Photoinduced Molecular Transformations. Part 142. One‐step syntheses of 1<i>H</i>‐benz[<i>f</i>]indole‐4,9‐diones and 1<i>H</i>‐indole‐4,7‐diones by a new regioselective photoaddition of 2‐amino‐1,4‐naphthoquinones and 2‐amino‐1,4‐benzoquinones with alkenes

Abstract

Abstract The 2,3‐dihydro‐1 H ‐benz[ f ]indole‐4,9‐diones 3a–d , h were formed in a one‐step reaction in 13–82% yield by an unprecedented [3 + 2] regioselective photoaddition of 2‐amino‐1,4‐naphthoquinone ( 1 ) with various electronrich alkenes 2 ( Scheme 1, Table ). The [3 + 2] photoadducts derived from 1 with vinyl ethers and vinyl acetate gave 1 H ‐benz[ f ]indole‐4,9‐diones 4e , f , i , in 33–72% yield, by spontaneous loss of the corresponding alcohol or AcOH from the resulting adducts; 4i has a kinamycin skeleton. The [3 + 2] photoaddition also took place on irradiation of the differently substituted amino‐1,4‐benzoquinones 6 , 7 , and 12 and excess alkenes 2 in benzene, giving 1 H ‐indole‐4,7‐dione derivatives 13 and 14 ( Scheme 3 ), 15a and 16 ( Scheme 4 ), and 18 ( Scheme 4 ), respectively. The initial products in these photoadditions were proved to be hydroquinones, the air oxidation of which yielded the heterocyclic quinones; 2,3‐dihydro‐2‐methoxy‐2‐methyl‐5‐phenyl‐1 H ‐indole‐1,4,7‐triyl triacetate ( 19 ) was isolated after treatment of the crude photoaddition mixture obtained from 2‐amino‐5‐phenyl‐1,4‐benzoquinone ( 7 ) and 2‐methoxyprop‐1‐ene ( 2f ) with Ac 2 O and pyridine under N 2 . A pathway leading to the annelated hydroquinones involving ionic intermediates arising from an electron transfer in these photoadditions is proposed ( Scheme 5 ).