Highly enantioselective intra- and intermolecular [2+2] photocycloaddition reactions of 2-quinolones mediated by a chiral lactam host: Host-guest interactions, product configuration, and the origin of the stereoselectivity in solution

The [2 + 2] photocycloaddition of 4-alkoxy-2-quinolones was conducted in the presence of the chiral lactams 5 or ent-5. At -60 degreesC in toluene as the solvent the intramolecular reaction of quinolones 6 and 8 as well as the intermolecular photocycloaddition of various alkenes 13 to quinolone 12 proceeded with excellent enantioselectivity (81-98% ee) and in high yields (61-89%). Styrene (13d) reacted sluggishly in the intermolecular reaction (29% yield, 83% ee). The absolute configuration of the intramolecular photocycloaddition products 7 and 9 was elucidated by single-crystal X-ray crystallography of the corresponding diastereomeric N-menthyloxycarbonyl derivatives. The relative configuration of the intermolecular photocycloaddition products 14 and 15 was assigned on the basis of NOESY experiments and on crystallographic evidence. The differentiation of the enantiotopic faces in the prochiral quinolones 6, 8, and 12 can be explained by assuming a coordination of these substrates to the lactams 5 or ent-5 via two hydrogen bonds. Upon binding to 5 the si-face is shielded by the bulky tetrahydronaphthalene backbone, and the re-face is exposed to an intra- or intermolecular attack. On the basis of the association constant (K-a) for the coordination of quinolone to host 5 an interpretation of the observed enantiomeric excess has been put forward. The parent quinolone 17 was employed as substrate for microcalorimetric and NMR titration experiments. From the data obtained for K-a and DeltaH(a) the expected enantiomeric excess was calculated for two given temperatures (-15 and -60 degreesC). The calculated values fit the observed data within reasonable limits and prove that two-point hydrogen bonding can be sufficient to achieve a preparatively useful face differentiation in solution phase photochemistry.