Diels-Alder reaction cubane

The following additional examples offer a hint of the utility of organic photochemistry in s)mthesis. A [2 + 2] photochemical cycloaddition was a key step in the synthesis of cubane. S)mthesis of 2-bromocyclopenta-2,4-dienone resulted in a spontaneous Diels-Alder reaction, which produced 132. The monoketal of the dimer (133) underwent intramolecular photocycloaddition to produce 134 (equation 12.85). The Favorskii reaction of 134 gave 135, which was decarboxylated to 136 (equation 12.86). Hydrolysis of the ketal gave 137. Subsequent Favorskii reaction to 138 and then decarboxylation produced cubane, 139. Similarly, de Meijere and co-workers reported that the intramolecular [2 + 2] photochemical cycloaddition of 140 produced octacyclopropylcubane (141, equation 12.87) Meder and coworkers utilized the reaction sequence in equation 12.88 to convert the diazo compound 142 into tetra-f-butylcyclobutadiene (143) and tetra-f-butyltetra-hedrane (144). " ... 

An elegant example of the use of the Favorskii rearrangement was in the first deliberate synthesis of the cubane carbon skeleton in 1964 by Eaton and Cole. " Their sequence begins with 2-cyclopentenone 10, which is first mono-brominated with NBS and then di-brominated using Br2 to give 11. Double dehydrobromination is achieved using diethylamine to form transient species 12, which immediately self-dimerizes via a Diels-Alder reaction to form 13. Subsequent ultraviolet light irradiation in the presence of HCl... 

In cases where the diene is unstable, a dihalo starting material may be used with the iron carbonyl acting as a reducing agent. Both the trimethylene methane complex 10.5 (Scheme 10.3) and the cyclobutadiene complex 10.7 (Scheme 10.4) have been made in this way. The cyclobutadiene-iron complex 10.7 is a convenient storable form of this highly unstable diene. It can be liberated by oxidation and, if this is done in the presence of a dienophile, the Diels-Alder product is obtained. The Diels-Alder reaction with 2,5-dibromobenzoquinone gave the expected e do-product 10.8. An intramolecular photochemical [2-1-2] cycloaddition, followed by a Favorskii reaction, gave a cubane dicarboxylic acid 10.10. ... 

After Buchi confirmed this finding in the 19S0s, numerous other investigators immediately recognized the rapidity with which complex molecular architectures could be constructed by this reaction. Cookson irradiated the Diels-Alder adduct (3) of cyclopentadiene and benzoquinone to produce the caged diketone (4) (equation 2) while Eaton utilized a photochemical cycloaddition of diene (5) in a synthesis of the platonic solid cubane (6) (equation 3). ... 

A convenient synthesis of a cubane system, in which the cyclobutadiene transfer reaction plays a key role, was reported by Pettit et al. (Scheme 11). By the oxidative decomposition of cyclobutadiene-iron tricarbonyl 56 with Ce + ion in the presence of a dienophile, a molecule of cyclobutadiene can be transferred from the iron to the dienophile. Decomposition of 56 in the presence of 2,5-dibromobenzoquinone 57 yielded the Diels-Alder adduct 58 with e do-configuration. Irradiation of 58 in benzene with a high-pressure Hg lamp afforded the bishomocubane derivative 59, which gave cubane-1,3-dicarboxylic acid 60 (80%) by treatment of 59 with aqueous KOH at 100°C. 

---