The selectivities of typical transformations of the p-benzoquinone Diels-Alder adduct 2 and its dihydro derivative 3 are shown to be highly dependent on the mechanistic path followed. To avoid ambiguities and to make sure of clearly defined regioselectivity, the monoketal 13 was examined and Since Diels-Alder adduct 2 is formed smoothly and quantitatively from p-benzoquinone and the enantiopure cyclopentadiene 1 , we became interested in studying the regioselectivity of its transformations. Having observed a quite remarkable regioselectivity with anhydride and imide adducts, we expected 2 to show a similar behaviour and envisaged it as being an excellent source of enantiopure cyclohexenones (see below).
This was borne out by experiment. ο¬-Selectride reduction of the dihydro compound 3, as well as of the epoxide 4, proved to be completely regioselective with the carbonyl group distant from the phenyl substituent again being attacked preferentially (Scheme 1).
Subsequent pyrolysis furnished the enantiopure hydroxycyclohexenones 7 and 8 cleanly and with ee's ΟΎ 98%.
If the regioselectivities of oxidations were to parallel those of reductions, the enantiomer of 8 should be obtainable from diol 9, which can be obtained in a highly diastereoselective manner by DIBAH reduction of adduct 2. With this diol at hand, we were in a position to study the regioselectivities of the oxidation processes. The best results were obtained with the Bobbit reagent . As is clear from the formula 10, this space-demanding reagent indeed attacked at the same position as found previously in the ο¬-Selectride reduction (Scheme 2). On subsequent pyrolysis, the enantiomer of 8 was formed as predicted .
Compared to these two operations, only modest regioselectivity was observed in the enolate alkylation of the dihydro derivative 3 (Table ).