Generation and hetero-Diels-Alder reactions of an o-quinone methide under mild, anionic conditions: rapid synthesis of mono-benzannelated spiroketals.

Deprotonation of o-hydroxybenzyl acetate with (i)PrMgCl provides a method of generating an o-quinone methide under mild, anionic conditions, such that highly sensitive exo-enol ethers can be employed as 2pi partners in hetero-Diels-Alder reactions. This process results in mono-benzannelated spiroketals such as those found in the natural products berkelic acid, the chaetoquadrins or cephalostatin 6.