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

摘要

The thermal addition of N-carbobenzyloxyisoindole (N-Z isoindole) 11a, generated by the reaction of 3,6-di(2-pyridyl)-s-tetrazine 9 with N-Z 7-azabenzonorbomadiene Sa, onto dimethyl tricyclo[4.2.1.0(2.5)]nona-3,7-diene-3,4-dicarboxylate 17 occurred site selectively at the cyclobutene pi -bond to form a stereoisomeric mixture of 1 : 1-adducts 18 and 19, in which the bent-frame isomer 19 was dominant (ratio 5: 1). In contrast, N-benzyl tetrafluoroisoindole 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 extended-frame isomer 18c was dominant and the accompanying bent-frame product 19c reverted to starting materials soon after isolation. These same stereoselectivities were used to prepare "windscreen wiper" compound 28c having two mobile N-benzyl substituents attached to a rigid scaffold by the reaction 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-(cyclobutene-1,2-diester) 6 reacted with N-benzyl tetrafluoroisoindole 11c twice over to produce cavity structure 36 with two O- and two N-benzyl bridges on the inner face, whereas the narrower cavity bis-alkene 32 stopped at the 1 : 1-addition stage. The dynamics of the Z-group in the dual adducts 26a-28a are discussed briefly and key adducts and cavity systems have been structurally evaluated by X-ray crystallography, VT NMR, and molecular modeling. [References: 37]