Thermal Electrocyclic Reaction Research Articles

Photochemically Produced Aminocyclobutanes as Masked Dienes in Thermal Electrocyclic Cascade Reactions.

Cyclobutane products of a triplet sensitized enamide-alkene intramolecular [2 + 2] photocycloaddition have been shown to undergo fragmentation under acidic conditions. This lability has been exploited by inducing a complexity-generating thermal electrocyclic cascade sequence involving the in situ formation of a cyclobutene, followed by electrocyclic ring opening, Diels-Alder cycloaddition, and subsequent lactamization. This combination of excited state photochemistry and thermal electrocyclic cascade reactions allows simple planar molecules to be rapidly transformed into sp3-rich scaffolds.

Computational and Experimental Evidence of Two Competing Thermal Electrocyclization Pathways for Vinylheptafulvene.

The thermal electrocyclic ring-closure reaction of vinylheptafulvene (VHF) to form dihydroazulene (DHA) is elucidated herein by using DFT and 1 H NMR spectroscopy. Two different transition states were found computationally; one corresponds to a disrotatory pathway, which is allowed according to the Woodward-Hoffmann selection rules, whereas the other corresponds to a conrotatory pathway. The conrotatory pathway is found to be zwitterionic in the transition state, whereas the disrotatory transition state varies in zwitterionic character depending on solvent and substituents in the molecular framework. The conrotatory and disrotatory transition states are found to have similar energy and their relative stability varies with solvent polarity and functionalization at the C1 position. To support these findings, we chemically ring-opened diastereomerically pure 1-(benzothiazol-2-yl)-DHA to give the VHF form, then subsequently thermally reconverted the VHF to DHA in a range of solvents with various polarities. We found that, depending on solvent polarity, different ratios of anti- and syn-diastereoisomers of DHA were formed in a systematic manner, which supports the existence of two distinct thermal ring-closure pathways for VHF.

The Dewar Isomer of 1,2-Dihydro-1,2-azaborinines: Isolation, Fragmentation, and Energy Storage.

The photochemistry of 1,2-dihydro-1,2-azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring-opening reaction of the Dewar valence isomer back to the 1,2-dihydro-1-tert-butyldimethylsilyl-2-mesityl-1,2-azaborinine has an activation barrier of (27.0±1.2) kcal mol-1 . In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(-48±1) kcal mol-1 ) at room temperature.

Synthetic Studies of Bioactive Heterocyclic Natural Products and Fused Heterocyclic Compounds Based on the Thermal Electrocyclic or Azaelectocyclic Reaction of 6π-Electron or Aza-6π-electron Systems

Since 1979, synthetic studies of bioactive heterocyclic natural products and condensed heteroaromatic compounds based on the thermal electrocyclic reaction of 6π-electron or aza-6π-electron systems incorporating the double bond of the principal aromatic or heteroaromatic ring have been conducted by our research group. In this review, five types of electrocyclic and azaelectrocyclic reaction are described: 1) the synthesis of the carbazole alkaloids hyellazole and 6-chlorohyellazole through the electrocyclic reaction of 2,3-bisalkenylindoles; 2) synthetic studies of the pyridocarbazole alkaloids ellipticine and olivacine through the electrocyclic reactions of the indole-2,3- and pyridine-3,4-quinodimethane intermediates; 3) synthetic studies of polysubstituted carbazole alkaloids through the allene-mediated electrocyclic reactions involving the indole 2,3-bond; 4) synthetic studies of fused pyridine rings through the azaelectrocyclic reaction of the 1-aza-6π-electron system using the oxime or oxime ether; and 5) synthetic studies of fused pyridine rings through the azaelectrocyclic reaction of the 2-aza-6π-electron system using a carbodiimide or isocyanate.

