Koshima Hideko, Fukano Michitaro, Ojima Naoko, Johmoto Kohei, Uekusa Hidehiro, Shiro Motoo
Department of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University , Matsuyama, Ehime 790-8577, Japan.
J Org Chem. 2014 Apr 4;79(7):3088-93. doi: 10.1021/jo5000913. Epub 2014 Mar 24.
Although 4-(2,4,6-triisopropylbenzoyl)benzylbenzamide is an achiral molecule, chiral crystals can form through spontaneous crystallization in a methanol solution. In the M crystal, twofold helical hydrogen-bond chains form in a counterclockwise direction among the molecules along the a axis to generate crystal chirality. The solid-state circular dichroism spectra of the two enantiomorphous crystals as Nujol mulls show a good mirror-image relationship. UV irradiation of the M crystal at >290 nm caused highly enantioselective Norrish type II photocyclization to yield the (R)-cyclobutenol with 94% ee in 100% yield as the sole product, resulting in successful absolute asymmetric synthesis. In contrast, the (S)-cyclobutenol was obtained from the P crystal with 95% ee in 100% yield. The high enantiodifferentiation in the crystalline-state photocyclization is attributed to the shorter distance between the carbonyl oxygen atom and one of the methine γ-hydrogen atoms of the two o-isopropyl groups as well as the smooth transformation with minimum molecular motion because of the similar shapes of the reactant and product molecules. UV irradiation of the platelike crystals resulted in a crack in the direction perpendicular to the long axis (the a axis of the unit cell), likely because the hydrogen-bond chains were broken during the photocyclization.
尽管4-(2,4,6-三异丙基苯甲酰基)苄基苯甲酰胺是一种非手性分子，但在手性晶体可以通过在甲醇溶液中自发结晶形成。在M晶体中，沿着a轴的分子之间以逆时针方向形成双螺旋氢键链，从而产生晶体手性。两种对映异构晶体作为石蜡糊的固态圆二色光谱呈现出良好的镜像关系。在>290nm波长下对M晶体进行紫外线照射，会引发高度对映选择性的Norrish II型光环化反应，以100%的产率生成ee值为94%的(R)-环丁醇作为唯一产物，从而实现了成功的绝对不对称合成。相比之下，从P晶体中获得了ee值为95%、产率为100%的(S)-环丁醇。晶体状态光环化反应中的高对映体区分归因于羰基氧原子与两个邻异丙基的次甲基γ-氢原子之一之间较短的距离，以及由于反应物和产物分子形状相似，分子运动最小化的平滑转变。对板状晶体进行紫外线照射会导致在垂直于长轴（晶胞的a轴）的方向上出现裂纹，这可能是因为光环化过程中氢键链被破坏。
