Neuartige Cycloadditionsreaktionen von 2‐ und 4‐Vinylpyridin mit N‐Alkyl‐maleinimiden

4-Vinylpyridine (1a) combines with 3 moles of dienophilic N-alkyl-maleinimides (2) in the presence of polymerization inhibitors. The first step of the reaction probably consists of 1:1-addition with participation of an aromatic double bond, comparable to the analogous behavior of styrene and its derivatives under similar conditions. The unstable intermediates 3, like other Schiff bases (imines), add 2 further moles of the N-alkyl-maleinimides forming the spiro compounds 4. These are split in an acidic medium into the N-alkyl-5,6,7,8-tetrahydroisoquinoline-7,8-dicarboximides (5), and N,N′-dialkyl-2-butene-1,2,3,4-tetracarboxylic 1,2,:3,4-diimides (6). LiAlH4 reduction of these two types of compounds leads to N-alkyl-1 H-(3,4-d)-pyrrolo-2,3,3a,4,5,9b-hexahydroisoquinolines (7) and to N,N′dialkyl-3,3′-bipyrrolidyls (8A) and their dehydro-products 8B, respectively. From the reaction of 2-vinylpyridine (1b) with N-alkyl-maleinimides (2) the 1:2-addition products 9 can be isolated in the presence of polymerization inhibitors, which are derivatives of N-alkyl-5,6,7,8-tetrahydroquinoline-5,6-dicarboximides (9). This again corresponds to the reaction type of cycloadditions with styrene. Furthermore 1:3 adducts are formed which according to 1H- and 13C-NMR.-data most likely have the structure 10, representing a new type of cycloaddition involving the pyridine nitrogen.