The polyketide chain produced then undergoes a ring closure to form the large
lactone ring and a
Diels–Alder reaction to form the fused
cyclohexane/
cyclohexene rings. The oxygen atoms attached to carbons in positions that do not correspond to polyketides—carbons 8 and 13 (the ether bridge), carbon 2 (the
methoxy substituent), and carbon 18 (on the hydroxyethyl chain attached to the lactone ring) are derived from molecular oxygen.[7]
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