Dual Behavior of Masked o-Benzoquinones in Intramolecular Diels-Alder Reactions.Expedient Synthesis of Highly Functionalized cis-Decalins from 2-Methoxyphenols

摘要

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 stereo-selective 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 cts-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.