Macrocycles by Ring-Closing Metathesis

Abstract

The ruthenium carbene complexes 1 and 2 (0.05-5 mol%) catalyse highly efficient macrocyclization reactions of 1,ω-dienes by ring-closing metathesis (RCM). Key parameters for successful RCM are (i) the presence of a functional group which serves as a relay entity that assembles the reacting sites, (ii) an appropriate distance between this polar group and the alkenes to be metathesized, and (iii) low steric congestion near the double bonds. Contrary to previous assumptions, however, the ring size formed and the conformational predisposition of the substrates for ring closure turned out to be of minor importance. These aspects are illustrated by some straightforward syntheses of macrocyclic lactones, lactams, ethers and ketones, including the musk odored perfume ingredients Exaltolide, Exaltone and Arova 16, of the macrolide recifeiolide (24), as well as of the alkaloids epilachnene (40) and its homologue 9-propyl-10-azacyclododecan-12-olide (39), which are active principles of the defense secretions of the pupae of the mexican beetle Epilachnar varivestis .