Enantioselective Intramolecular [2+2] Photocycloaddition Reactions of 4‐Substituted Coumarins Catalyzed by a Chiral Lewis Acid
作者:Richard Brimioulle、Hao Guo、Thorsten Bach
DOI:10.1002/chem.201104032
日期:2012.6.11
terminal alkene tethered by a CH2XCH2 group to their 4‐position (X=CH2, CMe2, O, S, NBoc, NZ, NTs, NBn), were synthesized in overall yields of 51–80 %. Starting materials for the syntheses were either commercially available 4‐hydroxycoumarin or 4‐formylcoumarin. The intramolecular [2+2] photocycloaddition of these coumarins gave diastereoselectively products with a tetracyclic 3,3a,4,4a‐tetrahydro‐1H‐cyclopenta[2
合成了八种香豆素,它们带有通过CH 2 XCH 2基团拴在其4位上的末端烯烃(X = CH 2,CMe 2,O,S,NBoc,NZ,NTs,NBn),总产率为51 –80%。合成的起始原料是市售的4-羟基香豆素或4-甲酰基香豆素。这些香豆素的分子内[2 + 2]光环加成产生具有四环3,3a,4,4a-四氢-1 H-环戊基[2,3]环丁[1,2-c] chromen-5(2 H)的非对映选择性产物)-一个骨架。在λ = 300 nm的二氯甲烷中直接照射(c = 10 m M)导致大多数底物以高收率形成产物,大概是通过单重激发态中间体。由于在λ > 350 nm处香豆素吸收率低,因此在λ = 366 nm处照射时光环加成反应很慢。添加基于手性恶唑硼烷的路易斯酸(50摩尔%)提高了在λ = 366 nm处的反应速率,并在[2 + 2]光环加成反应中诱导了显着的对映选择性。八种香豆素底物中的六种(X