Fused derivatives of (iso)steviol via pericyclic reactions

Recently, the diterpenoids steviol and isosteviol, respectively the aglycone and its rearrangement product of the steviol glycosides, have gained a growing interest. Their biological properties have encouraged many scientists to synthesize a considerable amount of analogues. In this Letter, we present three novel (iso)steviol derivatives in which the complex ring structures of the ent-kaurene steviol and ent-beyerane isosteviol are expanded via two highly stereoselective reaction pathways. For steviol, we expanded the structure by carbonyl ene reaction, followed by either formation of a lactone or a pyrane-3(2H)-one in good yields. Isosteviol was converted into a diene, followed by Diels–Alder reaction.