Disrotatory electrocyclic ring closure

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 direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

Upon exposure to UV light, a-tropolone methyl ether (142), included within chirally modified Y zeolite, has been found to undergo 4 7r-electron disrotatory electrocyclic ring closure to afford " the bicyclic photo-isomer (143). 

The initial photochemical reaction is intramolecular ortho cycloaddition leading to bicyclo[4.2.0]octa-2,4-diene derivatives. These normally are not detected because they undergo rapid thermal disrotatory ring opening to all-cis cyclooctatrienes which have large extinction coefficients and absorb a second photon to undergo electrocyclic ring closure to the photostable bicyclo[4.2.0] octa-2,7-dienes. 

Ring strain is important in preventing reaction that would otherwise change your view of a lot of the chemistry you know. Ally cations are conjugated systems containing 2k electrons, so if you knew no other chemistry than what is in this chapter you might expect them to cyclize via disrotatory electrocyclic ring closure. 

Tropolone ether 35 undergoes a disrotatory 4tt electrocyclic ring closure to yield the corresponding bicyclo[3.2.0] product 36 (Scheme 18) [284,285]. The chirality... 

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 coniotatory (con) ring closure for the thermally or photochemically induced processes, reqwctively. The same modes (dis or con) are predicted for the reverse ring opening processes...

The Robinson annulation has three distinct steps the Michael addition of the enol or enolate across the double bond of the a,(3-unsaturated ketone to produce a 1,5-diketone (Michael adduct), followed by an intramolecular aldol reaction, which affords a cyclic (3-hydroxy ketone (keto alcohol), and finally a base-catalyzed dehydration which gives rise to the substituted cyclohexenone. An alternative mechanism via disrotatory electrocyclic ring closure is possible. ... 

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

Figure 7.20. Comparison of the AO interactions in the photochemical chain abridgement in a polysilane (top) and in the disrotatory electrocyclic ring closure of butadiene (bottom) (by permission from Michl and Balaji, 1991).

This looks as though each of the C—C bonds is independently the result of both HOMO/LUMO interactions, with an endo selectivity as well. In the presence of dienes, these species behave as allyl cations (see p. 259) and undergo clean [4 + 2] cycloadditions, as in the reaction of the oxyallyl 6.372 giving the tricyclic ketone 6.373, which is similar to the diene 6.369. Normally, oxyallyls are in equilibrium by disrotatory electrocyclic ring closures with cyclopropanones and with allene oxides, but the presence of the five-membered ring in these particular examples makes these pathways counter-thermodynamic. 

The thermal disrotatory [n6] electrocyclization of cis-1,3,5-hexatriene systems has been extensively employed for the synthesis of cyclic hydrocarbons. The average enthalpy of activation is in the range of about 120 kJ mol 1 [36]. The incorporation of two of the hexatriene double bonds in phenyl rings (stil-bene, 1) stabilizes the precursor significantly and necessitates temperatures of 1050°C to obtain a 30% yield of phenanthrene (2, see Scheme 2, [37]). An enthalpy of activation of (250 20) kJ mok was estimated for the conversion of 9,9 -bifluorenylidene (3) to benz[e]indeno[l,2,3-hi]acephenanthrylene (4), a reaction that is accompanied by the radical initiated isomerization of 3 to diben-zo[g,p]chrysene (5, Scheme 2, [38]). It is assumed that both reactions 1 —> 2 and 3 —> 4 are initiated by an electrocyclic ring closure forming a 4 a,4 fr-dihydro-phenanthrene (la) intermediate. 

Electrocyclic ring closures occur in the gas phase and in solution, are relatively insensitive to solvent polarity and do not require catalysis. Typically, they are also stereospecific bond formation between terminal carbons of triene systems occurs in a disrotatory manner, and bond formation between termini of conjugated diene and tetraene systems occurs in a con-rotatory manner Kinetic data on electrocyclic ring closures are summarized in Table . 

Dienyl cations and dienyl anions both undergo electrocyclic ring closure—a nice example occurs when this cyclooctadiene is deprotonated with butyllithium. There are still five p orbitals involved in the cyclization, but now there are six n electrons, so the reaction is disrotatory. 

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