Abstract:

Our group has previously demonstrated that osmium tetroxide and TMEDA generate stable crystalline adducts with alkenes that facilitate X-ray analyses and structure assignments. Alternatively, osmate esters can be prepared from diols, potassium osmate, and TMEDA·2TsOH in a non-oxidative condensation reaction. The first half of the talk will describe how this new approach provides a convenient route to form stable, crystalline osmate (VI) esters for X-ray analysis. Because it is redox neutral, it works with a variety of diol substrates, including 1,3-diols, that cannot be prepared from alkenes. a-Hydroxy acids also form stable osmate esters in reasonable yield and readily crystallize. An alternative ligand screen was performed to assess improved crystallinity from substituted TMEDA analogues. The enhanced crystallinity of osmate esters and the incorporation of a heavy atom make reliable determination of structure and absolute configuration routine.

Populusene A, a cembranoid-type diterpene was isolated from the tree resin of Populus euphratica by Yun-Yun Liu et al. Populusene A contains a structurally uncommon anti-Bredt bridgehead alkene in a bicyclo[8.4.1]pentadecane core. NMR methods assigned the structure and relative configurations, and its absolute configuration was proposed based on ECD calculations. The natural product displays sub-micromolar activity against the expression of pro-inflammatory proteins and transcription factors responsible for the regulation of inflammatory cytokines. The second half of the talk will cover our synthetic strategies towards populusene A.