Synthesis of 6,9-Dipropyl-1,4-Dioxaspiro[4.5] Deca-6,8-Diene-2,10-Dione and its Diels-Alder Reactions with Substituted Acetylenes

The title compound, a masked 3,6-di-n-propyl-o-benzoquinone, was synthesized from 3,6-di-n-propylcatechol in 82% yield. Its Diels-Alder reactions with methyl propiolate, phenylacetylene, 1-octyne, dimethyl acetylenedicarboxylate, diphenylacetylene and 3-hexyne were studied. The yields of the adducts were excellent except for the last two cases in which the unimolecular decomposition of the title compound to generate 3,6-di-n-propylcatechol methylene ether predominates. The regiochemistry of the adducts derived from monosubstituted acetylenes were determined by the correlation of ¹³C chemical shifts of the adducts and the corresponding bicyclo[2.2.2]octa-5,7-diene-2,3-diones obtained from the hydrolysis of the spirolactone ring of the Diels-Alder adducts. Photolysis of these α-diketones gave the corresponding aromatic compounds in high yields. These synthetic sequences provide an effective entry to bicyclo[2.2.2]octa-5,7-diene-2,3-diones and polysubstituted benzene derivatives.