Entry to new spiroheterocycles via tandem Rh(II)-catalyzed O-H insertion/base-promoted cyclization involving diazoarylidene succinimides

A facile approach to novel medicinally relevant spiro heterocyclic scaffolds (namely furan-2(5H)-ones, tetrahydrofurans and pyrans spiro-conjugated with the succinimide ring) has been developed. The protocol consists of Rh(II)-catalyzed insertion of heterocyclic carbenes derived from diazoarylidene succinimides (DAS) into the O-H bond of propiolic/allenic acids or brominated alcohols, followed by base-promoted cyclization to afford the target spirocyclic compounds in good to high yields.

Cyclizations of Alkenyl(Alkynyl)-Functionalized Quinazolinones and their Heteroanalogues: A Powerful Strategy for the Construction of Polyheterocyclic Structures

Quinazolin-4-one, its heteroanalogues, and derivatives represent an outstandingly important class of compounds in modern organic, medicinal, and pharmaceutical chemistry, as these molecular structures are noted for their wide synthetic and pharmacological potential. In the last years, ever-increasing research attention has been paid to quinazolinone derivatives bearing alkenyl and alkynyl substituents on the pyrimidinone nucleus. The original structural combination of synthetically powerful endocyclic amidine (or amidine-related) and exocyclic unsaturated moieties provides a driving force for cyclizations, which offer an efficient toolkit to construct a variety of fused pyrimidine systems with saturated N- and N,S-heterocycles. In this connection, the present review article is mainly aimed at systematic coverage of the progress in using alkenyl(alkynyl)quinazolinones and their heteroanalogues as convenient bifunctional substrates for regioselective annulation of small- and medium-sized heterocyclic nuclei. Much attention is paid to elucidating the structural and electronic effects of reagents on the regio- and stereoselectivity of the cyclizations as well as to clarifying the relevant reaction mechanisms.

An Operationally Simple Approach to Indole Derivatives from 2-Alkenylanilines Utilizing an Oxidation-Intramolecular Cyclization-Elimination Sequence

Herein we describe a novel route to indole derivatives from a variety of N-substituted 2-alkenylanilines. This route features three operationally simple steps: (1) oxidation to convert N-substituted 2-alkenylanilines into epoxide intermediates, (2) intramolecular cyclization, and (3) the acid-catalyzed elimination of water.

Synthesis and stereodynamics of intramolecular hemiacetals in biaryl aldehyde-alcohols

Soai's asymmetric autocatalysis represents a highly remarkable example for spontaneous symmetry breaking and enantioselective amplification in the enantioselective alkylation of pyrimidine-5-carbaldehydes to the corresponding chiral pyrimidine alcohols. Recently, zinc hemiacetalate complexes, formed from pyrimidine-5-carbaldehydes and the chiral product alcohol, were identified by in situ high-resolution mass spectrometric measurements as highly active transient asymmetric catalysts in this autocatalytic transformation. To study the formation of such hemiacetals and their stereodynamic properties, we focused on the synthesis of coumarin homolog biaryl systems with carbaldehyde and alcohol substituents. Such systems are able to form hemiacetals by intramolecular cyclization. An interesting feature of the substituted biaryl backbone is that tropos and atropos systems can be obtained, enabling or disabling the intramolecular cyclization to hemiacetals. Biaryl structures with various functional groups were synthesized, and the equilibrium and stereodynamics between the closed and open structures were investigated by dynamic enantioselective HPLC (DHPLC). The enantiomerization barriers ΔG^ǂ and activation parameters ΔH^ǂ and ΔS^ǂ were determined from temperature dependent kinetic measurements.

