The total synthesis of (+)-ryanodol. Part II. Model studies for rings B and C of (+)-anhydroryanodol. Preparation of a key pentacyclic intermediate

This paper reports several model studies that were necessary for the rational conception of a simple four-step synthesis (6 + (S)-74 → 81a–b → 83 [Formula: see text] 87 → 89) (Scheme 11) of the carbonate derivative 89 of the optically active pentacyclic dihydroxy ketoaldehyde 87, an important key intermediate for the synthesis of (+)-ryanodol (5). The optically active vinyl ketone (S)-74 that was used as starting material was prepared in four steps from d-carvone ((S)-94) (Scheme 13). The preparation of the other starting material, the o-spirolactone dienone 6, was reported in Part I.