Seventeen mostly new, skin
irritant diterpene esters (DTE) of the
daphnane and 1 alpha-alkyldaphnane types were isolated from roots of Synaptolepis kirkii and Synaptolepis retusa. The parent
alcohols of the
daphnane types are shown to be 5 beta-hydroxyresiniferonol-6 alpha,7 alpha-
oxide [1] and 5 beta, 12 beta-dihydroxyresiniferonol-6 alpha,7 alpha-
oxide [2]. Ten of the
daphnane types are 9,13,14-orthoesters and three are conventional
esters involving tertiary or secondary
hydroxyl groups at C-13 or C-14, respectively. The latter may be considered immediate precursors of corresponding orthoesters. The four 1 alpha-alkyldaphnane types are intramolecular 9,13,14-ortho-(2-hexadecenoic acid)-esters in which, formally, the second to last C atom of the orthoester moiety is linked covalently to C-1 alpha of the
diterpene parent
alcohols 1 or 2. Thus, in the new structure, a macrocyclic ring bridges the alpha side of the
diterpene moiety in an "ansa" type manner. The irritancies on the mouse ear of the DTE obtained cover a wide range (I24 = 0.05-670 nmole-1). Some of them are considerably more
irritant than the
daphnane type standard
simplexin. Structure/activity investigations reveal that an
ester group instead of a free
hydroxyl group at C-20 ("cryptic types"), or presence of a hydroxy or an acetoxy group in position 12 diminishes the irritancies of the
daphnane types isolated, similar to what is known in corresponding
tigliane types. In the standardized initiation/promotion protocol on the back skin of mice, some of the
irritant DTE exhibit
tumor-promoting activities higher than that of
simplexin.