Absolute asymmetric photocyclization of isopropylbenzophenone derivatives using a cocrystal approach involving single-crystal-to-single-crystal transformation.
Absolute asymmetric photocyclization of isopropylbenzophenone derivatives was achieved by means of a cocrystal approach. Three chiral salt crystals formed by carboxylic acid derivatives with achiral amines could be prepared by spontaneous crystallization. In the M-crystal of 4-(2,5-diisopropylbenzoyl)benzoic acid with 2,4-dichlorobenzylamine, a twofold helical arrangement occurs in a counterclockwise direction to generate the crystal chirality. Conversely, the clockwise helix exists alone in the P-crystal. Irradiation of the M-crystal at >290 nm caused highly enantioselective Norrish type II cyclization to give the (R,R)-cyclopentenol, (R)-cyclobutenol, and (R)-hydrol in a 6:3:1 molar ratio, resulting in successful absolute asymmetric synthesis, while irradiation at around 350 nm afforded the (R,R)-cyclopentenol as the sole product. The reaction proceeded via single-crystal-to-single-crystal transformation, and therefore the reaction path producing the (R,R)-cyclopentenol could be traced by X-ray crystallographic analysis before and after irradiation.
Koshima
Hideko
H
Department of Applied Chemistry, Faculty of Engineering, Ehime University, Matsuyama 790-8577, Japan. koshima@eng.ehime-u.ac.jp
Kawanishi
Hirohisa
H
Nagano
Masaki
M
Yu
Haitao
H
Shiro
Motoo
M
Hosoya
Takaaki
T
Uekusa
Hidehiro
H
Ohashi
Yuji
Y