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.