Total Syntheses, Absolute Configuration, and Cytotoxicity Evaluation of Ugonins L, S, U, and Z from the Rhizomes of <i>Helminthostachys zeylanica</i>

This study describes the first and efficient syntheses of naturally occurring ugonins L (<b>1a</b>), S (<b>2a</b>, <b>2b</b>), U (<b>3</b>), and Z (<b>4</b>). Naturally occurring ugonin S has two stereoisomers. On the basis of their NMR and specific rotation data, the absolute configuration of <i>trans</i>-ugonin L (<b>1a</b>) was determined as 10<i>R</i>,11<i>R</i>, while the absolute configurations of <i>trans</i>-ugonin S (<b>2a</b>) and <i>cis</i>-ugonin S (<b>2b</b>) were determined to be 10<i>R</i>,11<i>R</i> and 10<i>R</i>,11<i>S</i>, respectively. The naturally occurring <i>cis</i>-ugonin U (<b>3</b>) presented the 10<i>S</i>,11<i>R</i> configuration. The naturally occurring <i>cis</i>-ugonin Z (<b>4</b>) was suggested to have a 10<i>S</i>,11<i>R</i> configuration. Four isomers of compound <b>2</b> and two isomers of compound <b>3</b> showed moderate cytotoxic activities against the CEM and H460 human cancer cell lines.