Substituents effect on thermal electrocyclic reaction of dihydroazulene–vinylheptafulvene photoswitch: a DFT study to improve the photoswitch

Thermal cyclization for a series of substituted vinylheptafulvenes (VHFs) to dihydroazulenes (DHAs) was studied at PBE0 method of density functional theory in the gas phase and in the acetonitrile solvent (through PCM). Judicious control of the thermal reaction through substituent is quite necessary to design thermally robust DHA–VHF photoswitches. For most of the substituents, DHA was predicted thermodynamically stable over VHF except for amino (in gas phase and solvent) and hydroxyl (in acetonitrile), where DHA isomers were calculated thermally unstable compared to VHF. Activation barriers for thermal electrocyclic reaction in both media showed positive correlation with Taft’s σ R values at positions 7 and 5, however, a negative correlation was observed at position 4 and 6. The latter unprecedented behavior is proposed to arise from the delocalization of negative charges on the seven membered ring. Activation barriers for amino-substituted VHFs were generally lower than expected from Taft’s σ R. A fluoro group at the position 7 was quite effective in imparting very high activation barrier (31.73 kcal mol−1) for the thermal cyclization in the gas phase. However, in the acetonitrile solvent, the highest activation barriers were observed for electron withdrawing CHO (28.10 kcal mol−1) and NO2 (28.13 kcal mol−1) groups at positions 7.

Thermal 8π electrocyclic reaction of heteroarene tetramers: new efficient access to π-extended cyclooctatetraenes

Bromo-substituted acyclic thiazole tetramers selectively underwent an 8π thermal electrocyclic reaction at high temperatures of 280–300 °C without solvent, producing π-extended thiazole-fused cyclooctatetraenes (COTs) in high yields. A quantum chemical study provided a rationale for the lower activation barrier and high selectivity of the 8π electrocyclic reaction mode over a competitive 6π electrocyclic reaction. The terminal bromine atom acts as a leaving group to trigger the rearomatization of the electrocyclized intermediate, which leads to the formation of the energetically stable COT products. This synthetic protocol enables us to synthesize a series of thiazole-fused COTs, including a derivative that can form loosely stacked columnar arrays in the crystalline state and shows fluorescence in the solid state.

ChemInform Abstract: Synthetic Studies on Nitrogen‐Containing Fused‐Heterocyclic Compounds Based on Thermal Electrocyclic Reactions of 6π‐Electron and Aza 6π‐Electron Systems

AbstractReview: 62 refs.

Synthetic Studies on Nitrogen-Containing Fused-Heterocyclic Compounds Based on Thermal Electrocyclic Reactions of 6π-Electron and Aza 6π-Electron Systems

Synthetic Studies on Nitrogen-Containing Fused-Heterocyclic Compounds Based on Thermal Electrocyclic Reactions of 6π-Electron and Aza 6π-Electron Systems

ChemInform Abstract: Synthesis of Ellipticine and Olivacine by the Thermal Electrocyclic Reaction via Pyridine 3,4-Quinodimethane Intermediates.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Synthesis of Mutagenic Amino-α-carbolines AαC and MeA. alpha.C by the Thermal Electrocyclic Reaction of 2-Azahexa-1,3,5- triene Intermediates.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: New Regioselective Total Syntheses of Antibiotic Renierol, Renierol Acetate, and Renierol Propionate from the 5-Oxygenated Isoquinoline.

New total syntheses of renierol (3), renierol acetate (4), and renierol propionate (5) were completed by the synthesis of 5-oxygenated isoquinoline (6) based on the thermal electrocyclic reaction of the 1-azahexatriene system followed by regioselective oxidations of 5-hydroxyisoquinolines (6).

ChemInform Abstract: Total Syntheses of Novel Cytocidal β-Carboline Alkaloids, Oxopropalines D and G.

