CAMPHOR-DERIVED, CHELATING AUXILIARIES FOR THE HIGHLY DIASTEREOSELECTIVE INTERMOLECULAR PAUSON-KHAND REACTION - EXPERIMENTAL AND COMPUTATIONAL STUDIES

A family of enantiomerically pure (2R)-10-(alkylthio)isoborneols [meth ylthio (1), neopentylthio (2), phenylthio (3)], specifically designed as chiral auxiliaries suitable for chirality transfer to cobalt in Pau son-Khand reactions, has been synthesized. The dicobalt hexacarbonyl c omplexes of the alkoxyacetylenes derived from these alcohols (10a-12a) can be converted to the rather stable, internally chelated, pentacarb onyl complexes 10b-12b by treatment with NMO. The intermolecular Pauso n-Khand reactions of 10b-12b with strained olefins take place with syn thetically useful rates at low temperatures (down to -20 degrees C), w ith high yields and diastereoselectivities: norbornene (77%; 92.8), no rbornadiene (82%; 96:4), bicyclo[3.2.0]hept-6-ene (91%; 93:7). The maj or diastereomer of the adduct of 10b with norbornadiene, 14, has been used as the starting point for a synthesis of (S)-(-)-4-alkyl-2-cyclop entenones through a sequence consisting of completely diastereoselecti ve conjugate addition, reductive cleavage with recovery (>95%) of the chiral auxiliary, and retro Diels-Alder reaction. The stereochemical c ourse of the reaction of 10b with norbornadiene has been analyzed and rationalized by theoretical means by using a combined semiempirical [P M3-(tm)]/density functional theory [VWN-Perdew-Wang 91] approach.