Stepwise Cyclization Research Articles

Synthesis of 1,2,4-diazaphospholes via base-promoted cyclization reaction of hydrazonoyl chlorides and [Bu4N][P(SiCl3)2

Aromatic 1,2,4-diazaphospholes featuring distinct hybrid-mode nitrogen atoms (N(λ3σ2), N(λ3σ3)) and low-valent phosphorus atoms (λ3σ2) exhibited the characteristic of serving as unique hybrid ligands. This study presented a one-pot reaction involving the base-promoted stepwise cyclization of hydrazonoyl chlorides and [Bu4N][P(SiCl3)2] to yield 1,2,4-diazaphospholes, providing an effective method for synthesizing such compounds.

Functional Characterization and Cyclization Mechanism of a Diterpene Synthase Catalyzing the Skeleton Formation of Cephalotane‐Type Diterpenoids

AbstractCsCTS, a new diterpene synthase from Cephalotaxus sinensis responsible for forming cephalotene, the core skeleton of cephalotane‐type diterpenoids with a highly rigid 6/6/5/7 tetracyclic ring system, was functionally characterized. The stepwise cyclization mechanism is proposed mainly based on structural investigation of its derailment products, and further demonstrated through isotopic labeling experiments and density functional theory calculations. Homology modeling and molecular dynamics simulation combined with site‐directed mutagenesis revealed the critical amino acid residues for the unique carbocation‐driven cascade cyclization mechanism of CsCTS. Altogether, this study reports the discovery of the diterpene synthase that catalyzes the first committed step of cephalotane‐type diterpenoid biosynthesis and delineates its cyclization mechanism, laying the foundation to decipher and artificially construct the complete biosynthetic pathway of this type diterpenoids.

Functional Characterization and Cyclization Mechanism of a Diterpene Synthase Catalyzing the Skeleton Formation of Cephalotane-Type Diterpenoids.

CsCTS, a new diterpene synthase from Cephalotaxus sinensis responsible for forming cephalotene, the core skeleton of cephalotane-type diterpenoids with a highly rigid 6/6/5/7 tetracyclic ring system, was functionally characterized. The stepwise cyclization mechanism is proposed mainly based on structural investigation of its derailment products, and further demonstrated through isotopic labeling experiments and density functional theory calculations. Homology modeling and molecular dynamics simulation combined with site-directed mutagenesis revealed the critical amino acid residues for the unique carbocation-driven cascade cyclization mechanism of CsCTS. Altogether, this study reports the discovery of the diterpene synthase that catalyzes the first committed step of cephalotane-type diterpenoid biosynthesis and delineates its cyclization mechanism, laying the foundation to decipher and artificially construct the complete biosynthetic pathway of this type diterpenoids.

Kinetic Rationalization of Nonlinear Effects in Asymmetric Catalytic Cascade Reactions under Curtin-Hammett Conditions.

Observations of nonlinear effects of catalyst enantiopurity on product enantiomeric excess in asymmetric catalysis are often used to infer that more than one catalyst species is involved in one or more reaction steps. We demonstrate here, however, that in the case of asymmetric catalytic cascade reactions, a nonlinear effect may be observed in the absence of any higher order catalyst species or any reaction step involving two catalyst species. We illustrate this concept with an example from a recent report of an organocatalytic enantioselective [10 + 2] stepwise cyclization reaction. The disruption of pre-equilibria (Curtin–Hammett equilibrium) in reversible steps occurring prior to the final irreversible product formation step can result in an alteration of the final product ee from what would be expected based on a linear relationship with the enantiopure catalyst. The treatment accounts for either positive or negative nonlinear effects in systems over a wide range of conditions including “major-minor” kinetics or the more conventional “lock-and-key” kinetics. The mechanistic scenario proposed here may apply generally to other cascade reaction systems exhibiting similar kinetic features and should be considered as a viable alternative model whenever a nonlinear effect is observed in a cascade sequence of reactions.

In Situ Allene Formation via Alkyne Tautomerization to Promote [4 + 2]-Cycloadditions with a Pendant Alkyne or Nitrile.

