Stereoselective synthesis of radiolabelled avermectins

The natural products avemectin B _1a 1a and B _1b 1b were each site‐specifically ¹⁴C labelled at carbon 23 in a convergent synthesis for metabolism, residue, and environmental studies. The 12‐step radiosynthesis involved a stereoselective aldol condensation and later a coupling to a protected avermectin degradate 2 in a one‐pot Wittig condensation/spiroketalization to reconstitute the dioxaspirane configuration found in the natural products. Copyright © 2004 John Wiley & Sons, Ltd.