Synthesis of Methyl- and Nitro-Substituted Pentacyclo2,6.03,10.05,9>undecane-8,11-diones

Article abstract of DOI:10.1021/jo00178a019

Diels-Alder cycloaddition of an appropriately substituted cyclopentadiene to an appropriately substituted p-benzoquinone (1a-c) followed by photocyclization of the resulting endo cycloadduct 2a-d was employed to synthesize the following monomethylated pentacyclo<5.4.0.0^2,6.0^3,10.0^5,9>undecane-8,11-diones: 1-methyl (3a), 2-methyl (3b), and 3-methyl (3c).Single-crystal X-ray structural analysis was performed on 3c. 2-Nitrobenzoquinone, generated via silver(I) oxide promoted oxidation of 2-nitrohydroquinone, was trapped in situ by cyclopentadiene, affording four products: 4a-nitro-1,4,4a,8a-tetrahydro-endo-1,4-methanonaphthalene-5,8-dione (4, 40percent), 4a-nitro-1,4,4a,8a-tetrahydro-exo-1,4-methanonaphthalene-5,8-dione (5, 7percent), and two 2:1 diene:dienophile cycloadducts <6 (2percent, from further reaction of 4 with cyclopentadiene) and 7 (4percent, from further reaction of 5 with cyclopentadiene)>.The assignment of endo configuration for 4 was confirmed via its facile intramolecular photocyclisation to 9-nitropentacyclo<5.4.0.0^2,6.0^3,10.0^5,9>undecane-8,11-dione (8).Attempted column chromatographic purification of 4 on either alumina or silica gel resulted in the formation of 1,4-dihydro-1,4-methano-5,8-naphthoquinone (10) via elimination of nitrous acid from 4.Reduction of 4 with methanolic sodium borohydride in the presence of cerous chloride afforded 4a-nitro-1,4,4a,8a-tetrahydro-endo-1,4-methanonaphthalene-5,8-diol (9) in 75percent yield.