Total synthesis of cyathin A(3) and cyathin B(2).

A stereoselective synthesis of cyathin A(3) and cyathin B(2) has been achieved by a Prins-type reaction of a cycloalkenyl cyclopropanol. Particularly noteworthy is the use of a spirocyclobutanone moiety as a convenient scaffold for an efficient ring-closing metathesis to stereoselectively construct a suitably functionalized seven-membered ring (see scheme).