Preparation of 4- and 10-fluorobenzo(j)fluoranthene via cyclodehydration of acetals and cyclopropanecarboxaldehydes

Two procedures have been developed for the synthesis of 4-fluorobenzo(j)fluoranthene (4-fluoroBjF) and 10-fluoroBjF. Reaction of 9-fluoro-11H-benzo(a)fluoren-11-one (5) with the Grignard reagent prepared from 2-(2-bromoethyl)-1,3-dioxane provided hydroxyl acetal 6 in quantitative yield. Cyclodehydration with polyphosphoric acid (PPA) at 110C gave 4-fluoroBjF in 35% yield. This represents an improvement over previous methods for preparing BjF derivatives substituted in the B ring. The preparation of 10-fluoroBjF represents a new synthetic for preparing BjF derivatives substituted in the B ring. The preparation of 10-fluoroBjF represents a new synthetic entry into the BjF ring system. 1-(4-Fluorophenyl)acenaphtylene (8) was treated with ethyl diazoacetate in the presence of copper bronze to give a mixture of anti- and syn-cyclopropanecarboxylates 9 and 10 in the ratio of 2:1. Reaction with iodotrimethylsilane gave the ring-opened ester attached at the 2-position of the substituted acenaphthylene. Reduction to the aldehyde followed by cyclodehydration with PPA at 100C gave 10-fluoroBjF acenaphthylene. Reduction to the aldehyde followed by cyclodehydration with PPA at 100C gave 10-fluoroBjF in 58% yield. Alternatively, the cyclopropyl esters could be reduced directly to the aldehydes, which underwent efficient ring opening and cyclodehydration in PPA at 100C to 10-fluoroBjF in 53-57% yield.