We have explored the net-[4 + 2]-cycloadditions of a variety of aryl (or alkenyl) alkynes. Tautomerization via base-catalyzed alkyne-to-allene isomerization produces a transient allene, which undergoes stepwise cyclization with not only a pendant alkyne but also a nitrile. The operative mechanisms for these reactions were studied by density functional theory and compared with the slower thermal cyclization of the precursor alkyne.

Construction of 4-spiroannulated tetrahydroisoquinoline skeletonsviaa sequential ring opening of aziridines and Pictet–Spengler reaction

A stepwise cyclization involving sequential ring opening of aziridines and Pictet–Spengler reaction has been developed for the synthesis of 4-spiroannulated tetrahydroisoquinoline compounds (22 examples).

A Nanoboat with Fused Concave N-Heterotriangulene.

The surface extension of all-carbon based bowl-shaped molecules, such as corannulene and sumanene, to synthesize even larger buckybowls has been widely studied, leaving other concave compounds with heteroatoms less considered. Herein we present a highly curved molecule synthesized via stepwise cyclization of fjords of a bisacridone derivative. Crystallographic analysis unambiguously confirmed a boat-shaped structure with deformed bottom benzene ring. Theoretical calculation unravels an inversion process with an S-shaped transition structure rather than a planar one. The enlarged boat demonstrates interesting properties, such as red shifts in absorption and emission spectra, enhanced emission intensity, and convergent frontier molecular orbital energy levels, in comparison to the related concave N-heterotriangulene.

A Nanoboat with Fused Concave N‐Heterotriangulene

AbstractThe surface extension of all‐carbon based bowl‐shaped molecules, such as corannulene and sumanene, to synthesize even larger buckybowls has been widely studied, leaving other concave compounds with heteroatoms less considered. Herein we present a highly curved molecule synthesized via stepwise cyclization of fjords of a bisacridone derivative. Crystallographic analysis unambiguously confirmed a boat‐shaped structure with deformed bottom benzene ring. Theoretical calculation unravels an inversion process with an S‐shaped transition structure rather than a planar one. The enlarged boat demonstrates interesting properties, such as red shifts in absorption and emission spectra, enhanced emission intensity, and convergent frontier molecular orbital energy levels, in comparison to the related concave N‐heterotriangulene.

Divergent Cyclization Reactions of Morita-Baylis-Hillman Carbonates of 2-Cyclohexenone and Isatylidene Malononitriles.

Zwitterionic dienolates generated from Morita-Baylis-Hillman carbonates of cyclohexen-2-one and a phenolic tertiary amine catalyst underwent divergent cyclization reactions with isatylidene malononitriles. A new [4 + 2] stepwise cyclization process was disclosed to deliver complex bridged spirooxindoles after the initial δ'-regioselective Rauhut-Currier-type reaction with N-methyl electrophiles by the catalysis of β-isocupreidine, while spirooxindoles incorporating an aromatic chromene motif were generated with N-MOM acceptors in the presence of α-isocupreine through different domino transformations.

Divergent Total Syntheses to Azafluor­anthene and Dehydroaporphine Alkaloids

AbstractFacile divergent total syntheses for azafluoranthene and dehydroaporphine alkaloids have been successfully developed. A common intermediate, a biarylsulfonamide‐protected amino aldehyde, underwent either a cascade or a stepwise cyclization to furnish a tetracyclic skeleton related to the azafluoranthene alkaloids. Natural products, triclisine and telitoxine, were prepared to illustrate the use of this approach. Subsequent C‐homologation of the aldehyde moiety on the same intermediate by means of a Wittig reaction allowed the synthesis of aporphine alkaloids, as exemplified by the preparation of dehydronornuciferine. This synthetic approach could be applicable to the syntheses of other azafluoranthene‐related as well as aporphine‐related alkaloids.

ChemInform Abstract: Gold(I)‐Promoted Heterocyclization of Internal Alkynes: A Comparative Study of Direct Metallate 5‐endo‐dig Cyclization versus a Stepwise Cyclization.

AbstractThe title reaction affords five‐ and six‐membered ring nitrogen heterocycles.

