Diels–Alder reaction of unsymmetrical dienes with unsymmetrical p-benzoquinones

Abstract

A number of Diels-Alder reactions between 1 -phenyl-, 1 -p-methoxyphenyl-, and 1 -p-nitrophenyl-2.3-dimethylbutadiene and methoxycarbonyl-, 2.6-dimethyL.2.5-dimethyl-, 2-methoxy-6-methyl-, 5-methoxy-2-methoxycarbonyl-3-methyl-, and 2-cyano-3.5-dimethyl-p-benzoquinone are reported. The structures of the major products are established and their formation is interpreted on the basis of a transition state having some diradical character.

Long-Range Strategies in the LHASA Program: The Quinone Diels−Alder Transform

“…However, here are some more substituents to be examined than with the site selectivity assessment. Interestingly enough, the influence of substituents with respect to orientation selectivity cannot be directly related to their electron-withdrawing or -donating properties . The effect of substituents on the A ring is such that substituents on the diene (atoms 1, 2, 3, and 4) tend to end up “ortho” or “para” to a substituent on the fusion (atom 5 or 6). − A rough measure is used here to describe the directing effect of a group, e.g., hydrogen = 0, alkyl = 2, acetoxy = 3, sulfide = 4, and ester = 5.…”

“…Note that all numbers are positive, simply reflecting the fact that no group on either diene or dienophile is known to exhibit a marked “meta”-directing effect. The substituent effect is larger with the positions next to the broken bonds (atoms 1, 4, 5, and 6) . For the directing effect of the more remote positions (atoms 2, 3, 8, and 9) a weighting factor of 0.25 is used in the calculation.…”

“…The substituent effect is larger with the positions next to the broken bonds (atoms 1, 4, 5, and 6). 53 For the directing effect of the more remote positions (atoms 2, 3, 8, and 9) a weighting factor of 0.25 is used in the calculation. The orientation selectivity is expressed as (4…”