2-Amino-1,3-benzothiazole: Endo N-Alkylation with α-Iodo Methyl Ketones Followed by Cyclization

Reactions of 2-amino-1,3-benzothiazole with aliphatic, aromatic and heteroaromatic α-iodoketones in the absence of bases or catalysts have been studied. The reaction proceeds by N-alkylation of the endocyclic nitrogen atom followed by intramolecular dehydrative cyclization. The regioselectivity is explained and the mechanism of the reaction is proposed. A number of new linear and cyclic iodide and triiodide benzothiazolium salts have been obtained and their structure proved by NMR and UV spectroscopy.

Organocatalytic Desymmetric Conjugate Addition to Cyclobutanone: An Enantio- and Diastereoselective Synthesis of [6,5,4]-Fused Carbocycles

The first organocatalytic, non-destructive desymmetrisation of 3-substituted cyclobutanone followed by intramolecular cyclisation is described. Owing to the slow activation of the pronucleophile by the weak Brønsted base, the α-functionalization of ketones with tertiary-amine-derived Brønsted base catalysis has been chemically challenging in asymmetric transformations. Herein, we demonstrate that the cinchona-derived squaramide catalyst works effectively for the intramolecular Michael addition of cyclobutanone to enones and affords the architecturally fascinating [6,5,4]-fused carbocyclic skeleton in good yields and excellent diastereo- and enantioselectivities (up to >20 : 1 dr and 96 % ee). Furthermore, as a synthetic application, this method provides an alternative pathway to enantioenriched high-value products like γ-lactones and indan-2-ol derivatives.

Catalytic aza-Nazarov cyclization reactions to access α-methylene-γ-lactam heterocycles

We have developed a catalytic aza-Nazarov reaction of N-acyliminium salts generated in situ from the reaction of a variety of cyclic and acyclic imines with α,β-unsaturated acyl chlorides to afford substituted α-methylene-γ-lactam heterocycles. The reactions proceed effectively in the presence of catalytic (20 mol %) amounts of AgOTf as an anion exchange agent or hydrogen-bond donors such as squaramides and thioureas as anion-binding organocatalysts. The aza-Nazarov cyclization of 3,4-dihydroisoquinolines with α,β-unsaturated acyl chlorides gives tricyclic lactam products 7 in up to 79% yield with full diastereocontrol (dr = >99:1). The use of acyclic imines in a similar catalytic aza-Nazarov reaction with 20 mol % of AgOTf results in the formation of α-methylene-γ-lactam heterocycles 19 in up to 76% yield and with good to high diastereoselectivities (4.3:1 to 16:1). We have demonstrated the scalability of the reaction with a gram-scale example. The relative stereochemistry of the α-methylene-γ-lactam products 19 has been determined via the single-crystal X-ray analysis of lactam 19l. In order to shed light on the details of the reaction mechanism, we have performed carefully designed mechanistic studies which consist of experiments on the effect of β-silicon stabilization, the alkene geometry of the α,β-unsaturated acyl chloride reactants, and adventitious water on the success of the catalytic aza-Nazarov reaction.

Cyclization Reaction of 3,5-Diacetyl-2,6-dimethylpyridine with Salicylic Aldehyde and Its Derivatives: Quantum-Chemical Study and Molecular Docking

Computational study of some details of the cyclization reaction between 3,5-diacetyl-2,6-dimethylpyridine and salicylic aldehyde in an acidic medium was performed by the DFT RB3LYP/6-31G method using the Gaussian-2016 software package. It was shown that protonation of the pyridine nitrogen atom leads to a significant increase in the charge of the hydrogen atom of the 2-methyl group of pyridine and the methyl acetyl group. This leads to the growth of the methyl group CH-acidity and enolization of the acetyl group. It was also found that the protonated tautomeric enol form of 3,5-diacetyl-2,6-dimethylpyridine gives a stable pre-reaction complex with salicylic aldehyde due to the formation of three hydrogen bonds. The formation of this pre-reaction complex, apparently, leads to the implementation of the Knoevenagel reaction, instead of the alternative possible Claisen–Schmidt reaction of salicylic aldehyde at the acetyl group of pyridine. The possible biological activity of the previously obtained cyclization products was evaluated by molecular docking using the AutoDock Vina software. Some cyclization products showed higher values of the binding affinity with the selected target proteins in comparison with the known antiviral drugs Nevirapine and Favipiravir. The results obtained confirm the correctness of the proposed cyclization mechanism between 3,5-diacetyl-2,6-dimethylpyridine and salicylic aldehyde. This also makes it possible to assess the prospects of previously obtained derivatives of epoxybenzo[7,8]oxocino[4,3-b]pyridine as synthetic analogs of natural integrastatins A, B for further synthesis and study of their antiviral activity.

