Appendix 1. Diels-Alder Reactions

One of the most efficient methods (high yield, controlled stereochemistry, diverse functionality) to construct rings from smaller fragments is via cycloaddition reactions. The reverse reaction, namely splitting of a ring into smaller fragments is termed a cycloreversion reaction is also an important synthetic tool. (For example, in the present experiment it is the cracking of cyclopentadiene dimer to cyclopentadiene monomer.) The construction of six-membered rings built from a fragment of four atoms linked together by two conjugated double bonds (a diene) and a fragment containing two atoms linked by a double bond or triple bond (a dienophile) is known as the Diels-Alder (D-A) reaction. It is often shorthanded as a [4+2]-p-electron cycloaddition. During the retro-D-A, a six-membered ring is split into a diene (often an aromatic compound) and a homonuclear (alkene, alkyne) or heteronuclear (CO) p bond. The simplest example of a D-A reaction is the reaction between butadiene and ethylene to form cyclohexene. Both the diene and the dienophile display a very low reactivity. Please note the very sluggish reaction conditions: high pressure and an unusual high butadiene: ethylene molar ratio (Swiss patent, Chem. Abstract 89, 59709).