Chirality and helical conformation

Absolute asymmetric synthesis was observed in the solid-state photoreaction of benzoylbenzamide 59 to phthalide 60 however, the reaction mechanism was completely different from that of thioester 57. [35] Recrystallization of these amides 59a-c from the chloroform-hexane solution afforded colorless prisms in all cases. X-ray crystallographic analysis revealed that all prochiral amides 59a-c adopted orthorhombic chiral space group P2 2 2 and were frozen in chiral and helical conformation in the crystal lattice. 

Fortunately, both 3a and 3b crystallized in a chiral space group, P2i2i2i, and the constituent molecules adopted a chiral and helical conformation in the crystal lattice. The rate of racemization of 3 after dissolving the chiral crystals in a solvent was measured based on the changes in the CD spectra The activation free energy (AG ) was calculated from the temperature dependence of the kinetic constant between 5 and 15°C as 21.2 0.2 kcal moH in THF. These facts indicate that the racemization of 3b is too fast to be resolved in the usual manner. The lifetime can be lengthened by lowering the temperature so that the racemization is sufficiently slow and the reaction can be used to accomplish asymmetric synthesis. 

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See also in sourсe #XX -- [ ]

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