Current Organic Synthesis - Volume 15, Issue 7, 2018

Background: Among the numerous bond-forming patterns, C–C bond formation is one of the most useful tools for building molecules for the chemical industry as well as life sciences. Recently, one of the most challenging topics is the study of the direct coupling reactions via multiple C–H bond cleavage/activation processes. A number of excellent reviews on modern C–H direct functionalization have been reported by Bergman, Bercaw, Yu and others in recent years. Among the large number of available methodologies, Pdcatalyzed reactions and hypervalent iodine reagent mediated reactions represent the most popular metal and non-metal involved transformations. However, the comprehensive summary of the comparison of metal and non-metal mediated transformations is still not available. Objective: The review focuses on comparing these two types of reactions (Pd-catalyzed reactions and hypervalent iodine reagent mediated reactions) based on the ways of forming new C–C bonds, as well as the scope and limitations on the demonstration of their synthetic applications. Conclusion: Comparing the Pd-catalyzed strategies and hypervalent iodine reagent mediated methodologies for the direct C–C bond formation from activation of C-H bonds, we clearly noticed that both strategies are powerful tools for directly obtaining the corresponding pruducts. On one hand, the hypervalent iodine reagents mediated reactions are normally under mild conditions and give the molecular diversity without the presence of transition-metal, while the Pd-catalyzed approaches have a broader scope for the wide synthetic applications. On the other hand, unlike Pd-catalyzed C-C bond formation reactions, the study towards hypervalent iodine reagent mediated methodology mainly focused on the stoichiometric amount of hypervalent iodine reagent, while few catalytic reactions have been reported. Meanwhile, hypervalent iodine strategy has been proved to be more efficient in intramolecular medium-ring construction, while there are less successful examples on C(sp3)–C(sp3) bond formation. In summary, we have demonstrated a number of selected approaches for the formation of a new C–C bond under the utilization of Pd-catalyzed reaction conditions or hyperiodine reagents. The direct activations of sp2 or sp3 hybridized C–H bonds are believed to be important strategies for the future molecular design as well as useful chemical entity synthesis.

Background: Spiro skeleton is an important structural motif in natural products, usually accounting for their formidable architectures and remarkable biological profiles. Recent advances demonstrated that a diverse range of scenarios held the promise for the assembly of spirocenter based on ortho-dearomatization strategy. In this article, we describe the latest development of ortho-dearomatization reaction in constructing spirocyclic scaffolds with an all-carbon ring junction from a methodological standpoint. Objective: The review focuses on recent progress in the area of ortho-dearomatization reaction in constructing spirocyclic scaffolds with an all-carbon ring junction. Conclusion: In summary, we have summarized the capability of ortho-dearomatization reaction to construct spirocyclic skeleton, a common structural pattern found at the core of numerous natural products with broad structural diversities and important bioactivities. Success in this area will benefit not only the area of synthetic chemistry through methodological development, but also medicinal chemistry and chemical biology by providing access to rapid assembly of bioactive compounds' core scaffolds.

Background: Pictet-Spengler reactions is an irreplaceable part of cyclization reaction leading to the formation of indispensable heterocyclic moieties including imidazole, benzoxazole, pyrrole, indole and others having immense biological and chemical significance. Researchers have explored this reaction using different types of catalysts and reactions conditions (including solvents, acids, etc.) to ensure the better selectivity, less reaction time and high product yields. A total of five Pictet-Spenglerases have been discovered from various sources including plants, animals, fungi, and microbes, and are responsible for the synthesis of various important alkaloids of biological medicinal importance. Objective: The present review is a strenuous effort to assemble information mainly focusing on synthetic as well as biological Pictet-Spengler reactions catalysed by enzymes called Pictet-Spenglerase. Conclusion: In the present review, the recent advances in the PS-mediated synthesis of diverse heterocycles such as tetrahydroisoquinoline, tetrahydro-β-carbolines, tetrahydroimidazopyridines and other fused heterocycles via chemical as well as enzymatic pathways have been covered. The compounds find their scope as medicinal agents for the treatment of cancer, tuberculosis, bacterial infection, leishmanial, etc. The compilation is expected to provide a mechanistic insight to chemists to enhance the reaction condition, yields and another parameter to ensure the safe and inexpensive reaction conditions considering the “Green-Concept” of chemistry.