A new type of b -carboline nucleus, N-methoxymethyl-4-methyl-b -carboline (4) was synthesized by thermal electrocyclic reaction of a 1-azahexatriene system, involving the indole 2,3-bond. The key compound Nmethoxymethyl-1-methoxycarbonyl-4-methyl-b -carboline (2) was then prepared in a four-step sequence. The total synthesis of oxopropaline G (1e) was achieved from this key compound in four steps. Furthermore, the enantioselective total syntheses of (1)-oxopropaline D (1c) and its enantiomer were also achieved by application of the Sharpless oxidation-procedure in nine steps from 2.

A Preferred Disrotatory 4n Electron Möbius Aromatic Transition State for a Thermal Electrocyclic Reaction

Not forbidden: Thermal 4n electron electrocyclic reactions of Hückel topology structures proceed via "allowed" conrotatory pathways. However, for a Möbius topology, the Woodward-Hoffmann rules may be reversed and a "forbidden" disrotatory pathway can be preferred as shown theoretically for dodecahexaene 1 that transforms via a Heilbronner-Möbius aromatic transition structure 2 into a cyclic polyene 3.

Novel synthesis of the 2-azaanthraquinone alkaloid, scorpinone, based on two microwave-assisted pericyclic reactions

The total synthesis of the 2-azaanthraquinone alkaloid, scorpinone ( 4), isolated from the mycelium of a Bispora-like tropical fungus, has been completed in nine steps. The two key steps involve a microwave-assisted thermal electrocyclic reaction for the synthesis of an 8-oxygenated isoquinoline skeleton from a 1-aza 6π-hexatriene system, and a regioselective microwave-assisted [4+2] cycloaddition for the construction of a 2-azaanthraquinone framework.

The Enantioselective Total Synthesis of a .BETA.-Carboline Alkaloid, (S)-(-)-Dichotomine C

The first enantioselective synthesis of a beta-carboline alkaloid, dichotomine C, possessing antiallergic effects, was achieved by constructing a beta-carboline framework based on the microwave-assisted thermal electrocyclic reaction of a 1-azahexatriene system, followed by the Sharpless asymmetric dihydroxylation.

Synthesis of the New Furo[3,2‐h]isoquinoline Alkaloid, TMC‐120B from Aspergillus ustus.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Highly Efficient Synthesis of Trispheridine through a Construction of Tetrahydrophenanthridine Based on a Microwave-assisted Thermal Electrocyclic Reaction of an Aza 6π-Electron System

A highly efficient synthesis of a phenanthridine alkaloid, trispheridine (1) has been achieved in a 74% overall yield in a four-step sequence. The key step is the construction of a tetrahydrophenanthridine based on a microwave -assisted thermal electrocyclic reaction of an aza 631-electron system including the benzene double bond.

Synthesis of the New Furo[3,2-h]isoquinoline Alkaloid, TMC-120B from Aspergillus ustus

A total synthesis of a new furo[3,2-h]isoquinoline alkaloid TMC120-B (2), isolated from Aspergillus ustus together with two related compounds, has been completed in sixteen steps. The key step is the synthesis of the appropriate 3,7,8-trisubstituted isoquinoline framework (23) based on a thermal electrocyclic reaction of the 1-aza 6pi-electron system involving the benzene double bond. In addition, the microwave assisted electrocyclic reaction of this system was newly performed.

Syntheses of the Antibiotic Alkaloids Renierone, Mimocin, Renierol, Renierol Acetate, Renierol Propionate, and 7‐Methoxy‐1,6‐dimethylisoquinoline‐5,8‐dione.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Syntheses of the antibiotic alkaloids renierone, mimocin, renierol, renierol acetate, renierol propionate, and 7-methoxy-1,6-dimethylisoquinoline-5,8-dione

The total synthesis of renierone, mimocin, renierol, renierol acetate, renierol propionate, and 7-methoxy-1,6-dimethylisoquinoline-5,8-dione was successfully achieved by the regioselective oxidation of 5-oxygenated isoquinoline. The synthetic method of the 5-oxygenated isoquinoline is based on the thermal electrocyclic reaction of 1-azahexatriene system involving the benzene 1,2-bond.