.

Functionalized indoles are synthezised under mild conditions in a tartaric acid-dimethylurea melt. The melt serves as the solvent and as the catalyst. Under these reaction conditions, sensitive functional groups such as N-Boc, N-Cbz, or azides are stable, and indolenines are obtained regioselectively in excellent yields. The practical use of the method is demonstrated in the synthesis of the hormone melatonin.

Concise enantioselective construction of a bridged azatricyclic framework via domino semipinacol-Schmidt reaction.

A TiCl(4)-promoted domino semipinacol-Schmidt reaction of oxaspiropentane-azide provides an easy access to bridged azatricyclic ring systems, which possess the azaquaternary center, present in the immunosupressant FR901483 and platelet aggregation inhibitor daphlongeranine B.