Synthesis and properties of furo[4,3,2‐ de ][1]benzopyran

A new heterocycle, furo[4,3,2‐ de ][1]benzopyran ( 2 ), was synthesized. A key step in the sequence was the allylic bromination of 3,4‐dihydrofuro[4,3,2‐ de ][1]benzopyran ( 8 ) to give 3‐bromo‐3,4‐dihydrofuro[4,3,2‐ de ][1]‐benzopyran ( 10 ) using N ‐bromosuccinimide under irradiation and high dilution conditions. Bromide 10 was dealt with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene to afford compound 2 . Several reactions of 2 were examined. Protonation of 2 in trifluoroacetic acid occurred at the 2‐position to form a pyrylium ion 12 . Catalytic hydrogenation of 2 with palladium on charcoal proceeded smoothly to give 8 . Reduction of 2 by sodium and ethanol afforded 3‐ethyl‐4‐hydroxybenzofuran ( 14 ). Electrophilic substitutions of 2 such as formylation, acetylation, and bromination, occurred easily at the 2‐position. The above results show that compound 2 has both properties of benzofuran and 4‐methylenepyran.