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By Tanmoy Kanti Das and Subhamoy Maitra

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The difficulties encountered in the hydrolysis of 13-acyl ingenols are not uncommon for hindered tertiary hydroxy groups, while the unusual reactivity of the tertiary 13-hydroxy functionalities of phorbol might be mediated by acyl migration to the adjacent secondary C-12 hydroxy group. Acyl groups at the primary and allylic 20-hydroxy group can be selectively removed by solvolysis in acidic methanol [105]. The rate of the reaction is critically dependent on the size of the ester group, sharply decreasing with the growing length of the acyl group.

Therefore, LiHMDS co-activates in a complementary fashion both the 3-hydroxylated site of ingenol 5,20-acetonide, by acting as a Brønsted base, and the acyl carbonyl of angelic anhydride, by behaving as a Lewis acid. Removal of the acetonide protection was uneventful, and I3A (5) was eventually obtained in 62% yield (Fig. 26). HNR2 O O R2N O O O ROH 117 R2N OR O 118 - HNR2 OR O O 119 120 Fig. 25 Possible mechanism of the angelate to tigliate isomerization during acylation reactions O O O 1. LHMDS, angelic anhydride (86%) acetone PTSA (73%) HO OH HO OH 3 (ingenol) 5 HO 2.

Esula E. cornigera E. esula E. kansui Ref. [211] [212] [213] [207, 214] [213] [207] [214] [213] [104] [82] [82] [213] [214] [212] [212] [82] [212] [211] 16-Hydroxyingenol Esters and 17-Hydroxyingenol Esters In the early studies on ingenoids, hydroxylation at C-16 was established based exclusively on considerations of chemical shift differences with the parent polyol ingenol. Whenever rigorous 2D-NMR measurements were done, the hydroxylation site on the geminal methyls was, however, always located at C-17.

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