Electrocyclic formation

Acid-catalyzed electrocyclic formation of cyclopentenone from di-vinyl ketone. 

From conventional homogeneous to green homogeneous and heterogeneous catalysis with Lewis acids (ring opening and electrocyclic formation of heterocycles, reactions in ionic liquids, 7V,7V -dialkylimidazolium and Y-alkylpyridinium salts) 03CRV4307. 

Three enantiomerically pure starting materials ensure remote stereochemical control Tandem iminium ion formation and vinyl silane cyclisation Part IV - Tandem Pericyclic Reactions Electrocyclic Formation of a Diene for Diels-Alder Reaction Tandem Ene Reactions Tandem [3,3]-Sigmatropic Rearrangements Tandem Aza-Diels-Alder and Aza-Ene Reactions... 

Electrocyclic Formation of a Diene for Diels-Alder Reaction... 

The indolin-3-one 115 is conveniently prepared by FVP of the benzi-soxazole 113 at 700 °C (Scheme 24). Electrocyclic formation of the nitrene 114, which then inserts intramolecularly into CH, is the most likely mechanism (2015UP3). There has been considerable progress in generating heterocycle-fiised analogs of such indolones, so-called heteroindoxyls. 

An increasing number of examples of ring formation through 1,5-electrocyclization of appropriate carbanions are illustrated in Scheme 27. In the last example the use of a chiral alkoxide (R = menthyl or bornyl) results in the formation of chiral indolines with optical purities ranging from 17 to 31%. 

Methylvinyldiazirine (199) rearranges at room temperature in the course of some days. Formation of the linear isomer is followed by electrocyclic ring closure to give 3-methyl-pyrazole. The linear diazo compound could be trapped by its reaction with acids to form esters, while the starting diazirine (199) is inert towards acids (B-71MI50801). 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

The proposed mechanism for the Conrad-Limpach reaction is shown below. Condensation of an aniline with a 3-keto-ester (i.e., ethyl acetoacetate 5) with loss of water provides enamino-ester 6. Enolization furnishes 10 which undergoes thermal cyclization, analogous to the Gould-Jacobs reaction, via 6n electrocyclization to yield intermediate 11. Compound 11 suffers loss of alcohol followed by tautomerization to give 4-hydroxy-2-methylquinoline 7. An alternative to the proposed formation of 10 is ejection of alcohol from 6 furnishing ketene 13, which then undergoes 671 electrocyclization to provide 12. 

Upon treatment of a divinyl ketone 1 with a protic acid or a Lewis acid, an electrocyclic ring closure can take place to yield a cyclopentenone 3. This reaction is called the Nazarov cyclization Protonation at the carbonyl oxygen of the divinyl ketone 1 leads to formation of a hydroxypentadienyl cation 2, which can undergo a thermally allowed, conrotatory electrocyclic ring closure reaction to give a cyclopentenyl cation 4. Through subsequent loss of a proton a mixture of isomeric cyclopentenones 5 and 6 is obtained ... 

Irradiation of triazafulvene 2 or 3 leads to the formation of an azacyclobutadienc intermediate which, after dimerization and two subsequent electrocyclic ring-opening reactions, gives the hexaphenyl-substituted 1.5-diazocine.48,49... 

Formation of 59 and 60 is postulated to occur via an electrocyclic ring opening reaction giving the open chain cation 63 which can then go either... 

Trauner and colleagues [39] recently found a striking contrast in the thermal and catalyzed reactions of a triene. Thermal reaction of a trienolate readily underwent disrotatory electrocyclization to afford cyclohexadiene (delocalization band in Scheme 8) in accordance with the Woodward-Hoffmann rule. Surprisingly, treatment of the trienolate with Lewis acid did not result in the formation of the cyclohexadiene but rather gave bicyclo[3.1.0]hexene in a [4n +2nJ manner (pseudoexcitation band in Scheme 8). The catalyzed reaction is similar to the photochemical reaction in the delocalization band. 

---