Synthesis of Indoloquinolines: An Intramolecular Cyclization Leading to Advanced Perophoramidine-Relevant Intermediates

The bioactive natural product perophoramidine has proved a challenging synthetic target. An alternative route to its indolo[2,3-b]quinolone core structure involving a N-chlorosuccinimde-mediated intramolecular cyclization reaction is reported. Attempts to progress towards the natural product are also discussed with an unexpected deep-seated rearrangement of the core structure occurring during an attempted iodoetherification reaction. X-ray crystallographic analysis provides important analytical confirmation of assigned structures.

Recent advances in the syntheses of anthracene derivatives

Anthracene and anthracene derivatives have been extensively studied over the years because of their interesting photophysical, photochemical, and biological properties. They are currently the subject of research in several areas, which investigate their use in the biological field and their application in OLEDs, OFETs, polymeric materials, solar cells, and many other organic materials. Their synthesis remains challenging, but some important preparative methods have been reported, especially in the last decade. This review presents an update of the recent strategies that have been employed to prepare anthracene derivatives. It encompasses papers published over the last twelve years (2008–2020) and focuses on direct and indirect methods to construct anthracene and anthraquinone frameworks.

In Vivo Degradation Mechanism and Biocompatibility of a Biodegradable Aliphatic Polycarbonate: Poly(Trimethylene Carbonate-co-5-Hydroxy Trimethylene Carbonate)

A recently developed viscous liquid aliphatic polycarbonate, poly(trimethylene carbonate-co-5-hydroxy trimethylene carbonate), has advantageous properties for the delivery of acid-sensitive drugs such as proteins and peptides. This copolymer degrades in vitro via an alkaline-catalyzed intramolecular cyclization reaction yielding oligo (trimethylene carbonate), glycerol, and carbon dioxide, but its in vivo degradation mechanisms are presently unknown. The in vivo degradation mechanism and tissue response to this copolymer were investigated following subcutaneous implantation in Wistar rats. The molecular weight and composition of the copolymer varied in the same manner following subcutaneous implantation as observed in vitro. These findings suggest that the copolymer also degraded in vivo principally via intramolecular cyclization. The tissue response in terms of the inflammatory zone cell density, fibrous capsule thickness, and macrophage response was intermediate to that of two clinically used biodegradable sutures, Vicryl and Monocryl, indicating that the copolymer can be considered biotolerable. Collectively, the data show that further development of this copolymer as a drug delivery material is warranted.

One-Pot Synthesis of Novel Multisubstituted 1-Alkoxyindoles

Studies on a one-pot synthesis of novel multisubstituted 1-alkoxyindoles 1 and their mechanistic investigations are presented. The synthesis of 1 was successfully achieved through consecutive four step reactions from substrates 2. The substrates 2, prepared through a two-step synthetic sequence, underwent three consecutive reactions of nitro reduction, intramolecular condensation, and nucleophilic 1,5-addition to provide the intermediates, 1-hydroxyindoles 8, which then were alkylated in situ with alkyl halide to afford the novel target products 1. We optimized the reaction conditions for 1 focusing on the alkylation step, along with the consideration of formation of intermediates 8. The optimized condition was SnCl₂·2H₂O (3.3 eq) and alcohols (R¹OH, 2.0 eq) for 1–2 h at 40 °C and then, base (10 eq) and alkyl halides (R²Y, 2.0 eq) for 1–4 h at 25–50 °C. Notably, all four step reactions were performed in one-pot to give 1 in good to modest yields. Furthermore, the mechanistic aspects were also discussed regarding the reaction pathways and the formation of side products. The significance lies in development of efficient one-pot reactions and in generation of new 1-alkoxyindoles.

