Electrocyclic ring-closure conrotatory

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 ... 

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

The first electrocyclic ring closure involves eight electrons, so it is conrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the tetraene, which are both in, become trans. The second electrocyclic ring closure involves six electrons, so it is disrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the triene, which are both out, become cis. This is the arrangement observed in the natural product. 

The electrocyclic ring-closure reaction proceeds exclusively in the S state and yields solely the trans product by a conrotatory mode of reaction, as is to be expected from Table 7.3 for a 6 r-electron system. Competing reactions are cis-trans isomerization (cf. Section 7.1.4) and intersystem crossing to T,. From the T, state, generally only cis-trans isomerization is observed. Stilbenes with substituents that enhance spin inversion, such as Br, RCO, and NO2, do not undergo the cyclization reaction efficiently. 

Which thermal electrocyclic ring closure mode, conrotatory or disrotatory, would the cation in problem 12.1 follow ... 

Figure 1 The four stereochemically distinct modes of electrocyclic ring closure wherein the notations inward and outward refer to the axes of rotation as viewed with respect to ( ). The principle of the conservation of orbital symmetry predicts disrotatory (dis) or conrotatory (con) ring closure for the thermally or photochemically induced processes, respectively. The same modes (dis or con) are predicted for the reverse ring opening processes...

The cycloaddition of 8 with dimethyl acetylene dicarboxylate 9 to generate 10 [2, 3], the electrocyclic ring closures 11 > 12 and 14 > 15 (8 electron conrotatory... 

Electrocyclic ring closures are reversible, and only those which yield significant amounts of cyclic products at equilibrium have been studied. Most butadiene-cyclobutene equilibria greatly favor the butadienes (see below). An exception is the cyclization of irans, a.s-l,2,3,4-tetraphenyl-l-bromo-l,3-butadiene, which undergoes the predicted conrotatory cyclization to a significant extent. ... 

In this case, it is the conrotatory photochemical cyclization that is prevented by strain (it was tried—cyclooctadienyl anion is stable for at least a week at -78 °C in broad daylight) as the product would be a 5,5 traws-fused system. The same strain prevents thermal electrocyclic ring closure of cyclooctadienyl cations. 

There is another approach to predicting the stereochemistry of electrocyclic ring closures. One simply looks at the ends of the HOMO to conclude the proper direction for rotation of the bond by creating in-phase interactions during closure. Show that this method also predicts conrotatory closure for butadiene and disrotatory closure for hexatriene. 

Theoretical studies have been carried out on the conrotatory and disrotatory reaction pathways of hexa-l,3,5-triene to cyclohexadiene, and the effect of solvent and salt effects on the rates of the electrocyclic ring closure of (IZ, 3Z, 5 )-l,2,6-triphenylhexa-l,3,5-triene has been determined. An ab initio study of the electro-cyclization of (Z)-hexa-l,3,5-triene and its hetero-substituted analogues has been undertaken.The involvement of a lone pair on the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the ring. It has been reported that the intramoiecular aza-Diels-Alder reaction of an a./S-unsaturated hydrazones to a quinone, as in (197), is followed by an unprecedented rearrangement in which the aminoaryi moiety formally undergoes a 1,2-shift to yield benzo- or pyrido-[fc]acridine-6,11-dienes (198). 

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