A polyene cyclization converts a chain containing several carbon–carbon double bonds into one or more rings. A suitable initiating event can generate a reactive centre that attacks a later double bond, producing another reactive centre and allowing a cascade of ring closures.
The folded chain's geometry helps determine which rings and stereogenic centres form. In biosynthesis, enzyme-controlled cyclization of precursors related to squalene constructs complex steroid frameworks. The double bonds need not all belong to one continuous conjugated system. A plausible drawn cascade also needs compatible reaction kinetics and a three-dimensional conformation; connectivity alone does not guarantee that a proposed sequence occurs.
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