Regio- and stereoselectivities in Diels–Alder cyclodimerizations of orthoquinonoid cyclohexa-2,4-dienones

The [4+2] cyclodimerization of cyclohexa-2,4-dienone derivatives of the orthoquinone monoketal and orthoquinol types has been the topic of numerous investigations over the last 50 years in the aim of rationalizing the extraordinary level of regio-, site-, and stereoselectivities observed in these processes. In particular, the double diastereo-π-facial differentiation expressed in cyclodimerizations of chiral orthoquinols (i.e., 6-alkyl-6-hydroxycyclohexa-2,4-dienones) is an important aspect of these transformations, for they relate to the construction of several natural products. The experimental and theoretical results that are described in this article offer a comprehensive understanding of the factors controlling these site-specific regio- and diastereoselectivities. Our interpretation of these results relies on a combination of Woodward–Hoffmann and Salem–Houk secondary orbital interactions and Cieplak-type hyperconjugative effects in bispericyclic C2-symmetric transition states.