Graphene-like Molecules Based on Tetraphenylethene Oligomers: Synthesis, Characterization, and Applications

Graphene-like molecules were prepared by oxidative cyclodehydrogenation of tetraphenylethene(TPE) oligomers using iron(III) chloride as the catalyst under mild conditions. All the oxidized samples can be separated effectively from the stepwise ring-closing reaction that highly related to the reaction time. For example, the model compounds obtained from the stepwise cyclization reaction show a regular red-shift in UV/vis absorption and photoluminescence (PL) spectra. This result reveals that the molecular conjugation length will extend with the stepwise ring-closing reaction going on. Interestingly, we successfully obtained a series of colorful luminogens with blue, cyan, and green emission during this stepwise and accurate ring closing process. Cyclic voltammetry measurements taken give the corresponding band gap, which supports the results obtained from optical spectroscopy. For the strong intermolecular interaction, our graphene molecules can self-assemble to form a red-colored and hexagonal fiber. Furt...

Gold(I)-promoted heterocyclization of internal alkynes: a comparative study of direct metallate 5-endo-dig cyclization versus a stepwise cyclization.

With cationic gold catalysts, internal alkynes bearing both propargylic acyloxy groups and tosylamide pronucleophiles were found to cyclize to give either five- or six-membered ring nitrogen heterocycles. A wide variety of gold catalysts, counterions, and solvents were examined to elucidate their effect on product distribution. In most cases, the direct 5-endo-dig cyclization was found to be the major pathway leading to good yields of dehydropyrrolidine products. Alkyne substrates bearing additional normal alkyl substituents at the propargylic position gave dehydropiperidines as the major product. This pathway is thought to proceed by way of a 1,2- Rautenstrauch rearrangement to produce a vinyl gold(I) carbene, which undergoes conjugate addition by the nitrogen pronucleophile. Structural and electronic factors were studied in the nitrogen pronucleophile and in the migrating acyloxy group. Each was found to have a minor effect on the product ratio.

Efficient synthesis of aminomethylated azaindoles and corresponding pyrrole-fused derivatives by copper-catalyzed domino multicomponent coupling and cyclization

Efficient methods for the synthesis of aminomethylated azaindole derivatives via domino copper-catalyzed multicomponent coupling and cyclization have been developed. Using various secondary amines and aldehydes, N-substituted 3-ethynyl-4-aminopyridine was converted to substituted azaindoles in moderate to excellent yields. By use of a 3,4-diaminopyridine derivative bearing two alkynyl groups, the corresponding pyrrole-fused azaindoles were synthesized by controlled stepwise cyclization.

Reactions of heterocumulenes with organometallic reagents: XVIII. Quantum-chemical study on the mechanisms of formation of thiophene ring from lithium and potassium buta-2,3-dienimidothioates

Thermodynamic stabilities of different isomers and tautomers of lithium and potassium buta-2,3-dienimidothioates (ambident N,S-centered anionic adducts derived from methoxyallenyl carbanion and methyl isothiocyanate) as real or potential precursors of 3-methoxy-N,N-dimethylthiophen-2-amine were estimated by quantum-chemical calculations in terms of the density functional theory. Gradient channels corresponding to concerted (one-step) and stepwise (two-step) mechanisms of formation of thiophene ring from the most stable conformers and tautomers were localized. The formation of thiophene structure from lithium and potassium buta-2,3-dienimidothioates in two steps via intramolecular cyclization to thiophen-2(5H)-ylidene anion and its isomerization into thienylamide was found to be more probable. The activation barriers in both stepwise and concerted cyclization mechanisms considerably decrease in going from lithium to potassium as counterion.

The evolution of poly(hydroxyamide amic acid) to poly(benzoxazole) via stepwise thermal cyclization: Structural changes and gas transport properties

