Assembly of tricyclic compounds that include the spiro[4.4]nonane subunit

Spiroannulation of ketals derived from unsaturated cyclohexenone derivatives by the Lewis acid-catalyzed reaction of 1,2-bis((trimethylsilyl)oxy)cyclobutene 5 led to spiro[4.5]decene-diones 9 and 30. These were transformed into spiro[4.4]nonene derivatives 25 and 35/36 via similar sequences involving ozonolysis and aldol ring reclosure. Reduction of an annular double bond allowed facile closure of a third ring, also by an aldol reaction. This work led to a single oxygenated angular triquinane 28 from 9, and to the tricyclo[6.2.1.01,5]undecane derivative 41 from 30. An oxatricyclo[5.3.1.01,5]undecane derivative 19 was also obtained from 9. Keywords: spiroannulation, geminal acylation, sesquiterpene synthesis.