Silver(I) Oxide-/DBU-Promoted Synthesis of Dihydrofuran Units through Allenyl Silver Formation

A formal [3+2] cyclization mediated by silver(I) oxide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is described herein. Through a broad variety of carbonyl compounds, this system can promote cyclization reactions with high yield (up to 85 %) and diastereoselectivity (up to 95:5) for a straightforward access to complex and congested dihydrofuran derivatives in one step under mild conditions. Based on DFT studies, the proposed mechanism would involve an allenyl silver intermediate.

Halogen-Induced Controllable Cyclizations as Diverse Heterocycle Synthetic Strategy

In organic synthesis, due to their high electrophilicity and leaving group properties, halogens play pivotal roles in the activation and structural derivations of organic compounds. Recently, cyclizations induced by halogen groups that allow the production of diverse targets and the structural reorganization of organic molecules have attracted significant attention from synthetic chemists. Electrophilic halogen atoms activate unsaturated and saturated hydrocarbon moieties by generating halonium intermediates, followed by the attack of carbon-containing, nitrogen-containing, oxygen-containing, and sulfur-containing nucleophiles to give highly functionalized carbocycles and heterocycles. New transformations of halogenated organic molecules that can control the formation and stereoselectivity of the products, according to the difference in the size and number of halogen atoms, have recently been discovered. These unique cyclizations may possibly be used as efficient synthetic strategies with future advances. In this review, innovative reactions controlled by halogen groups are discussed as a new concept in the field of organic synthesis.

Synthesis of Medium-Sized Heterocycles by Transition-Metal-Catalyzed Intramolecular Cyclization

Medium-sized heterocycles (with 8 to 11 atoms) constitute important structural components of several biologically active natural compounds and represent promising scaffolds in medicinal chemistry. However, they are under-represented in the screening of chemical libraries as a consequence of being difficult to access. In particular, methods involving intramolecular bond formation are challenging due to unfavorable enthalpic and entropic factors, such as transannular interactions and conformational constraints. The present review focuses on the synthesis of medium-sized heterocycles by transition-metal-catalyzed intramolecular cyclization, which despite its drawbacks remains a straightforward and attractive synthesis strategy. The obtained heterocycles differ in their nature, number of heteroatoms, and ring size. The methods are classified according to the metal used (palladium, copper, gold, silver), then subdivided according to the type of bond formed, namely carbon-carbon or carbon-heteroatom.

Design, synthesis and anticancer evaluation of β-carboline-1-one hydantoins

Aim: Cancer is a major health burden and a leading cause of death worldwide. We sought to discover potential anticancer molecules with novel scaffold for further development of more active agents to address the issue. Methodology: A series of β-carboline-1-one hydantoins were designed according to a conformational restriction strategy, synthesized via a one-pot Knoevenagel condensation-intramolecular cyclization, and tested in cytotoxicity assays. Results: The study culminated in the identification of 6b and 6c, both of which were found to potently inhibit breast and lung cancer cell lines. Of particular interest was 6c, which was 83 times more potent an inhibitor than 5-fluorouracil in inhibiting MCF-7. Conclusion: This work establishes β-carboline-1-one hydantoin as a promising scaffold in the investigation of anticancer agents.