Background: Due to significant biological activity associated with N-, O- and S-heterocycles, a number of reports for their synthesis have appeared in recent decades. Traditional approaches require expensive or highly specialized equipment or would be of limited use to the synthetic organic chemist due to their highly inconvenient approaches. This review summarizes the applications of copper catalysts with the emphasis on their synthetic applications for nitrogen bearing polyheterocylces. In summary, this review article describes the synthesis of a number of five-membered poly heterocyclic rings. Objective: Nowadays new approaches that employ atom-economical and efficient pathway have been developed. The researchers are following natural models to design and synthesize heterocycles. The transition metal catalyzed protocols have attracted the attention as compared to other synthetic methodologies because they use easily available substrates to build multiple substituted complicated molecules directly under mild conditions. In organic synthesis, constituted by transition metal catalyzed coupling transformations are one of the most powerful and useful protocols. The N-heterocycles are synthesized by this convenient and useful tool. Conclusion: The efficient and chemoselective synthesis of heterocycles by this technique has appeared as an important tool. This review shows a highly dynamic research field and the employment of copper catalysts in organic synthesis. Several strategies have been pointed out in the past few years, to meet more sustainable, efficient and environmentally benign chemical products and procedures. The catalytic strategies have been the focus of intense research because they avoid the use of toxic reagents. Among these catalytic strategies, highly rewarding and an important method in heterocycles synthesis is metal catalyzed synthesis.

Background: Benzofurans are very important structural motifs found in a great number of natural products and biologically active compounds. Many commercially available drugs, including citalopram, dronedarone, saprisartan, darifenacin, and galantamine are derived from benzofuran core entities. Due to the diversity of benzofuran derivatives in the therapeutic response profile, developing novel and truly efficient methods for their synthesis from low-cost and easily accessible starting materials in one-step has been the subject of number of papers in recent years. Objective: Propargylic ethers are one of the most specific class of heteroatom-containing alkynes showing a large application as intermediates in organic synthesis. These compounds were successfully transformed into various organic compounds, including chromenes, carbazoles, cyclopentanones, 1, 2-dihydropyridines, α, β- unsaturated ketones, alcohols, allenes and many more. Synthesis of benzofurans via intramolecular cyclization of aryl propargyl ethers has become one of the most popular applications of theses versatile compounds. In this review we will highlight the recent discoveries and advances in this interesting research arena. Method: The review is divided into two major sections. The first will discuss cyclization of ortho-halo aryl propargylic ethers, while the second focuses exclusively on cyclization of ortho-unsubstituted aryl propargyl ethers. It is should be mentioned that special emphasis is placed on the mechanistic aspects of these reactions. Conclusion: Synthesis of benzofuran derivatives via intramolecular cyclization of corresponding aryl propargyl ethers have witnessed rapid and comprehensive development in recent years. The main advantages of this synthetic strategy include the use of simple, inexpensive, non-toxic, and readily accessible starting materials, and its pot, atom, and step economy.

Aim and Objectives: Development of simple synthetic routes for widely used organic compounds from readily available reagents is one of the major tasks in organic chemistry. Therefore, new approaches for increasing the molecular diversity of simple starting materials, are needed. Herein, an efficient synthesis of imidazo[1,2-a]pyridine, pyrido[1,2-a]pyrimidine and pyrido[1,2-a][1,3]diazepine derivatives is described. Materials and Methods: A one-pot, multi-component reaction of nitro ketene aminal derived from the addition of various 1,n-diamines to 1,1-bis(methylthio)-2-nitroethene with cyanoacetamide and aromatic aldehydes is described. The reactions are completed within 2-5 h, in ethanol at reflux, in good to high yields (70-93%). The structures of products were deduced from their IR, mass, 1H NMR, and 13C NMR spectra. Results: Optimal reaction conditions for the synthesis of products were obtained, when ethanol was used as the solvent at reflux. This protocol involves Michael reaction, imine–enamine tautomerization, and cyclization sequences. Conclusion: This work represents an efficient synthesis of imidazo[1,2-a]pyridine, pyrido[1,2-a]pyrimidine and pyrido[1,2-a][1,3]diazepine derivatives via a one-pot, multi-component reaction. The advantages of this protocol are mild conditions, easy accessibility of reactants, absence of catalyst, high atom economy, simple work-up and purification process with no chromatographic technique.

