Clerodanes, Pimaranes, and Rosanes

The occurrence of the rosane lactones e.g., 249) raises the interesting question of whether a carbonium ion intermediate (248, R = CO2H) is neutralized by lactonization immediately following the shift of the methyl group from Cio to C9. If so, oxidation of the methyl group would obviously have to precede cyclization. Separate incorporation of mevalonate bearing tritium at positions 4 and 5 into rosenonolactone 

Although relatively small amounts of the A -rosadienes (255) and (256) are produced initially, the proportion of these rearranged structures increases to 23—50% with longer reaction times, stronger acids, and/or higher temperatures (312, 313). Small amounts (5—11%) of the natural rosadiene, rimuene (250), are also formed (311, 313). As in the pimaradienes, rimuene is isomerized (HCI/CHCI3) to its more stable 

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