Intramolecular cyclization of the antimicrobial peptide Polybia-MPI with triazole stapling: influence on stability and bioactivity

Cationic antimicrobial peptides have attracted increasing attention as a novel class of antibiotics to treat infectious diseases caused by pathogenic bacteria. However, susceptibility to protease is a shortcoming in their development. Cyclization is one approach to increase the proteolytic resistance of peptides. Therefore, to improve the proteolytic resistance of Polybia-MPI, we have synthesized the MPI cyclic analogs C-MPI-1 (i-to-i+4) and C-MPI-2 (i-to-i+6) by copper(I)-catalyzed azide-alkyne cycloaddition. Compared with MPI, C-MPI-1 displayed sustained antimicrobial activity and had enhanced anti-trypsin resistance, while C-MPI-2 displayed no antimicrobial activity. The relationship between peptide structure and bioactivity was further investigated by probing the secondary structure of the peptides by circular dichroism. This showed that C-MPI-1 adopted an α-helical structure in aqueous solution and, interestingly, had increased α-helical conformation in 30 mM sodium dodecyl sulfate and 50% trifluoroethyl alcohol compared with MPI. C-MPI-2 that was not α-helical in structure, suggesting that the propensity for α-helix conformation may play an important role in cyclic peptide design. In addition, scanning electron microscopy, propidium iodide uptake, and membrane permeabilization assays indicated that MPI and the optimized analog C-MPI-1 had membrane-active action modes, indicating that the peptides would not be susceptible to conventional resistance mechanisms. Our study provides additional insight into the influence of intramolecular cyclization at various positions on peptide structure and biological activity. In conclusion, the design and synthesis of cyclic analogs via click chemistry offer a new strategy for the development of stable antimicrobial agents. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

A BODIPY-Based Fluorescent Probe for Detection of Subnanomolar Phosgene with Rapid Response and High Selectivity

A new type of phosgene probe with a limit of detection down to 0.12 nM, response time of less than 1.5 s, and high selectivity over other similarly reactive toxic chemicals was developed using ethylenediamine as the recognition moiety and 8-substituted BODIPY unit as the fluorescence signaling component. The probe undergoes sequential phosgene-mediated nucleophilic substitution reaction and intramolecular cyclization reaction with high rate, yielding a product with the intramolecular charge transfer (ICT) process from amine to the BODIPY core significantly inhibited. Owing to the emission feature of 8-substituted BODIPY that is highly sensitive to the substituent's electronic nature, such inhibition on the ICT process strikingly generates strong fluorescence contrast by a factor of more than 23 300, and therefore creates the superhigh sensitivity of the probe for phosgene. Owing to the high reactivity of ethylenediamine of the probe in nucleophilic substitution reactions, the probe displays a very fast response rate to phosgene.

Synthesis of pyrrolo[2,1-f][1,2,4]triazin-4(3H)-ones: Rearrangement of pyrrolo[1,2-d][1,3,4]oxadiazines and regioselective intramolecular cyclization of 1,2-biscarbamoyl-substituted 1H-pyrroles

Pyrrolo[2,1-f][1,2,4]triazin-4(3H)-ones 12 have been easily prepared via nucleophile-induced rearrangement of pyrrolooxadiazines 11 and regioselective intramolecular cyclization of 1,2-biscarbamoyl-substituted 1H-pyrroles 10. In this work, we demonstrated that the described synthetic approaches can be considered to be more facile and practical than previously reported procedures.

Catalytic Formation of Disulfide Bonds in Peptides by Molecularly Imprinted Microgels at Oil/Water Interfaces