The thermally induced structure transformation and polymer chain rearrangement of a thermally rearranged (TR) material for gas separation has been explored in this work. A tremendous enhancement as high as 215 folds in CO2 permeability has been achieved by converting the pristine poly(hydroxyamide amic acid) (PHAA) to the final polybenzoxazole (PBO) via thermal cyclization at 400 °C for 2 h. The evolution of the pristine polymer PHAA derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluropropane (BisAPAF) and trimellitic anhydride chloride (TAC) by thermal treatment from 130 °C to 400 °C has been examined by various characterization techniques including elemental analysis, TGA, TGA-IR, DSC, ATR-FTIR, XPS, XRD and Positron Annihilation Lifetime Spectroscopy (PALS). The stepwise cyclization process commences with cyclodehydration followed by cyclodecarboxylation. At the first step, PHAA transforms to poly(imide benzoxazole) (PIBO) up to 300 °C, while at the second step, the final structure of PBO is formed at 400 °C. Following the changes in the cyclization process, gas transport properties also show the stepwise changes. The significant enlargements of polymer inter-chain distance and free volume cavity radius provide the fundamental understanding for the changes of gas transport properties at the molecular level.

Catalytic Dicyanative [4 + 2] Cycloaddition Triggered by Cyanopalladation Using Ene−Enynes and Cyclic Enynes with Methyl Acrylate

Palladium-catalyzed dicyanative [4 + 2] cycloaddition using various ene-enynes was investigated. The key species in this process is a cyanoallene intermediate that is obtained by the cyanopalladation of conjugated enynes followed by 5-exo-cyclization. To achieve an efficient [4 + 2] cycloaddition reaction, both the smooth generation of this species and critical control of regioselectivity in the 6-endo-cyclization step are quite important. A study of the substrate scope revealed that the reaction is strongly affected by the steric bulk of the substituents on the enyne and alkene units and prefers to give trans-fused cycloadducts. The stereochemistry of olefins was reasonably transferred to the corresponding products. Further study proved that this transformation includes not a thermal [4 + 2] cycloaddition process via 1,2-dicyanoalkenes generated in situ but rather a palladium-mediated stepwise cyclization sequence to control a maximum of five contiguous stereogenic centers in a single operation. An intermolecular version using methyl acrylate with conjugated cyclic enynes and TMSCN also gave the corresponding [4 + 2] cycloadducts in a regioselective manner.

Synthesis of Binuclear Complexes Bound in an Enlarged Tetraphosphamacrocycle: Two Diphosphine Metal Units Linked in Front-to-Front Style

A large-hole tetraphosphamacrocycle 2, with four phosphorus centers separated at the corners of a 3.7 A wide and 9.7 A long rectangle, was synthesized by a stepwise cyclization reaction between PCl-bridged [1.1]ferrocenophane and bisphenol A in a 2:2 ratio. The macrocycle 2 could incorporate two Ag(+) or Pt(0) fragments in the hole to provide binuclear complexes, which were identified as mu-2-[Ag(NCMe)(2)](2)(BF(4))(2) (3) and mu-2-[Pt(PhCCPh)](2) (5), respectively, using X-ray and spectroscopic analysis. The X-ray structure of 3 demonstrates that the macrocycle 2 serves as a framework in which two diphosphine silver units are aligned in a front-to-front style, while that of 5 indicates that 2 can also bind two bulky Pt(PhCCPh) fragments by the flexible change of its conformation.

Controlled Release of Volatile Secondary and Tertiary Alcohols by Neighboring Group Participation: Stepwise Cyclization and Re‐Opening of 2,2′‐Bis(carbamoyl)dibenzoates at Neutral pH

Bioactive, volatile, secondary and tertiary fragrance alcohols are efficiently released by intramolecular neighboring-group-assisted hydrolysis of 2,2'-bis(carbamoyl)dibenzoates at neutral pH. The stepwise cyclization of 2,2'-[(methylimino)bis(propane-3,1-diylcarbamoyl)]dibenzoates is followed by the re-opening of the intermediately formed diphthalimide and proceeds in an overall four-step consecutive reaction sequence. Kinetic rate constants for all four reaction steps could be determined pairwise by reversed-phase HPLC. At neutral pH, secondary alcohols were released by one order of magnitude faster than the tertiary alcohols, and the rate constants for the re-opening of the diphthalimides were found to be in the same order of magnitude as the release of the tertiary alcohols. Dynamic headspace analysis on a dry cotton surface finally confirmed the efficient release of tertiary alcohols under mild reaction conditions generally encountered for applications in functional perfumery.

Synthesis of (‐)‐Quinocarcin by Directed Condensation of α‐Amino Aldehydes.

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