Highly Rigid Labdane-Type Diterpenoids from a Chinese Liverwort and Light-Driven Structure Diversification

Abstract

Two unprecedented labdane-type diterpenoids haplomintrins A (<b>1</b>) and B (<b>2</b>) with six rings system were isolated from a Chinese liverwort <i>Haplomitrium mnioides</i>. Light-driven reaction of homologous haplomitrenonolides C (<b>6</b>), A (<b>4</b>), and D (<b>3</b>) afforded haplomintrins A–C (<b>1</b>, <b>2</b>, and <b>7</b>), respectively, while <b>4</b> was converted to more complex congeners haplomintrins D–G (<b>8</b>–<b>11</b>) through intramolecular cyclization. Formation of <b>1</b> and <b>2</b> from compounds <b>6</b> and <b>4</b>, respectively, helps us to postulate that a photochemical reaction is involved in the biosynthetic pathway. These structure features can be used as molecular markers of <i>H. mnioides</i>, and their allelopathic effects are also preliminarily tested