This work describes the preparation and investigation of molecularly imprinted polymer (MIP) microgel (MG) stabilized Pickering emulsions (PEs) for their ability to catalyze the formation of disulfide bonds in peptides at the O/W interface. The MIP MGs were synthesized via precipitation polymerization and a programmed initiator change strategy. The MIP MGs were characterized using DLS analysis, SEM measurement, and optical microscopy analysis. The dry and wet MIP MGs showed a hydrodynamic diameter of 100 and 280 nm, respectively. A template rebinding experiment showed that the MIP MGs bound over two times more template (24 mg g^-1) compared to the uptake displayed by a nonimprinted reference polymer (NIP) MG (10 mg g^-1) at saturation. Using the MIP MGs as stabilizers, catalytic oxidation systems were prepared by emulsifying the oil phase and water phase in the presence of different oxidizing agents. During the cyclization, the isolation of the thiol precursors and the oxidizing reagents nonselectively decreased the formation of the byproducts, while the imprinted cavities on the MIP MGs selectively promoted the intramolecular cyclization of peptides. When I₂ was used as the oxidizing agent, the MIP-PE-I₂ system showed a product yield of 50%, corresponding to a nearly 2-fold increase compared to that of the nonimprinted polymer NIP-PE-I₂ system (26%). We believe the interfacial catalysis system presented in this work may offer significant benefits in synthetic peptide chemistry by raising productivity while suppressing the formation of byproducts.

Regioselective Synthesis of a Family of β-Lactams Bearing a Triazole Moiety as Potential Apoptosis Inhibitors

Apoptosis is a biological process important to several human diseases; it is strongly regulated through protein–protein interactions and complex formation. We previously reported the synthesis of apoptosis inhibitors bearing an exocyclic triazole amide isoster by using an Ugi four‐component coupling reaction (Ugi‐4CC), followed by a base‐promoted intramolecular cyclization. Depending on the substitution patterns and the reaction conditions, this cyclization forms the six‐ or four‐membered ring. Two compounds bearing the β‐lactam scaffold turned out to be the most potent inhibitors. This encouraged us to optimize the modulation of the cyclization, and prepare a library of 15 β‐lactams with total regioselectivity. Moreover, we aimed to improve the bioavailability of these compounds through the introduction of diversity at different substitution positions. The activity of these compounds as apoptosis inhibitors in cellular extracts has been evaluated, showing an increase in their potency.

Acetic acid-catalyzed formation of N-phenylphthalimide from phthalanilic acid: a computational study of the mechanism

In glacial acetic acid, phthalanilic acid and its monosubstituents are known to be converted to the corresponding phthalimides in relatively good yields. In this study, we computationally investigated the experimentally proposed two-step (addition-elimination or cyclization-dehydration) mechanism at the second-order Møller-Plesset perturbation (MP2) level of theory for the unsubstituted phthalanilic acid, with an explicit acetic acid molecule included in the calculations. In the first step, a gem-diol tetrahedral intermediate is formed by the nucleophilic attack of the amide nitrogen. The second step is dehydration of the intermediate to give N-phenylphthalimide. In agreement with experimental findings, the second step has been shown to be rate-determining. Most importantly, both of the steps are catalyzed by an acetic acid molecule, which acts both as proton donor and acceptor. The present findings, along with those from our previous studies, suggest that acetic acid and other carboxylic acids (in their undissociated forms) can catalyze intramolecular nucleophilic attacks by amide nitrogens and breakdown of the resulting tetrahedral intermediates, acting simultaneously as proton donor and acceptor. In other words, double proton transfers involving a carboxylic acid molecule can be part of an extensive bond reorganization process from cyclic hydrogen-bonded complexes.

Theoretical studies on the intramolecular cyclization of 2,4,6-t-Bu3C6H2P=C: and effects of conjugation between the P=C and aromatic moieties

The intramolecular C–H insertion of the Mes*-substituted phosphanylidenecarbene [Mes*P=C:] (Mes* = 2,4,6-t-Bu₃C₆H₂) and physicochemical properties of the cyclized product, 6,8-di-tert-butyl-3,4-dihydro-4,4-dimethyl-1-phosphanaphthalene were studied based on ab initio calculations. Whereas the alternative Fritsch–Buttenberg–Wiechell-type rearrangement requires almost no activation energy, the intramolecular cyclization needs an activation energy of 12.3 kcal/mol at the MP2(full)/6-31G(d) condition. DFT calculations supported that the optimized structure of the cyclization product of Mes*P=C: suggests remarkable conjugation effects between the nearly coplanar P=C skeleton and the aryl moiety.

