Three-dimensional conformation of the quassinoids aligned on the lactone substructure: (A) overlay of the biologically inactive picrasane quassinoids and (B) overlay of the biologically active chaparrinon-type quassinoids.  Notice the presence of the oxymethylene bridge between C8 and C11 in the active molecules and the absence of the oxygen bridge in the inactive molecules.
 

Electrostatic potential maps of the region of the oxymethylene bridge in (A) the biologically inactive picrasane-type quassinoids and (B) of the biologically active chaparrinone-type quassinoids.  Notice the increased electronegativity potential resulting from the presence of the oxymethylene bridge.  Electrostatic potentials range from electronegative (purple) to electropositive (red).
 

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