Daphnane diterpenes from Diarthron vesiculosum: vesiculosin and isovesiculosin

An alcoholic extract of Diarthron vesiculosum seed has yielded two novel daphnane diterpene esters, vesiculosin and isovesiculosin. Compounds 1 and 2 undergo rapid interconversion to an equilibrium mixture containing approximately 80% 1 and 20% 2 in solution at elevated temperatures. At 25 degrees or below, 1 and 2 may be isolated and stored for extended periods with minimal interconversion. Excoecariatoxin and simplexin are the major daphnane orthoesters present in D. vesiculosum, and 1 and 2 apparently are biosynthetic precursors of 3. Isolation of these compounds was monitored by bioassay against PS leukemia (in vivo), and structures were determined by 1H nmr, 13C nmr, ms, and comparison with known compounds.