BASE-CATALYZED SKELETAL ISOMERIZATION OF 6-ENDO-HYDROXY-9-THIABICYCLO[3.3.1]NONA-3,7-DIEN-2-ONE WITH THE EPIMERIZATION OF THE HYDROXY GROUP

Abstract

Studies on the base-catalyzed skeletal isomerization of (4) to (5) and (6) and the base-catalyzed epimerization of the hydroxy group in (5) and (6) are reported.The most plausible mechanism for the skeletal isomerization of (4) and the epimerization in (5) and (6) involves retro-aldol cleavages of (4), (5) and (6) to give an enolate (7) as the common intermediate.When 6-endo-acetoxy-9-thiabicyclo[3.3.1]nona-3,7-dien-2-one (1)1 was irradiated in methanol,2 formal 1,3-acyl3 and carbon shifts take place to give the isomeric compounds, 6-exo-acetoxy-2-thiabicyclo[3.3.1]nona-3,7-dien-9-one (2) (27%)5 and 9-endo-acetoxy-2-thiabicyclo[3.3.1]nona-3,7-dien-6-one (3) (9%)6 , respectively.In connection with the photochemical isomerization of (1) to (2), we found that a similar, but non-photochemical, skeletal isomerization takes place during hydrolysis of (1) by potassium carbonate in aqueous methanol solution. We wish to report here the remarkably smooth basecatalyzed isomerization of (1) with the epimerization of the hydroxy group to give a mixture of 6-exo-and 6-endo-hydroxy-2-thiabicyclo[3.3.1]nona-3,7-dien-9-ones (5 and 6).The keto acetate (1) was synthesized from 2,6-dihydroxy-9-thiabicyclo[3.3.1]nona-3,7-diene7 by acetylation followed by oxidation with active MnO2. The structural determination and the stereochemical assignment to (1) were derived unequivocally by the nmr spectrum.1