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

A new heterocycle, furo4,3,2-de]1]benzopyran ( 2 ), was synthesized. A key step in the sequence was the allylic bromination of 3,4-dihydrofuro4,3,2-de]1]benzopyran ( 8 ) to give 3-bromo-3,4-dihydrofuro4,3,2-de]1]-benzopyran ( 10 ) using N-bromosuccinimide under irradiation and high dilution conditions. Bromide 10 was dealt with 1,8-diazabicyclo5.4.0]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.