A novel route to synthesize libraries of quinoxalines via Petasis methodology in two synthetic operations

This communication reveals an innovative and facile procedure to prepare quinoxalines in two synthetic steps. The microwave assisted Petasis reaction is followed by the acid mediated unmasking of an internal amino nucleophile, cyclodehydration and oxidation to give collections of quinoxalines in good to excellent yields.

1,1'-(3-methyl-4-phenylthieno[2,3-b]thiophene-2,5-diyl)diethanone as a building block in heterocyclic synthesis. Novel synthesis of some pyrazole and pyrimidine derivatives

A series of new bis(heterocycles) featuring thieno[2,3-b]thiophene rings was synthesized in a combinatorial manner. Intramolecular cyclization of enaminone derivatives with appropriate N-nucleophiles afforded the target compounds. All compounds were characterized by ¹H-, ¹³C-NMR, GCMS, IR, and UV-Vis spectrometry. These compounds represent a new class of sulfur- and nitrogen-containing heterocycles that should also be of interest as new materials.

One Pot Spiropyrazoline Synthesis via Intramolecular Cyclization/Methylation

Regioisomeric spiropyrazolines were synthesized through a tandem intramolecular cyclization/methylation reaction of a functionalized 5,5-disubstituted pyrazoline in one reaction vessel. The 5,5-pyrazolines were constructed through a 1,3-dipolar cycloaddition reaction of aromatic ring containing nitrile imines and a disubstituted geminal alkene. An evaluation of the relative location of the nucleophilic and electrophilic functional groups on the pyrazoline was performed in order to ascertain the best pyrazoline system for the intramolecular cyclization/methylation reaction. Higher spiropyrazoline isolated yields were realized from pyrazolines with the electrophilic ester located further away from the pyrazoline when compared to pyrazolines with a directly bonded ester.

Synthesis and Neuroprotective Action of Optically Pure Neoechinulin A and Its Analogs

We developed an efficient, stereoselective synthetic method for the diketopiperazine moiety of neoechinulin A and its derivatives. The intramolecular cyclization at 80 ºC proceeded with minimal racemization of the stereogenic center at C-12 on neoechinulin A, even though the cyclization at 110 ºC caused partial racemization. In contrast with these results, the cyclization on diketopiperazine of 8,9-dihydroneoechinulin A derivatives did not cause epimerization of the stereogenic centers, even at 110 °C. We examined the structure-activity relationships for the cytoprotective activity against cytotoxicity induced by 3-morpholinosydnonimine (SIN-1) in nerve growth factor (NGF)-differentiated PC12 cells. The C-8/C-9 double bond, but not the stereogenic center derived from alanine, was found to play a key role in the cytoprotective activity.

Construction of Novel Spiroisoxazolines via Intramolecular Cyclization/Methylation

Improved yields for the syntheses of a variety of spiroisoxazolines were achieved through intramolecular cyclization/methylation reactions of functionalized 5,5-disubstituted isoxazolines in one reaction vessel. Aromatic ring containing nitrile oxides and disubstituted geminal alkenes reacted in a 1,3-dipolar fashion to afford the corresponding 5,5-isoxazoline. A comparison of the relative location of the nucleophile and electrophile on the isoxazoline and two different ester functional groups was performed in order to determine the best isoxazoline system for the intramolecular cyclization/methylation reaction.

A stable enol from a 6-substituted benzanthrone and its unexpected behaviour under acidic conditions

Treatment of benzanthrone (1) with biphenyl-2-yl lithium leads to the surprisingly stable enol 4, which is converted by dehydrogenation into the benzanthrone derivative 7. Under acidic conditions 4 isomerises to the spiro compound 11 and the bicyclo[4.3.1]decane derivative 12. Furthermore, the formation of 7 and the hydrogenated compound 13 is observed. A mechanism for the formation of the reaction products is proposed and supported by DFT calculations.