[4+2]‐Cycloadditionen von 1,2‐Dicyanocyclobuten und seine thermische Ringöffnung zum 2,3‐Dicyanobutadien‐1,3

Facile synthetic routes to 1,2‐dicyanocyclobutene ( 3 ), cyclobutene‐1,2‐dicarboxylic acid ( 56 ) and derivatives thereof are presented, starting from 1,2‐dicyanocyclobutane ( 1 ), a commercially available acrylonitrile cyclodimer. The favored mode of [4+2]‐cycloadditions of 3 to cyclic dienes with sp 3 carbon atoms is the endo ‐addition (above 90% relative yields of adducts with endo ‐cyclobutane ring). Exo ‐cycloaddition, however, is preferred by dienes having no sp 3 ‐carbon atom ( e . g . furane). Cyclisation reactions involving cis ‐vicinal substituents in [4+ 2]‐cycloadducts afford ( m . n . 2)‐azapropellanes 18 , 74 and 77 . 1 H‐ and 13 C‐NMR. spectra of the stereoisomeric adducts are discussed in detail. The structures of the furane adducts 14 and 15 were determined by 1 H‐NMR. using the shift reagent Eu(dpm) 3 . Reactive butadienes 32 , 53 ‐ 55 are obtained in high yield and purity by gasphase thermolysis (380–420°) of the correspondingly substituted cyclobutenes. 2,3‐Dicyanobutadiene‐l, 3 ( 32 ) gives good yields of [4 + 2]‐cycloadducts with strained cycloolefines, moderate yields with vinylethers and non‐activated olefins, and no adducts at all with electrophilic dienophiles (8.g. maleic anhydride, fumaronitrile). Thus, reactions of 32 are typical Diels ‐ Alder reactions with ‘inverse electron demand’. Some of these primary [4+2]‐cycloadducts ( 38 , 39 and 45 ) were dehydrogenated to new aromatic ortho ‐ dinitriles 46 ‐ 48 .