3-Oxooxa- and 3-oxoazacycloalk-4-enes were obtained with good yield from 1-(omega-alkenyloxy)- and 1-(omega-alkenylamino)-but-3-en-2-ones by using a ring-closing metathesis. This methodology has been used to synthesize an inhibitor of cathepsin K. (C) 2007 Elsevier Ltd. All rights reserved.