Electrocyclic reaction ring opening

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... 

Additionally, the same authors have isolated [l,4]diazepino[2,3-f]isoxazoles 27 <2000TL9319> from the same reagents which they suggest proceeds via fused tricyclic compound 26 followed by an electrocyclic ring-opening reaction (Scheme 2), although no yield for the reaction is given in the paper. 

Consider the electrocyclic ring-opening reaction of cyclobutene. The molecule is formally divided into two fragments the double bond and the single 0 bond which is cleaved.9 The frontier orbital interactions (0,7t ) and (0, it) relevant to the conrota-tory and disrotatory reactions are given in diagrams 4, 5, 6 and 7, respectively. The net overlap is positive for 4 and 5, but zero for 6 and 7. The conrotatory process is therefore allowed, and the disrotatory process forbidden. 

The construction of complex intermediates from simple and readily available starting materials has been accomplished using the electrocyclic ring-opening reaction of halocyclopropanes. This is typically achieved through interception of the cationic haloallyl intermediate by solvent, the silver(I) counteranion, or some alternate tethered heteroatom or carbon-based nucleophile. Examples of these processes are described below. 

The addition of carbonylated electrophiles to the 2-lithio derivative of 4-oxazolinyloxazole 132 allowed the efficient preparation of 5-phenyloxazoles 134 bearing a variety of hydroxyalkyl groups at C-2 position and a carboxyl (or formyl) function at C-4. This protocol suppresses the troublesome electrocyclic ring-opening reaction and allows access to the target compounds by simple chemical transformation of the oxazoline moiety of 133 <02JOC3601>. A direct chemoselective C-2 silylation of oxazoles was performed by treatment of the lithiated parent compounds with silyl triflates <02TL935>. 

The electrocyclic reactions of phenyl fulgides and l,8a-DHNs can be photo-induced by a conrotatory process. The electrocyclic ring-opening reaction of cyclohexadiene systems (l,8a-DHNs) can also be induced thermally via a disrota-tory process. 

Under basic conditions, attempts to achieve C-5-alkylation of 574 (Scheme 135) led instead to the N-alkylated product 576 after hydrolysis. Presumably a putative initial anion underwent electrocyclic ring opening reaction to generate the more stable A -complex 575 before alkylation took place <1997TL4359>. 

The decomposition of cyclic sulfones and sulfoxides has attracted increasing attention recently in view of their relationship to cyclic fragmentation reactions and of the dramatic impact of the Woodward-Hoffman rules for intermolecular cycloaddition and electrocyclic ring opening reactions. The simplest cyclic sulfone known is 2,3-diphenyl thiirene-1,1-dioxide (A) which was isolated by Carpino and McAdams by the Ramberg-Backlund base catalyzed dehydrohalogenation of a, a -dibromodibenzylsulfone, viz. 

Fig. 33 a, b. Orbital correlation diagrams for an electrocyclic ring opening reaction proceeding by a) the disrotatory or b) the conrotatory route... 

Related electrocyclic ring opening reactions play an important role in photochromic systems. In this connection,... 

R. Huisgen, Electrocyclic Ring Opening Reactions of Ethylene Oxides , Angew. Chem. Internat. Edn., 1977,16, 572. 

First principles simulations of electrocyclic ring opening reactions... 

A review of the use of electrocyclic reactions in synthesis has been presented" and of asymmetric electrocyclic reactions that result in diastereo- or enantio-selectivity selection as a consequence of the influence of a chiral component, be it substrate or catalyst, on the electrocyclic bond-forming process." Computational studies of 0 electrocyclic ring-opening reactions of 2-pyrone and 6-fluoro-2-pyrone are pseudoper-icyclic 6-fluoro-2-pyranol are a borderline case and 2-pyranol, pyran, and 6-fluoro pyran reactions are pericyclic in character." ... 

The torquoselectivity of the 4 r electrocyclic ring-opening reaction of 2-azetines has been reported to be controlled by the Brpnsted acidity of the catalyst and the polarity of the solvent." A review of the origin of the torquoselectivity in the thermal ring opening of cyclobutenes based on the Cplex-isoelectronic theory and expanded Robinson electronic theory has been reported." Cplex-isoelectronic theory is said to provide a rational for the torquoselectivities in the ring opening of cyclobutenes." ... 

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