(±)‐cis‐Chrysanthemic acid is readily lactonized in boiling dilute sulphuric acid to the crystalline (±)‐cis‐dihydrochrysanthemo‐δD‐lactone. The lactone ring is opened by alkali hydrolysis and a derivative of the (±)‐cis‐δD‐hydroxydihydrochrysanthemic acid is obtained. With methanolic sulphuric acid the lactone ring is opened to give mainly methyl (±)‐cis‐±‐methoxydihydrochrysanthemate together with methyl (±)‐cis‐chrysanthemate. In contrast, (±)‐trans‐chrysanthemic acid is converted under similar conditions into the crystalline (±)‐trans‐δ‐hydroxydihydrochrysanthemic acid. The mechanisms of these reactions and their bearing on the Seil and the Wilcoxon–Holaday methods of pyrethrum assay are discussed. On heating at 300° C. in sealed tubes (±)‐cis‐and (±)‐trans‐chrysanthemic acids and (±)‐cis‐dihydrochrysanthemo‐δ‐lactone are converted into an isomeric crystalline lactone, (±)‐pyrocin. This is shown by oxidative degradation not to be a dihydrochrysanthemo‐lactone but to be (±)‐β‐isobutenylisohexano‐σ‐lactone. These lactones are substantially non‐toxic to houseflies.

In boiling dilute sulphuric acid hydration of (±)‐trans‐chrysanthemic acid occurs to give the crystalline (±)‐trans‐δ‐hydroxydihydrochrysanthemic acid. In like manner (+)‐and (‐)‐trans‐chrysanthemic acids yield the crystalline (‐)‐ and (+)‐trans‐δ‐hydroxy‐dihydrochrysanthemic acids respectively. No racemization occurs as the recovered ( + )‐and ( ‐ )‐trans‐chrysanthemic acids have unchanged specific rotation. The lactonization of (±)‐cis‐chrysanthemic acid to (±)‐cis‐dihydrochrysanthemo‐δ‐lactone in boiling dilute sulphuric acid is shown to be reversible and to constitute an example of lacto‐enoic tautomerism. ( + )‐ and ( ‐ )‐cis‐Chrysanthemic acids are similarly lactonized to the crystalline (+)‐ and (‐)‐cis‐dihydrochrysanthemo‐δ‐lactones. The latter lactone is not identical with (‐)‐pyrocin, whose structure is confirmed as (‐)‐β‐isobutenyliso‐hexano‐γ‐lactone.

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