Aim and Objective: During the investigation of sodium nitrite-catalyzed oxidative coupling reaction of aryls, an unprecedented C(sp2)-H and C(sp3)-H coupling of substituted 2-aryl acetonitrile was found. Materials and Methods: The structure of the coupled product was confirmed by 1H and 13C NMR spectroscopy and high-resolution mass spectrometry (HRMS), and comparison of its derivatives with known compounds. The effects of methoxy group in the benzene ring on the reaction were evaluated. Results: The optimized reaction conditions are summarized as follows: CF3SO3H/substrate = 1.5 equiv., NaNO2/substrate = 0.3 equiv., CH3CN as solvent. 2-(4-Methoxyphenyl)acetonitrile and 2-(3,4,5- trimethoxyphenyl)acetonitrile could also generate C(sp2)-H and C(sp3)-H coupling. The coupling reaction occurred as a typical radial mechanism. Conclusion: An unprecedented cyano-induced, NaNO2-catalyzed oxidative C(sp3)-H and C(sp2)-H coupling was reported. The reaction proceeded under very mild conditions, using O2 in the air as terminal oxidant. The unique oxidative manner might provide more inspiration for the development of intriguing oxidative coupling reactions.

Aim and Objective: To develop efficient method for the synthesis of unusual oxetanes bicycle and diversely functionalized oxetane motifs bearing medicinally relevant functional group in new chemical space and evaluation as cytotoxic agents. Materials and Methods: General procedure for the synthesis of 2, 3, 9 and 11: A mixture of L-ascorbic acid derivative 1 or 8 or 10 (10 mmol) and benzophenone (12 mmol) in dry benzene (125 mL) was purged with dry nitrogen for 5 min. The solution was then irradiated for 20-24 h with UV light (125 W, medium-pressure mercury lamp) in an immersion-well photoreactor at ambient temperature under nitrogen. After completion of the reaction, the solution was washed with aqueous NaHCO3 (10%), dried (Na2SO4), and concentrated under reduced pressure to give crude product which was purified by column chromatography using ethyl acetate: n-hexane (5-20%) as eluent to yield pure product as white solid. Results: 3-O and 2-O methyl derivatives of 5,6-O-isopropylidene-L-ascorbic acid 8 and 10 were irradiated with Benzophenone to give single regioisomer 9 and 11 in 78 % and 69 % yield respectively, while in case of 5,6-O-isopropylidene-2,3-di-O-methyl-L-ascorbic acid, it gave both regioisomer 2 and 3. The presence of only oxetanes moiety improved the cytotoxic activity compared to oxetanes bicycle moiety. It was scrutinized that all the compounds in series of 2 and 4 displayed more MCF-7 cell proliferation than their regioisomers 3 and 6. Conclusion: We have developed a versatile strategy to prepare diversely functionalized fused oxetane bicycles involving alkoxy, hydroxy methyl, alkyl, and aryl substituents. A wide variety of functional groups have been introduced on the oxetane ring, accessing new chemical space. These compounds were tested for growth inhibition against MCF-7 breast cancer cell line, compounds 2d, 4b, 4d and 6d showed comparable cytotoxic activity with L-ascorbic acid. These oxetane motifs will provide interesting new structural elements for medicinal chemistry programs as well as synthesis.

Aim and Objective: Reaction of phthalic anhydride (1) with thiocarbohydrazide (2) in methanol gave the intermediate (6) whose boiling in water or ethanol gave N-(1,3-dioxoisoindol-2-yl)aminothiocarbohydrazide (5); where its Condensation with aldehydes and aldoses 9a-d afforded the respective thiosemicarbazide derivatives. The manno-derivative 10, the D-galacto derivative 11 and the L-arabino derivative 12 exist in the cyclic pyranosyl structures undergo equilibration, in solution of DMSO, with their acyclic structures. Dehydrative cyclization to the thiadiazoline derivatives and the acyclo C-nucleoside was done by refluxing in acetic anhydride. Materials and Methods: Commercially available solvents and reagents were purified according to the standard procedures. Thin layer chromatography (TLC) was performed on plastic plates Silica Gel 60 F254 (E-Merk, layer thickness 0.2 mm) with detection by UV light absorption. IR spectra were recorded for the compounds in a KBr matrix with a Unicam SP 1025 spectrophotometer. NMR spectra were measured with Jeol spectrometer (500 MHz). Chemical shifts (δ) are given in ppm relative to the signal for TMS as internal standard, and coupling constants in Hz. The 13C NMR spectra were recorded with JEOL spectrometer at 125.7 MHz. The assignments of 1H NMR spectra were based on chemical-shift correlation DQFCOSY spectra, while the assignment of 13C NMR spectra were based on heteronuclear multiple quantum coherence, HMQC experiments. Results: The N-(1,3-dioxoisoindol-2-yl)aminothiocarbohydrazide (5) was prepared, which incorborated the 1,3-dioxoisoindole ring that linked to thiosemicarbazide moiety. Such feature found to be excellent precursor for the synthesis of hybrid of bi-heterocycles, the dioxoisoindolyl and thiadiazoline rings which could of potential biological activity. The reaction of thiocarbohydrazide with phthalic anhydride in a detailed manner and investigate its further reaction with aromatic aldehydes and sugars, then converting them to the target biheterocycles and nucleosides was studied. The conformations of the acyclic alditolyl residues or the acyclic Cnucleosides have been explained. Conclusion: The new functionalized isoindolyl ring with thiosemicarbazide as in N-(1,3-dioxoisoindol-2- yl)aminothiocarbohydrazide (5) resulted from the reaction of phthalic anhydride with thiocarbohydrazide via the intermediate open chain derivative 6. It is a valuable precursor for the synthesis of thiosemicarbazones containing sugar moieties 10-13. Their solutions in DMSO-d6 showed that the products exist in one form after immediate dissolution that equilibrated with time to show a mixture of acyclic and cyclic forms. They have been exploited as scaffolds for generation of potential hybrids of thiadiazoline derivatives containing isoindolyl rings as well as their acyclo C-nucleosides 10a-13a and 14-15. The conformations of the acyclic alditolyl residues or the acyclic C-nucleosides have been deduced from their spectral analysis.

