SYNTHESES OF THEASPIRONE AND VITISPIRANE VIA PALLADIUM(II)-CATALYZED OXASPIROCYCLIZATION

Total syntheses of theaspirone (A and B) and vitispirane (A and B) are described. The key step in the syntheses is the palladium(II)-catalyz ed intramolecular oxaspirocyclization of diene alcohol 4 to either vit ispirane or the allylic alcohol 9. The outcome of the oxaspirocyclizat ion is very much dependent on the solvent employed. In water-acetic ac id (4:1) a 1:1 mixture of the diastereomeric alcohols 9A and 9B was ex clusively formed. In water with 8 equiv of a strong non-nucleophilic a cid, vitispiranes A and B (1:1) were obtained. An alternative procedur e to obtain vitispirane with the use of LiCl and K2CO3 is described. I n the latter reaction vitispirane B is formed preferentially. This res ult is explained by an equilibrium between the two possible pi-allyl c omplexes 5A and 5B, the kinetically favored 5B being transformed into vitispirane 3B before isomerization to 5A occurs.