Diels-Alder Reaction of Cyclopenta-1,3-diene and Anthracene to bis-Fumarates Derived from Menthol Analogues

Due to steric interactions of the overly bulky substituted 8‐position, the classic 8‐Ph‐menthol (‐)‐1 d is not the optimal chiral auxiliary for the asymmetric Diels‐Alder reaction of its bis fumarate (‐)‐2 d to dienes such as cyclopenta‐1,3‐diene 3 b, or anthracene 3 c. This prosthetic group (‐)‐1 d could efficiently be replaced by an intermediate smaller analog (‐)‐1 e, readily prepared in a single step by cyclopropanation of isopulegol (‐)‐1 b. This latter auxiliary, resulting from the industrial technology developed by Takasago for menthol (‐)‐1 a, does not necessitate the stereoselective ketone reduction and chromatographic separation of the usual Michael adducts, derived from (+)‐pulegone. This chiral auxiliary (‐)‐1 e, never used as an asymmetric Diels‐Alder promoter, is much more selective than menthol (‐)‐1 a itself, under uncatalyzed conditions, and is also more stable than the polymerizable isopulegyl derived dienophiles, under Lewis acid mediated conditions. This prosthetic group thus readily allows large scale operations in both accessible enantiomeric series.