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We herein report a serendipitously observed acid mediated rearrangement of monoethylene ketal of 2-methyl-2-(3-methylbut-2-en-1-yl)cyclohex-4-ene-1,3-diones to Dihydrobenzopyran and demonstrated the application of this methodology in the construction of core carbon scaffolds of dimethoxyajacareubin, cariphenone-A and crotamadine.

A regioselective synthesis of carbazole analogs belonging to both the categories of natural origin, viz., microorganisms and higher plant source is reported. The synthesis of carbazole derivatives possessing a methylester group at C-1 position has been achieved by Cadogan cyclization of nitro bi-phenyl derivatives, whereas, the carbazole analog possessing a methylester group at C-3 position was synthesized by Buchwald–Hartwig Pd-catalyzed cyclization of amino bi-phenyl derivatives. Suitably substituted bi-phenyl precursors were accessed from norbornyl α-diketones via Grob fragmentation, O-methylation and DBU aromatization reaction sequence. The reported carbazole derivatives possess structural features that are common with many carbazole natural products: mukonine, clausine-L, murrayafoline-A, and their sibling natural products.

An intermolecular FeCl3 catalyzed reaction between anilines and enol ethers is described. A variety of enol ethers and aromatic amines undergo a C C bond formation followed by cyclization via C N bond formation to afford the 2,3-disubstituted indoles, involving an unexpected aryl group migration. In this methodology, anilines act as bis-nucleophiles, wherein the initial attack occurs at the α-position of enol ether from the ortho position of aniline followed by the subsequent reaction of the amine moiety of aniline at the β-position, leading to the indole framework. This method is simple, obviates the use of expensive/hazardous transition-metal catalysts, and offers a wide range of substrate scope.

An efficient Bronsted acid-induced one-pot syntheses of 2-methylbenzofurans from norbornyl derivatives have been reported. The construction of new C-O bond proceeds via Grob-type fragmentation followed by cyclization and aromatization respectively. This synthetic strategy facilitates the efficient synthesis of 2-methylbenzofurans under acidic conditions. Significantly, this methodology is successfully applied to the synthesis of 6,7-bisaryl benzofurans.

An efficient metal-free synthesis of thiazolines, dihydrothiazole and enureas or thioureas from methyl enol ether with urea or thiourea derivatives in the presence of TFA (trifluoroacetic acid) are reported here. This synthetic protocol involves the formation of two bonds, C–N as well as C–S, in one step for the synthesis of thiazoline derivatives, dihydrothiazoles and a direct C–N bond for the synthesis of enamides with good to excellent yields. Further, this method was employed for the synthesis of wilsoniamine B alkaloid.

A facile and efficient base promoted one-pot dimerization of halo cyclohexene derivatives leading to a de novo synthesis of biologically important diarylmethane and diarylmethanone derivatives is reported. A plausible mechanism for the unexpected formation of the title compounds involves dehydrohalogenation, 1,6-conjugate addition, subsequent aromatization through C–C bond cleavage and dehydrohalogenation.

A macrocyclic furanobutenolide was crafted from readily available furan building-blocks to set-up a transannular Diels–Alder reaction which delivered the carbocyclic core present in a class of cembranoid diterpene natural products.

Total syntheses of two antimicrobial natural products (±)-cis-neocnidilide and (±)-trans-neocnidilide starting from a readily preparable cyclohexa[b]-fused 5-oxabicyclo[2.1.1]hexane derivative are presented. The diastereomeric tetrahydrofuran tricarboxylate epimers obtained from a BF3OEt2 promoted Grob-type fragmentation of the oxa-bicycle derivative were converted into title natural products by employing pyridinium dichromate/acetic anhydride mediated bis-oxidative cleavage reaction.

A formal synthesis of marine-derived antitumor natural product varitriol from a 5-oxabicyclo[2.1.1]hexane derivative is described. A tetrahydrofuran unit of varitriol embedded with four contiguous stereocenters was synthesized with an overall yield of 10.2% in 11 steps from an oxa-bicyclic system. An unprecedented oxidative cleavage reaction involving scissoring of two C C bonds at oxa-quaternary carbon of THFs leading to γ-butyrolactones was reported and a plausible mechanism has been proposed.

A full account of the total synthesis of aspidostomide B, C, their analogues and our synthetic efforts towards the synthesis of aspidostomide D, which led to the synthesis of regioisomeric N-methyl aspidostomide D, its analogues via epoxide opening strategy is presented. The synthesis of regioisomeric N-methyl aspidostomide D involves an efficient, five-step sequence, with 36.3% overall yield, starting from 3,4,5-tribromo-1H-pyrrole-2-carboxylic acid. The key features of this protocol are intramolecular cyclization, dehydration, oxidation, and a Lewis acid-mediated regioselective epoxide ring opening by C-3 position of 2,5-dibromo-1H-indole to furnish the title compounds.

A total synthesis of bioactive tetracyclic natural product paracaseolide A, embodying an architecturally unusual oxa-bowl framework, has been accomplished from commercially available 5-methyl-2-furfural. The key step involving a thermal [4+2]-dimerization of an appropriately crafted 5-methyl-3-alkenylbutenolide is shown to proceed in a stepwise manner.

A facile transformation of pentasubstituted tribromophenol derivatives to the corresponding alkyl-substituted dibromo-p-benzoquinones has been achieved using PbO2 as oxidizing agent in combination with 60% aq HClO4 in acetone in 70–72% yields. The electrochemical properties of these quinones were studied by means of cyclic voltammetry and compared with p-benzoquinone (BQ) and 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ) recorded under identical experimental conditions.

