21 Asymmetric Synthesis Involving Dynamic Enantioselective …
447
by a reversible photochemical reaction as a new asymmetric reaction system. It is
an absolute asymmetric synthesis of the PP-type by photochemical reaction and
photochemical racemization.
When prochiral ethyl 6-bromochromone-2-carboxylate 33 was used as a starting
material, the photodimer 34 was obtained as a conglomerate of the C2 crystal space
group. A reversible reaction process by light irradiation using the same wavelength
of light was also observed (Fig. 21.12) [63]. When a 0.01 M acetonitrile solution
of 33 was irradiated at 20 °C while gradually evaporating the solvent, crystalline
optically active dimer 34 was obtained; however, the enantiomeric purity was only
29% ee. It was possible to increase the ee to 50% by adding a small amount of seed
crystals during crystallization.
Next, the acetonitrile solution was irradiated at low temperature to promote crystallization without evaporation of solvent. When a 0.01 M acetonitrile solution of 33
was irradiated at −40 °C, dimer 34 was obtained with 38% ee without adding seed
crystals. The enantiomeric purity could be increased up to 80% ee by adding a small
amount of seed crystals.
We also examined the photoreaction using sunlight, because both monomer 33
and dimer 34 showed absorption in the long wavelength region over 400 nm. When a
Fig. 21.12 Asymmetric synthesis by reversible photodimerization of chromone carboxylates
followed by enantioselective crystallization
447
by a reversible photochemical reaction as a new asymmetric reaction system. It is
an absolute asymmetric synthesis of the PP-type by photochemical reaction and
photochemical racemization.
When prochiral ethyl 6-bromochromone-2-carboxylate 33 was used as a starting
material, the photodimer 34 was obtained as a conglomerate of the C2 crystal space
group. A reversible reaction process by light irradiation using the same wavelength
of light was also observed (Fig. 21.12) [63]. When a 0.01 M acetonitrile solution
of 33 was irradiated at 20 °C while gradually evaporating the solvent, crystalline
optically active dimer 34 was obtained; however, the enantiomeric purity was only
29% ee. It was possible to increase the ee to 50% by adding a small amount of seed
crystals during crystallization.
Next, the acetonitrile solution was irradiated at low temperature to promote crystallization without evaporation of solvent. When a 0.01 M acetonitrile solution of 33
was irradiated at −40 °C, dimer 34 was obtained with 38% ee without adding seed
crystals. The enantiomeric purity could be increased up to 80% ee by adding a small
amount of seed crystals.
We also examined the photoreaction using sunlight, because both monomer 33
and dimer 34 showed absorption in the long wavelength region over 400 nm. When a
Fig. 21.12 Asymmetric synthesis by reversible photodimerization of chromone carboxylates
followed by enantioselective crystallization