Background: Aziridine compounds are among the most fascinating intermediates in organic synthesis, acting as precursors of many complex molecules due to the strain incorporated in their skeletons and can be opened in a stereo controlled manner with various nucleophiles, providing access to a wide range of important nitrogen-containing products. Sonochemistry involves the use of ultrasound technique to promote organic reactions. In recent years sonochemistry has been widely used in organic syntheses as it offers a versatile and facile pathway for a bewildering range of organic reactions. Material and Method: The synthesis of some Schiff bases was done in according the previously reported procedure in literature. The synthesized Schiff bases in the presence of magnesium powder and CCl4 in anhydrous tetrahydrofuran was converted to gem-dichloroaziridines at ultrasonic irradiation. Finally, the synthesized 2,2- dichloro-1,3-bis(4-chlorophenyl) aziridine was treated with DMSO and H2O with ultrasonic irradiation to produce the N-(4-chlorophenyl)-N-[4-(chlorophenyl)hydroxy-methyl] formamides. Results: Here, total synthesis of N-(α-hydroxybenzyl)-formamides through ultrasonic promoted oxidation and hydrolysis reaction of gem-dichloroaziridine derivatives in the presence of dimethyl sulfoxide and water has been investigated. Conclusion: In this study, ultrasonic promoted oxidation and hydrolysis of gem-dichloroaziridines toward N- (α-hydroxybenzyl)-formamides in the presence of DMSO and H2O under neutral medium reaction conditions has been reported. Excellent yields of products were obtained through this method. Other advantages of this reaction are short reaction times, simplicity of the method and high purity of products.

Aim and Objective: Phosphonate possesses a broad range of applications ranging from agrochemistry to medicines. Because of the synthetic and biological importance of phosphonate, their chemistry has stimulated an increasing interest. Hence we developed an efficient and eco-friendly Mg(ClO4)2 catalyzed one pot, solvent free synthesis of phosphonate derivatives of chalcones. Materials and Methods: Magnesium perchlorate phosphonate derivatives of chalcones were synthesized. The reactions were executed at room temperature in the presence of Anhydrous Magnesium perchlorate Anhy. Mg(ClO4)2 under solvent free condition, affording high yield of the product. The cardinal significance of the method lies in its uncomplicated and eco- friendly experimental conditions. Results: The PMA method for P-C bond formation, using dialkyl phosphite with chalcone, was performed in the presence of Anhydrous Magnesium perchlorate under solvent free condition. Despite the solvent free condition, the methodology is efficient since it is required only in catalytic amount of Anhy. Mg(ClO4)2 in one pot reaction. Results demonstrated that the catalytic PMA of dimethyl phosphite, diethyl phosphite, diisopropyl phosphite and dibutyl phosphite with chalcone generates desired products with a percentage yield of 60-80%. Conclusion: We have developed a new efficient PMA method for P-C bond formation, using dialkyl phosphite with α,β-unsaturated ketone i.e. chalcone. To the best of our knowledge, Anhy. Mg(ClO4)2 catalyzed phosphamicheal addition of dialkyl phosphite with chalcones was found to be a new and efficient method for various biological applications.