Cardenolides in <i>Asclepias syriaca</i> Seeds: Exploring the Legacy of Tadeus Reichstein

The common milkweed <i>Asclepias syriaca</i> is widespread in North America and produces cardenolide toxins that deter herbivores by targeting the transmembrane enzyme Na⁺/K⁺-ATPase. In 1979, Nobel Laureate Tadeus Reichstein elucidated the structure of novel cardenolides isolated from <i>A. syriaca</i> roots and proposed structures for several other cardenolides that could not be confirmed. In this study, we investigate the cardenolide composition of <i>A. syriaca</i> seeds, focusing on their abundance and <i>in vitro</i> inhibitory potency on the sensitive porcine Na⁺/K⁺-ATPase and that of the highly resistant large milkweed bug, <i>Oncopeltus fasciatus</i>. We identify five previously unreported cardenolides (<b>1</b>-<b>5</b>), three of which are predominantly found in seeds, in addition to the known syrioside (<b>6</b>), aspecioside (<b>7</b>), and the 2-thiazoline ring-containing cardenolide labriformin (<b>8</b>). Glucopyranosyl-allomethylosyl-12-deoxy aspecioside (<b>5</b>) is distinguished by lack of oxidation at C-12, and compounds <b>2</b>, <b>3</b>, <b>6</b>, and <b>8</b> contain a rare 1,4-dioxane motif. Inhibitory efficacy of the isolated cardenolides for sensitive and resistant enzymes appears to be correlated. Finally, we confirmed the structure of compound <b>2</b>, originally proposed by Tadeus Reichstein, and are pleased to share his original 1979 handwritten manuscript.