The influence of steric hindrance caused by the dienophiles on the stereoselectivity of cycloadditions of 2H-pyran-2-ones with maleimides was investigated in this study. It was found that sufficiently bulky N-substituents on the maleimides (such as N-triphenylmethyl) can cause the cycloaddition to proceed differently than expected, thus yielding asymmetric exo,endo-bicyclo2.2.2octenes instead of the commonly obtained symmetric exo,exo products. Furthermore, the incorporation of an N-triphenylmethyl group, which induces highly diastereoselective formation of asymmetric exo,endo adducts and can later be easily removed under acidic conditions, can be described as an example of an efficient exo,endo-diastereoselective auxiliary.
Krivec et al. (Thu,) studied this question.