vs. CC chemoselectivity in favor of CS, while N-p-anisyl (11b) and N-benzyl (11c) derivatives display quite the reverse reactivity and selectivity. N-Phenylmonothiocitraconimide (11d) serves as a CS specific dienophile and reacts with dienes to furnish ortho,endo adducts 12 with excellent regio- and stereoselectivities (100% ortho,endo). Both hard (BF3 · OEt2, TiCl4) and soft (CuBr · SMe2) Lewis acids
The thiocarbonyl group of monothiomaleimide 1 serves as a more reactive dienophile than the electron-deficient CC double bond in the same molecule for the DielsâAlder reaction with dienes 2câg and provides ortho-endo products 3 exclusively or predominantly over the other possible adducts 4â10.