Ring opening of cyclobutanes

For Woodward-Hoffman allowed thermal reactions (such as the conrotatory ring opening of cyclobutane), orbital symmetry is conserved and there is no change in orbital occupancy. Even though bonds are made and broken, you can use the RHF wave function. 

Arenesulphinyl groups have been found to facilitate ring opening of cyclobutanes (equation 376)684. 

Many highly strained cage molecules undergo rearrangement when treated with metallic ions such as Ag+, Rh(I), or Pd(II).580 The bond rearrangements observed can be formally classified into two main types (1) 2 + 2 ring openings of cyclobutanes and (2) conversion... 

Butanediyl, tetramethylene. Using complete active space (CAS) calculations (below), Doubleday found ten stationary points with tetramethylene-like structures, in work connected with ring-opening of cyclobutane [61]. We saw that model chemistries simply lead to closure or dissociation of input tetramethylene-type structures. 

The density functional theory calculations mentioned earlier also modeled the ring-opening of cyclobutane radical cation [87]. This reaction proceeded via a distorted trapezoid transition state with two shorter (142.2, 147.1 pm) and two longer bonds (187.7, 208.4 pm), forming the "anti -complex (one long bond 221.6 pm), similar to that calculated earlier [168]. 

In addition to the 3,4-dimethylcyclobutene case discussed in Section 10.5.1, there are many other examples of electrocyclic ring opening of cyclobutanes, and cis- and fra 5-3,4-dichlorocyclobutene have been examined carefully. The products are those expected for conrotation. In the case of the franx-isomer, the product results from outward rotation of both chlorine atoms, in agreement with the calculated substituent effect. The c/x-isomer, in which one of the chlorines must rotate inward, has a substantially higher E. ... 

The similarity in the product ratios from the two 3.2.0 dideuterio isomers would appear to minimize the role of isotope effects here. It is possible that the 1,3- and 3,3-shift pathways are separate, but ring opening of cyclobutanes by the coupled motions depicted in Scheme 9.59 would give conformationally distinct, incompletely equilibrated, bisallyl biradicals which could rationalize the preferences. 

Figure 1 Transition structure for the electrocyclic ring opening of cyclobutane to trans-1,3-butadiene...

The cyclobutane ring was then cleaved by hydrolysis of the enamine and ring opening of the resulting (3-diketone. The relative configuration of the chiral centers is unaffected by subsequent transformations, so the overall sequence is stereoselective. Another key step in this synthesis is Step D, which corresponds to the transformation 10-IIa => 10-la in the retrosynthesis. A protected cyanohydrin was used as a nucleophilic acyl anion equivalent in this step. The final steps of the synthesis in Scheme 13.11 employed the C(2) carbonyl group to introduce the carboxy group and the C(l)-C(2) double bond. 

Scheme 1 UV-light induced formation of the two major photo lesions in DNA. T=T cyclobutane pyrimidine dimer. (6-4)-photo product (6-4)-lesion, formed after ring opening of an oxetane intermediate, which is the product of a Paterno-Buchi reaction...

A cyclobutane ring-opening of the photoadduct (426) by the reaction of BF3 Etfi in refluxing benzene gave isopropenylcyclohexenone (427). The reaction could be applied to more complex compounds. Thus, (428), (430), (432) and (434) were converted accordingly to (429), (431), (433) as well as (435) 145). 

The ciT-substituted cyclobutane ester 502 is of interest as intermediate for the synthesis of carbocyclic 503 (equation 218). Ring opening of the bicyclic (-l-)-Nopinone 501... 

Intramolecular [2n + 27r] cycloadditions leading to cyclobutanes formally belong to valence isomerizations. Alkene/alkcne, ketene/alkene, and allene/alkene cycloadditions have received detailed attention. These rearrangements provide powerful methods for the synthetic arsenal. An example is the facile synthesis of bieyelo[3.2.0]heptenones 3 by intramolecular ketene/alkene cycloaddition after electrocyclic ring opening of cyclobutenoncs l.89... 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

Isomerization reactions of organo-fluorine compounds are included in Houben-Weyl, Vol. 5/3, pp 346-353, where five types of isomerization reactions are covered (1) intramolecular cycli-zation of dienes, (2) opening of cyclobutene rings, (3) opening of cyclobutane rings, (4) isomerization of polyfluorocyclohexadienes, and (5) intramolecular rearrangement of chlorofluoro-and bromofluorocarbons. Much of the work reported is still of fundamental significance and is covered here, where appropriate. Several more later reviews also deal with various aspects of this area.1... 

In a similar manner, Mloston et al. prepared the thiadiazoline 88 and treated it with electron-deficient acetylenes giving 2,5-dihydrothiophenes 89 in generally good yields. Reaction of 89 with TFA at room temperature, resulted in the formation of thiophene 90 via isomerisation and ring-opening of the cyclobutane ring <02HCA451 >. 

A relevant extension of the ring opening of epoxides with a titanium salt can be seen in the concomitant scission of the C-C bond of cyclobutane illustrated in Eqs (285) and (286) [648]. Choice of each diastereoisomer enables highly selective construction of the trisubstituted double bond which finds application in terpene synthesis. The tandem cleavage of the C-O and C-C bonds was similarly feasible in the combination of oxetane and cyclobutane as illustrated in the stereoselective synthesis of a homoallyl alcohol (Eq. 287) [648]. 

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