| Abstract: | Stereoselective synthesis of 2-methylidene-3-[(Z)-(2-nitrophenylsulfenyl)methylidene]-7-oxabicyclo[2.2.1]-heptane ( 16 ), 1,4-epoxy-1,2,3,4-tetrahydro-5,8-dimethoxy-2-methylidene-3-[(Z)-(2-nitrophenylsulfenyl)methylidene]anthracene ( 18 ), and 1,4-epoxy-1,2,3,4-tetrahydro-5,8-dimethyoxy-2-methylidene-3-[(Z)-(phenylsulfenyl)-methylidene]anthracene ( 19 ) are presented. The Diels-Alder additions of these S-substituted dienes and those of 2,5-dimethylidene-3,6-bis{[(Z)-(2-nitrophenyl)sulfenyl]methylidene}-7-oxabicyclo[2.2.1]heptane ( 17 ) have been found to be face selective and ‘ortho’ regiospecific. The face selectivity depends on the nature of the dienophile. It is exo-face selective with bulky dienophiles such as ethylene-tetracarbonitrile (TCNE) and 2-nitro-1-butene and endo-face selective with methyl vinyl ketone, methyl acrylate, and 3-butyn-2-one. In the presence of a Lewis acid, the face selectivity of the Diels-Alder reaction can be reversed. The addition of the first equivalent of a dienophile to tetraene 17 is at least 100 times faster than the addition of the second equivalent of the same dienophile to the corresponding mono-adduct. The X-ray structure of the crystalline bis-adduct 43 , a 7-oxabicyclo[2.2.1]hepta-2,5-diene system annellated to two cyclohexene rings, resulting from the successive additions of methyl acrylate and methyl vinyl ketone to tetraene 17 is presented. Only one of the two endocyclic double bonds of the 7-oxabicyclo[2.2.1]hepta-2,5-diene deviates from planarity, the substituents bending towards the endo face by 5.7°. |
