Cyclization disrotatory

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Show, by constructing a correlation diagram, whether each of the following disrotatory cyclizations is symmetiy allowed ... 

This analysis shows that disrotatory cyclization is allowed, whereas eonrotation would lead to a highly exeited (r eonfiguration of eyelobutene. The same eonclusion is... 

Problem 30.2 Draw the products you would expect from conrotatory and disrotatory cyclizations of (2Z,4Z.6/)-2,4,6-octatriene. Which of the two paths would you expect the thermal reaction to follow ... 

In an interesting illustration of these reactions, the two disymmetric trienes (+)-33a and (—)-33b were found to preserve their chirality upon photolysis at 193 K and provide cyclohexadienes 34a and 34b, respectively (Scheme 8)18. Upon warming above 205 K, however, they lose their chiral integrity by competitive disrotatory cyclization to the achiral dienes 35a and 35b. The thermal disrotatory closure to the cis-fused ring isomer is generally found to be extremely facile in these systems. 

The thermal disrotatory cyclization of hexatrienes leads to cyclohexadienes (equations 11 and 12)11"13. 

When a cyclic polyene is large enough, it can exist in both cis- and iraws-forms. Our approach to polyene cyclization has tacitly assumed an all cis -n chain in the form of a band or ribbon that would slip smoothly on to the surface of a cylinder of appropriate diameter. Should the orbitals of the two polyenes in (36) have a mismatch in their orbital symmetries, a single twist in the tt band of one of them could remedy this (Fig. 10c). Cycloaddition would now be allowed and the reaction would proceed, provided other factors were favorable. Such cases of Mobius (Zimmerman, 1966), anti (Fukui and Fujimoto, 1966b) or axisymmetric (Lemal and McGregor, 1966), as opposed to Hiickel, syn, or sigma-symmetric ring closure are unknown (or, at least, rare). A Mobius form has, however, been proposed as the key intermediate in the photochemical transformations of benzene (Farenhorst, 1966) in (48) in place of the disrotatory cyclization proposed by van Tamelen (1965). 

Because there are already chiral centres in the molecule, there are, in fact, two possible diastereoiso-meric products from this reaction, both arising from disrotatory cyclization. One is endiandric acid D one is endiandric acid E. 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocyclic reactions, the rules regarding conrotatory and disrotatory cyclizations are reversed. 

It would be a good point here to remind you that, although all electrocyclic reactions are allowed both thermally and photochemically pro viding the rotation is right, the steric requirements for con- or disrotatory cyclization or ring opening may make one or both modes impossible. 

Disrotatory and conrotatory rotation The concerted rotation around two bonds in the same direction, either clockwise or counterclockwise, is described as conrotatory. In the electrocyclic ring opening, the terminal p-orbitals rotate (or twist, roughly 90°) in the same direction known as conrotation (comparable to antarafacial) to form a new ct-bond. In disrotatory cyclization (comparable to suprafacial) the terminal p-orbitals rotate in opposite directions. These two modes of the electrocyclic reaction are shown in Fig. 8.37. 

Problem 30.2 Draw tiic products you would expect from conrotatory anci disrotatory cyclizations... 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

The Nazarov cyclization is an example of a 4iT-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear p ictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constmeted to allow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with the pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatory) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

The electrocyclization reaction of 27 to produce 22 also represents a highly selective transformation. Application of orbital symmetry argue-ments 14> indicates that either 22 or 28 may be formed by a disrotatory cyclization reaction. HMO calculations do not allow for a choice between... 

Disrotatory cyclization of the reduced pyridine (40.1) occurs when it is refluxed in toluene. 

Why is the thermal stability of diarylethene derivatives enhanced by replacing phenyl groups with furan or thiophene groups In molecular orbitals calculation, the photochromic reaction is treated as a typical electrocyclic reaction between hexatriene and cyclohexadiene. The thermal reaction proceeds disrotatorily and the photoreaction, conrotatorily. Disrotatory cyclization of A to B requires an increase in free energy larger than 138 kJ/mol, and hence no thermal ring-closure occurs in the case of either phenyl- or furan-substituted molecules (see... 

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