None of the crystalline coumarins reported earlier have insecticidal activity able to account for that of the crude light petroleum extract of Mammea americana L. A crystalline mixture, shown to be of the two 4-alkyl-5,7-di-hydroxycoumarins (Xa and b), differing only in the possession of an 8-(3-methylbutyryl) or an 8-(2-methylbutyryl) substituent, has now been isolated. It is more active than the most effective noncrystalline fractions from which it came. There is also evidence in some samples of an 8-(butyryl) member (Xc). The distinguishing feature of the insecticides, relative to the already known extractives (cf. Parts I–IV), is the presence of a 4-(1-acetoxypropyl) side-chain. M. africana contains a similar insecticidal mixture.

Spectral and chemical findings concerning compounds (Xa–c) are discussed. The insecticidal compounds are uncouplers of oxidative phosphorylation, but this property appears widespread among other coumarins of the plant. From the structural conclusions drawn it appeared that surangin B from M. longifolia should be an insecticidal compound; this has been confirmed.