dominant role in the defensive chemistry of dytiscid beetles, and of the large water bug, Abedus herberti. This bug discharges a mixture of pregnanes in which desoxycorticosterone is the main component (12). A family of much more highly functionalized steroids, the lucibufagins, serves to render some species of firefly (lampyrid beetles) unpalatable to predatory spiders and birds (13-15). These cardiotonic steroids are closely related to the bufadienolides, whose only known occurrence among animals is in the poison glands of certain toads (16). The discovery
that the lucibufagins also show antiviral properties (17) has prompted us to seek a technique for joining a preformed a-pyrone nucleus to a steroidal framework, since up to now there has been no general, convenient synthetic route to these steroidal pyrones. Our search has recently met with success (18), using the Pd0-promoted coupling of 5-trimethylstannyl-2-pyrone with an enol triflate (Eq. 1).
This direct synthesis of steroidal pyrones should make a variety of structures related to the lucibufagins (as well as to the toad-derived bufadienolides) readily available for biological investigation for the first time. How the insects themselves manage to obtain their defensive pregnanes and steroidal pyrones remains a mystery, since insects are generally considered to lack the enzymatic machinery essential for steroid biosynthesis (19). In fact, we do not yet know whether these insect defensive steroids are produced de novo or whether they are derived