Dual Behavior of masked o-benzoquinones in intramolecular Diels-Alder reactions.: Expedient synthesis of highly functionalized cis-decalins from 2-methoxyphenols

The potential dual behavior as dienes and dienophiles of the diene moieties of masked o-benzoquinones (MOBS) 10a-e-12a-e, generated upon oxidation of 2-methoxyphenols 1-3 with BTIB in the presence of appropriate dienols, in their intramolecular Diels-Alder (IMDA) reactions has been examined. The IMDA reactions of MOBS 10a-d, 11a,b,d, and 12a,b,d resulted in highly functionalized oxatricyclic compounds 18a-d, 19a,b,d, and 20a,b,d, respectively, with concomitant formation of cis-decalin derivatives 21a-d, 22a,b,d, and 23a,b, d in a highly regio- and stereoselective manner. However, the MOBS 10e-12e provided exclusively oxatricyclic compounds 18e-20e. The formation of cis-decalins in these IMDA reactions illustrates the dienophilic character of MOBS, in addition to their behavior as dienes. The ratio of the two cycloadducts obtained in each reaction as a result of the dual character of MOBS depends on the nature and/or position of the substituents on both the cyclohexadienone moiety and the added 2,4-dienol. The majority of the cycloadducts resulted from the diene property of MOBS in intramolecular Diels-Alder reactions smoothly underwent Cope rearrangement to furnish cis-decalins as sole products in excellent to quantitative yields that provides a short and efficient entry to polyfunctionized cis-decalins from 2-methoxyphenols. Furthermore, the variation of dienophilic and diene characters of MOBS in the IMDA reactions with the electron-donating or electron-withdrawing substituent of both cyclohexadienone moiety and the added conjugated acyclic diene or 2,4-dienol has been studied in detail.