Thermal 6π-electrocyclization Research Articles

A Torquoselective Thermal 6π-Electrocyclization Approach to 1,4-Cyclohexadienes via Solvent-Aided Proton Transfer: Experimental and Theoretical Studies.

The thermal 6π-electrocyclization of hexatriene typically delivers 1,3-cyclohexadiene (1,3-CHD). However, there is only limited success in directly synthesizing 1,4-cyclohexadiene (1,4-CHD) using such an approach, probably due to the difficulty in realizing thermally-forbidden 1,3-hydride shift after electrocyclic ring closure. The present study shows that by heating (2E,4E,6E)-hexatrienes bearing ester or ketone substituents at the C1-position in a mixture of toluene/MeOH or EtOH (2 : 1) solvents at 90-100 °C, 1,4-CHDs can be selectively synthesized. This is achieved through a torquoselective disrotatory 6π-electrocyclic ring closure followed by a proton-transfer process. The success of this method depends on the polar protic solvent-assisted intramolecular proton transfer from 1,3-CHD to 1,4-CHD, which has been confirmed by deuterium-labeling experiments. There are no reports to date for such a solvent-assisted isomerization. Density functional theory (DFT) studies have suggested that forming 1,3-CHD and subsequent isomerization is a thermodynamically feasible process, regardless of the functional groups involved. Two possible successive polar solvent-assisted proton-transfer pathways have been identified for isomerization.

Enal-azomethine ylides: application in the synthesis of functionalized pyrroles.

Rhodium-catalyzed [3 + 2] annulation of diazoenals and N-alkyl imines resulted in N-alkyl-pyrrole-3-carbaldehyde derivatives. The reaction involves thermal 6π-electrocyclization and aromatization of a new class of enal-azomethine ylides (EAYs). The EAYs derived from dihydroisoquinoline and 2H-azirine gave fused-pyrrole and pyridine derivatives, respectively. The synthetic importance of pyrrole products has been demonstrated by one-step synthesis of the biologically relevant pyrrolo[3,2-c]quinoline scaffold as well as pyrrolo[2,1-a]isoquinoline which is a core structure of lamellarin alkaloids.

Synthesis of Sugar Diene and Its Pd-Catalyzed Transformation into Chromanes.

A route to synthesize 1,2-disubstituted glucals has been developed, which were further converted to substituted chromanes by thermal 6π-electrocyclization in HMPA followed by in situ aromatization. One of the key steps in the synthesis of chromane is metal-free generation of C1-substituted glucal from d-mannose, which was further converted to 1,2-disubstituted glucals by Pd-catalyzed Fujiwara-Moritani reaction with styrenes, acrylates, acrylamide, acrylonitrile, and ethyl vinyl ketone in good yields.

Tandem Reaction Approaches to Isoquinolones from 2-Vinylbenzaldehydes and Anilines via Imine Formation–6π-Electrocyclization–Aerobic Oxidation Sequence

Two distinctive transition-metal-promoted aerobic oxidation protocols have been developed for the synthesis of isoquinolones from 2-vinylbenzaldehydes and aniline derivatives. Thus, the one-pot tandem reaction sequence of imine formation, thermal 6π-electrocyclization, followed by either Cu(OAc)2-mediated or Pd(OAc)2-catalyzed aerobic oxidation protocol allowed the ready access to isoquinolone derivatives. The control experiments revealed that the 1,2-dihydroisoquinoline intermediates from the 6π-electrocyclization of 1-azatrienes were aerobically oxidized to isoquinolones in the presence of either Cu(OAc)2 or Pd(OAc)2 catalyst.

Ligand-controlled Regiodivergent C-H Alkenylation of Pyrazoles and its Application to the Synthesis of Indazoles.

Regioselective C4-, C5-, and di-alkenylations of pyrazoles were achieved. An electrophilic Pd catalyst generated by trifluoroacetic acid (TFA) and 4,5-diazafluoren-9-one (DAF) leads to C4-alkenylation, whereas KOAc and mono-protected amino acid (MPAA) ligand Ac-Val-OH give C5-alkenylation. A combination of palladium acetate, silver carbonate, and pivalic acid affords dialkenylation products. Annulation through sequential alkenylation, thermal 6π-electrocyclization, and oxidation gives functionalized indazoles. This comprehensive strategy greatly expands the range of readily accessible pyrazole and indazole derivatives, enabling useful regiodivergent C-H functionalization of pyrazoles and other heteroaromatic systems.

