Disrotatory opening

When 2-methoxy-l,6-methano[10]annulene 32 was subjected to a cyclopropana-tion with diazomethane and cuprous chloride as catalyst reaction occurred preferentially at the 5,6-, 6,7- and/or 1,10-bonds and the adducts spontaneously underwent disrotatory opening yielding the corresponding methoxybicyclo[5.4.1]dodecapen-taenes. Hydride abstraction with triphenylmethyl fluoroborate was performed on the mixture and the ions 33 and 34 so produced were treated with dilute aqueous potassium hydroxide. The annulenones 13 and 14 were then separated by chromatography. 

Figure B14.2. Orbital correlation diagram for electrocyclic opening of cyclopropyl cation S, A and solid lines indicate disrotatory opening S, A and dotted lines indicate conrotatory opening.

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

Figure 6.11 Orbital symmetry allowed (disrotatory) opening of a cyclobutyl cation. Note that the orbitals of the C—C bond being broken overlap with the back side of the orbital used for bonding to the departing group.

Photolysis of ene oxiranes not possessing an a-cyano group has also led to intramolecular carbonyl ylide cycloaddition, but in low yield.123 Thus, bicyclic tetrahydrofuran (223) was formed in 35% yield by photolytic disrotatory opening of the corresponding franr-diphenyloxirane. Thermolytic conrotatory opening of the isomeric m-oxirane also afforded tetrahydrofuran (223) in 25% yield accompanied by an aldehyde byproduct. 

The concerted disrotatory opening of the cyclobutene is much easier in the aldehyde than in the starting alcohol. Thus, while the alcohol could be easily isolated, its oxidation to aldehyde leads to a cyclobutene that could not be isolated, because it suffers a quick opening of the ring to the final product. 

The thermal ring opening of bicyclo[3.2.0]hepta-3,6-dien-2-one A to give tropolone B is forbidden (disrotatory opening of a cyclobutene). So, in spite of the strain in the starting material, it only occurs at 350 °C. 

When the latter reaction is carried out in MeOD, the distribution of the label in the products is consistent with attack of silver ion at the 7i-bond and disrotatory opening of an incipient cyclopropyl cation towards the centre of developing positive charge 284) ... 

Disrotatory opening of Dewar benzene, which would produce benzene, is thermally forbidden. Therefore, even though the conversion of Dewar benzene to benzene is quite exothermic, it might indeed prove possible to isolate Dewar benzene because there is no low-energy pathway for its conversion to benzene. 

On a strict symmetry basis, conversion of 1,2-dimethylenecyclobutane to the planar biallylene with preservation of a plane of symmetry is thermally allowed with a disrotatory opening 85>. Direct conversion to the perpendicular Dza species maintained aCa axis gives a thermally... 

As we saw in Section 2.B, the interconversion of butadiene and cyclobutene can occur through either a disrotatory or a conrotatory process. As the following diagram shows, the disrotatory opening is symmetrical with respect to a O plane, whereas conrotatory opening is symmetrical with respect to the C axis. 

The cycloaddition of alkenylidenecyclopropanes 5 with chlorosulfonyl isocyanate is described via electrophilic attack at C4, disrotatory opening of the cyclopropane ring to form a dipolar intermediate 8, and ring closure by nucleophilic attack of nitrogen or oxygen on either end of the allyl cation portion of the intermediate 8 which gives adducts 9-12. [27t -I- 2tc] Products 13 were also obtained. 

---