ENANTIOSELECTIVE SYNTHESIS OF 2-ALKYL-5-METHYLENE-1,3-DIOXOLAN-4-ONES AND EXO-SELECTIVE DIELS-ALDER REACTIONS WITH CYCLOPENTADIENE

Highly stereoselective syntheses of chiral dienophiles (R)-1 and (R)-2 are described. Diazotization of L-serine in the presence of HCI and then treatment of the resulting beta-hydroxy-alpha-chloropropionic acid (S)-7 with KOH provides potassium glycidate ((R)-8) in good yield and high enantiomeric purity. Treatment of (R)-8 with PhSH in MeOH then provides alpha-hydroxy acid (S)-10 that can be purified by recrystallization. Condensation of (S)-10 with either pivalaldehyde or cyclohexanecarboxaldehyde followed by oxidation to the sulfone and DBU-promoted elimination of benzenesulfinic acid then provides dienophiles (R)-1 and (R)-2, respectively. Highly exo-selective Diels-Alder reactions of (R)-1 and (R)-2 with cyclopentadiene are also described. The major cycloadduct (-)-15 (94% of total) from the Diels-Alder reaction of 1 was shown to have an enantiomeric purity of greater-than-or-equal-to 99% ee. This figure defines the lower limit of enantiomeric purity of (R)-1. The diastereofacial selectivity of the Diels-Alder reactions of 1 in the exo manifold (50:1) is greater than that of 2 (20:1), as would be expected on the basis of the different steric requirements of the tert-butyl and cyclohexyl substituents of the two reagents. Consequently, dienophile 1 is the preferred reagent for complex synthetic applications, either as a chiral ketene equivalent or in contexts in which the a-hydroxy acid functionality will be preserved in the ultimate synthetic target. Finally, the possible role of dipole effects on the exo selectivity of the Diels-Alder reactions of these and related dienophiles are briefly discussed.