Secoheliosphanes A and B and Secoheliospholane A, Three Diterpenoids with Unusual <i>seco</i>-Jatrophane and <i>seco</i>-Jatropholane Skeletons from <i>Euphorbia helioscopia</i>

Secoheliosphanes A (<b>1</b>) and B (<b>2</b>) and secoheliospholane A (<b>3</b>), possessing an unusual 7,8-<i>seco</i>-jatrophane skeleton and an unprecedented 9,10-<i>seco</i>-7,10-epoxyjatropholane skeleton, respectively, were isolated from the whole plants of <i>Euphorbia helioscopia</i>, along with two biogenetically precursors, a new jatrophane diterpene, 2-<i>epi</i>-euphornin I (<b>4</b>) and a known jatrophane diterpene, euphoscopin A (<b>5</b>). Structures of <b>1</b>–<b>4</b> including absolute configurations were elucidated on the basis of spectroscopic data, X-ray crystallography, and chemical conversion. Compounds <b>1</b> and <b>2</b> were prepared from <b>4</b> and <b>5</b>, respectively, confirming their structural assignments. Notably, <b>1</b> and <b>2</b> presented the first examples of <i>seco</i>-jatrophane-type diterpenoids and <b>3</b> featured a novel 5/6/7/7-fused tetracyclic ring skeleton. Among them, compound <b>2</b> showed modest activity against HSV-1 with IC₅₀ value of 6.41 μM.