Reductive Coupling of Benzofuran-2,3-diones with Ketones and Aldehydes by Low-Valent Titanium

The reductive coupling of 4-unsubstituted benzofuran-2,3-diones with ketones by Zn-TiCl4 in THF at 0 °C gave 3-hydoxy-3-(1-hydroxyalkyl)benzofuran-2(3H)-ones. The same reaction at 30 °C produced 3-alkylidenebenzofuran-2(3H)-ones. On the contrary, the reductive coupling of 4-substituted benzofuran-2,3-diones with ketones by Zn-TiCl4 in THF at 0 °C yielded 2-hydoxy-2-(1-hydroxyalkyl)benzofuran-3(2H)-ones. On the other hand, the same reaction with aldehydes at 0 °C and at 30 °C resulted in 3-hydoxy-3-(1-hydroxyalkyl)- and 3-alkylidenebenzofuran-2(3H)-ones, respectively, irrespective to the substituent at the 4-position in benzofuran-2,3-diones. The reductive coupling of 4-unsubstituted benzofuran-2,3-diones with ketones by Zn-TiCl4 in THF at 0 °C gave 3-hydoxy-3-(1-hydroxyalkyl)benzofuran-2(3H)-ones. The same reaction at 30 °C produced 3-alkylidenebenzofuran-2(3H)-ones. On the other hand, the same reaction with aldehydes at 0 °C and at 30 °C resulted in 3-hydoxy-3-(1-hydroxyalkyl)- and 3-alkylidenebenzofuran-2(3H)-ones, respectively, irrespective to the substituent at the 4-position in benzofuran-2,3-diones.