Thermal rearrangements of 2-allenyl- and 2-alkynyl-3-(2-ethenylphenyl)-4,4-dimethoxycyclobutenones were studied. At ambient temperature, the allenyl compounds undergo an electrocyclic cascade to give bicyclo[4.2.0]octadienyl-fused cyclobutenones. These unusual tetracyclic cyclobutenones were shown to be viable synthetic precursors to benzo[a]anthracene-7,12-diones, compounds representing the framework of the angucycline group of naturally occurring antibiotics. In contrast, the 2-alkynylcyclobutenones are stable at ambient temperature but undergo a facile rearrangement at 110 degrees C (toluene) to give the previously unknown naphthalene derivatives, 1,2-dihydro-2,2-dimethoxy-1-(3-alkenylidene)naphtho[2,1-b]furans.
A useful synthetic route to benzophenanthridines and annulated derivatives is reported. These
arise from the thermolysis (refluxing chlorobenzene) of squaric acid-derived 4-(3-N-methyl-N-arylpropynyl)-cyclobutenones via a mechanism which involves an electrocyclic ring opening of the
cyclobutenone to the corresponding enynylketenes. Subsequent ring closure to a diradical
intermediate followed by radical arylation gives the benzophenanthridines.
A reliable and scalable purification procedure for the easy removal of triphenylphosphine oxide by complexation with CaBr 2 in ethereal solvents or toluene was developed. The resulting insoluble precipitated complex was easily eliminated from crude reaction mixtures in high yields by filtration without the need of purification by column chromatography. The compatibility of this procedure with different organic solvents, salts, filtration aids, packing materials, and several molecules containing most common organic functional groups was examined to explore the scope and limitations of this method. Additionally, precipitation of TPPO by complexation with other halide salts was also explored.
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