<P> In "structurally complex organic molecules", it is the three - dimensional orientation of the carbon--carbon bonds at quaternary loci which dictates the shape of the molecule . Further, quaternary loci are found in many biologically active small molecules, such as cortisone and morphine . </P> <P> Carbon--carbon bond - forming reactions are organic reactions in which a new carbon--carbon bond is formed . They are important in the production of many man - made chemicals such as pharmaceuticals and plastics . </P> <P> Some examples of reactions which form carbon--carbon bonds are aldol reactions, Diels--Alder reaction, the addition of a Grignard reagent to a carbonyl group, a Heck reaction, a Michael reaction and a Wittig reaction . </P> <P> The directed synthesis of desired three - dimensional structures for tertiary carbons was largely solved during the late 20th century, but the same ability to direct quaternary carbon synthesis did not start to emerge until the first decade of the 21st century . </P>

List three other organic reactions that can be used to form carbon‐carbon bonds
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