Total Synthesis of Luotonin A and Rutaecarpine from an Aldimine via the Designed Cyclization.

The total synthesis of rutaecarpine (1) and luotonin A (2) is described through controlled cyclization of a common aldimine intermediate 5 derived from ethyl-2-aminocinnamate and quinazolinone-2-carbaldehyde. The cyanide-mediated imino-Stetter reaction of aldimine 5 provided the corresponding indole derivative 3, from which the total synthesis of rutaecarpine (1) was completed via the formation of a 6-membered C-ring. On the other hand, microwave-assisted thermal 6π-electrocyclization of the common intermediate 5, followed by the formation of a 5-membered C'-ring, allowed the completion of the total synthesis of luotonin A (2).

ChemInform Abstract: Synthesis of Functionalized Benzothiophenes and Dibenzothiophenes by Twofold Heck and Subsequent 6π‐Electrocyclization Reactions of 2,3‐Dibromothiophenes and 2,3‐Dibromobenzothiophenes.

Abstract Benzothiophenes and dibenzothiophenes were prepared by twofold Heck reactions of 2,3-dibromothiophene and 2,3-dibromobenzothiophene, respectively, and subsequent thermal 6π-electrocyclization. The Heck reaction of 2,3-dibromothiophene and 2,3-dibromobenzothiophene with 1 equiv of alkenes proceeded with different regioselectivities and afforded 2-alkenylthiophenes and 3-alkenylbenzothiophenes.

A New Access to 4 H‐Quinolizines from 2‐Vinylpyridine and Alkynes Promoted by Rhodium–N‐Heterocyclic‐Carbene Catalysts

Forging the lock that autolocks! Rh-NHC catalysts promote a new access to 4 H-quinolizine species from 2-vinylpyridine and terminal and internal alkynes through C-H activation and C-C coupling reactions (see figure). N-Bridgehead heterocycle formation is favored for internal- over terminal-substituted butadienylpyridine derivatives in a thermal 6π-electrocyclization process.

Synthesis of functionalized benzothiophenes and dibenzothiophenes by twofold Heck and subsequent 6π-electrocyclization reactions of 2,3-dibromothiophenes and 2,3-dibromobenzothiophenes

Benzothiophenes and dibenzothiophenes were prepared by twofold Heck reactions of 2,3-dibromothiophene and 2,3-dibromobenzothiophene, respectively, and subsequent thermal 6π-electrocyclization. The Heck reaction of 2,3-dibromothiophene and 2,3-dibromobenzothiophene with 1 equiv of alkenes proceeded with different regioselectivities and afforded 2-alkenylthiophenes and 3-alkenylbenzothiophenes.

A thermal 6π electrocyclization strategy towards taiwaniaquinoids. First enantiospecific synthesis of (−)-taiwaniaquinone G

The first route towards taiwaniaquinoid terpenes bearing an A/B trans-configuration has been developed through a sequence which includes a thermal 6pi electrocyclization.

DABCO‐Assisted Coupling of Some Activated Olefins and 2,3‐Dihalo‐1,4‐naphthoquinones and Conversion to the Anthraquinone Derivatives.

AbstractFor Abstract see ChemInform Abstract in Full Text.

DABCO-Assisted Coupling of Some Activated Olefins and 2,3-Dihalo-1,4-naphthoquinones and Conversion to the Anthraquinone Derivatives

A new synthesis of mono- and di-α-vinylnaphthoquinones based on stepwise or one-pot substitution of the halogens in 2,3-dihalo-1,4-naphthoquinones by DABCO-assisted enolate ion is described. Di-α-vinylnaphthoquinones undergo thermal 6π electrocyclization to yield 1,4-disubstituted anthraquinones readily.

A novel synthesis of 3-nitroindoles via electrocyclization of 2,3-(dialk-1-enyl)-4-nitropyrroles

3-Nitroindoles ( 10) are prepared in good yields via a thermal 6π-electrocyclization of 2,3-(dialkenyl)-4-nitropyrroles ( 4) in nitrobenzene, a solvent which causes in situ aromatization of the initially formed dihydroindoles ( 8). The corresponding reaction of 2-alkenyl-3-alkadienyl-4-nitropyrroles ( 5) also leads to 3-nitroindoles ( 11), however, now together with 3-nitrotetrahydroindole derivatives ( 12). The latter compounds are formed by a tandem 6π-electrocyclization - intramolecular Diels-Alder reaction, and are the predominant (or only) products when nitrobenzene is replaced by triglyme.