Influence of Lewis acids on the [4+2] cycloaddition of (2R,2′R)-N,N′-fumaroylbis[fenchane-8,2-sultam] to cyclopentadiene and cyclohexadiene

Compared to the analogous bornane-10,2-sultam derived dienophile (−)-1b, the reversed topology observed during the [4+2] cycloaddition of cyclopentadiene or cyclohexadiene to the (−)-1a–TiCl4 chelate can be rationalised on the basis of IR studies of their complexes with different Lewis acids. According to X-ray analyses, the origin of this differentiation resides in the loss of masked C2 symmetry, due to the pseudoequatorial ‘down’ orientation of the SO(1) bond in (−)-1a,c as compared to the pseudoequatorial ‘up’ direction adopted by the SO(2) bond in (−)-1b,d, associated with the steric influence of the apical Ti–Cl atoms. Dependent on the strength of the Lewis acid, the much higher constraint of the SO2/CO syn-s-cis conformer diminishes the chelating properties of this type of fenchane-8,2-sultam derived dienophiles (−)-1a and 1c.

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(−)-[(2R,3R)-Bicyclo[2.2.2]oct-5-ene-2,3-diyl]bis[[(3aS,6S,7aR)-1,4,5,6,7,7a-hexahydro-7,7-dimethyl-2,2-dioxido-3H-3a,6-methano-2,1-benzothiazol-1-yl]methanone]C30H42N2O6S2Ee = 100%[α]D20=-88.7 (c 0.5, CHCl3)Source of chirality: (−)-fenchone

(−)-[(2S,3S)-Bicyclo[2.2.2]oct-5-ene-2,3-diyl]bis[[(3aS,6S,7aR)-1,4,5,6,7,7a-hexahydro-7,7-dimethyl-2,2-dioxido-3H-3a,6-methano-2,1-benzothiazol-1-yl]methanone]C30H42N2O6S2Ee = 100%[α]D20=-4.5 (c 0.6, CHCl3)Source of chirality: (−)-fenchone