Scheme 26 Hetero Diels-Alder route to (±)-alantrypinone [268,269]

these adducts gave (±)-alantrypinone 24 and (±)-17-epi-alantrypinone 110, respectively (Scheme 26). Based on thermal equibration results in various conditions, the interesting epimerizations between 108 and 109, and 24 and 110 are rationalized by an intramolecular rearrangement involving an anionic retro-Mannich reaction [268,269]. (For synthesis of oxygen-briged spiro-type fumiquinazolines C, and H see [248,250])

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