Nucleophilic racemic amines have been synthesized utilizing a C 7n C 5x free-radical cascade reaction from a bis-allyl amine starting material. Being potentially useful organocatalytic bases as is evident from their screening in the Baylis–Hillman reaction, optically pure amines were also synthesized from optically pure aldehydes (−) or (+)- 4 . Bis-allyl amides under similar radical reaction condition resulted in C 7n cyclized products.

Palladium(II)-catalyzed oxidative cyclization of methyl 2-allyl-4,5,6-tribromo-3-hydroxybenzoate 4 gave a mixture of methyl 5,6,7-tribromo-2-methyl-benzofuran-4-carboxylate 5 and methyl 6,7,8-tribromo-2H-1-benzopyran-5-carboxylate 6; the ratio of the two products varied between 71:29 and 24:76 depending on the reaction conditions. On the other hand, NBS or NIS mediated cyclization of 4 followed by treatment with NaOMe or DBU furnished benzofuran derivative 5 in good overall yield.

An efficient stereoselective synthesis of both the diastereomers of diquinane-based conformationally constrained symmetric bis-γ-lactones starting from tricyclic derivative 9 is reported. The key step involves the intramolecular ring opening of an epoxide by the in situ formed hydrate of a diketone leading to the tetracyclic hemiacetal 10, which directly leads to the exo-hydroxy bis-γ-lactone derivatives 2 and 4 under basic hydrogen peroxide cleavage conditions. Conversion to the endo-hydroxy bis-γ-lactone derivatives 1 and 3 was accomplished through lithium hydroxide mediated SN2 displacements in dimesylate 7, or more expeditiously, via trimesylate 21 through alkaline H2O2 mediated cleavage.

A new reaction of diazomethane with norbornyl α-diketones in MeOH as solvent leading regioselectively to ketals is described. The mechanistic details of this intriguing reaction were investigated employing d4-MeOH, EtOH and d6-EtOH. The surprising observation of complete deuterium incorporation in the diazomethane-derived methoxy group was accounted for by sequential deuterium exchange between the initially formed hemiketal d4-18 and diazomethane in the presence of d4-MeOH as solvent.

A two-step synthesis of marine origin brominated alkaloids wilsoniamines A ( 1 ) and B ( 2 ), isolated from Australian bryozoan Amathia wilsoni Kirkpatrick, possessing a novel hexahydro-1 H -pyrrolo[1,2- c ]imidazol-1-one ring system is described. The core hexahydro-1 H -pyrrolo[1,2- c ]imidazol-1-one ring systems is constructed through a condensation reaction between (2,4,6-tribromo-3-methoxyphenyl)acetaldehyde and ( S )- N -methylpyrrolidine-2-carboxamide as key steps. The orientation of substituted benzyl side-chain in the two epimeric natural products was controlled by carrying out the key condensation reaction under kinetic or thermodynamic conditions.

Knoevenagel condensation proceeds efficiently in recyclable [bmim]PF 6 and [bmim]BF 4 without any catalyst, and hydrotalcites in ionic liquid serve as a safe and recyclable reaction system for both Knoevenagel as well as nitroaldol condensations.

A simple protocol for the synthesis of an aesthetically pleasing ‘oxa-basket’ (5-methoxy-2,8,13-trioxapentacyclo[7.2.1.1.5,120.4,1106,10]tridecane-3,7-dione), possessing 9-oxa-noradamantane core, is described. The readily available diquinane based dichloro bis-γ-lactone precursor upon treatment with MeSO3H furnished dichloro ‘oxa-basket’ which was dechlorinated using TBTH/AIBN to obtain the title compound. The key role of the chlorines was demonstrated by replacement with hydrogen or allyl substituents which redirects the cascade to simple hydrolysis products.

A simple and useful protocol for the reduction of nitro arenes to their corresponding azoxy derivatives by employing zinc and NH4Cl in a mixture of [bmim][BF4] and water is described. The selective reduction of nitro to azoxy is attributed to the hitherto unknown moderating effect of oxygen on zinc metal.

Radical mediated selective carboncarbon bond forming processes and hydrodehalogenation at the bridgehead positions of tetrahalonorbornenes (which are readily accessible, inexpensive and precursors for numerous applications) is described.

Synthesis of a compound with bowl-like, symmetrical molecular architecture, starting from a Diels-Alder adduct 7, derived from a reaction between 2,5-dihydro-2,5-dimethoxyfuran and 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene, is reported.

A concise stereoselective route to (±)-pentenomycin 1 in 33% overall yield starting from the readily accessible Diels–Alder adduct 4 is reported. The key reaction involves decarbonylation of β-methoxy-α,β-unsaturated aldehyde 8 obtained from β-hydroxy-dimethylketal 6.

Electronically/electrostatically dictated face-selection in endo -substituted 7-norbornenones can be altered/reversed by means of host-guest interaction with β-CD. Reversal in face-selectivity is also observed in the reduction of 5,6- endo , endo -substituted 7-norbornenone 2a in solid state.

The X-ray structure of Image and MNDO optimized geometries of related 7-norbornenone derivatives show a clear tilt of the carbonyl bridge away from the C=C double bond. The preferred reduction from the more hindered face of the diester reveals the electron/electrostatic origin of π - facial selectivity in these systems. X-ray structure and MNDO calculations reveal the dominance of electronic effects in determining the π-facial selectivity in 4a.

The pronounced syn -face selectivity exhibited by 7-norbornenone in hydride reduction is dramatically altered (reversed) by the remote electron withdrawing substituents at the C2,C3- endo -position.