Isoindole cycloadditions. Part III: The synthesis of "Windscreen wiper" and other N-bridged cavity systems

The thermal addition of N-carbobenzyloxyisoindole (N-Z isoindole) 11a, gene rated by the reaction of 3,6-di(2-pyridyl)-s-tetrazine 9 with N-Z 7-azabenz onorbomadiene Sa, onto dimethyl tricyclo[4.2.1.0(2.5)]nona-3,7-diene-3,4-di carboxylate 17 occurred site selectively at the cyclobutene pi -bond to for m a stereoisomeric mixture of 1 : 1-adducts 18 and 19, in which the bent-fr ame isomer 19 was dominant (ratio 5: 1). In contrast, N-benzyl tetrafluoroi soindole 11c reacted with 17 only under high-pressure conditions (14 kbar, RT, 4 days) to afford I : I-adducts at the cyclobutene site, in which the e xtended-frame isomer 18c was dominant and the accompanying bent-frame produ ct 19c reverted to starting materials soon after isolation. These same ster eoselectivities were used to prepare "windscreen wiper" compound 28c having two mobile N-benzyl substituents attached to a rigid scaffold by the react ion of N-benzyl tetrafluoroisoindole 11c with tetramethyl tetracyclo[4.4.1. 0.(2,5).0(7.10)]undeca-3,8-diene-3,4,7,8-tetracarboxylate 23. Cavity bis-(c yclobutene-1,2-diester) 6 reacted with N-benzyl tetrafluoroisoindole 11c tw ice over to produce cavity structure 36 with two O- and two N-benzyl bridge s on the inner face, whereas the narrower cavity bis-alkene 32 stopped at t he 1 : 1-addition stage. The dynamics of the Z-group in the dual adducts 26 a-28a are discussed briefly and key adducts and cavity systems have been st ructurally evaluated by X-ray crystallography, VT NMR, and molecular modeli ng.