Spiromeroterpenoids from the Higher Plants.

Meroterpenoids are a distinctive class of natural products found in various organisms, including animals, plants, bacteria, algae, and particularly fungi. Among them, spiromeroterpenoids, which have a spiro-ring connecting a terpenoid and a non-terpenoid moiety, are markedly unique. Currently, only a limited number of plants from the families Myrtaceae, Hypericaceae, Annonaceae, Asteraceae, and Lauraceae are known to biosynthesize spiromeroterpenoids. The non-terpene moiety of plant-derived spiromeroterpenoids is generally a polyketide, mainly a functionalized phloroglucinol derivative such as syncarpic acid and tasmanone. However, a flavanone, as found in the syzygioblanes isolated from Syzygium oblanceolatum (Myrtaceae), is another rare non-terpene component. The terpene moieties are restricted to monoterpenes or sesquiterpenes. The spiro-ring is generally formed by [m + n] cyclization or, in some cases, by radical or ionic cyclization.