1-MAGNESIOTETRAHYDROISOQUINOLYLOXAZOLINES AS CHIRAL NUCLEOPHILES IN STEREOSELECTIVE ADDITIONS TO ALDEHYDES - AUXILIARY OPTIMIZATION, ASYMMETRIC-SYNTHESIS OF (-CORLUMINE, (+)-BICUCULLINE, (+)-EGENINE, AND (+)-CORYTENSINE, AND PRELIMINARY C-13 NMR-STUDIES OF 1-LITHIOTETRAHYDROISOQUINOLYLOXAZOLINES AND 1-MAGNESIOTETRAHYDROISOQUINOLYLOXAZOLINES())

Transmetalation of 1-lithiotetrahydroisoquinolyloxazolines with magnes ium halides affords Grignard reagents that add to aldehydes with up to 80% selectivity for one of the four possible diastereomeric products. An oxazoline chiral auxiliary derived from camphor provides an optima l blend of diastereoselectivity and isomer separability. Synthetic app lications of the optimal auxiliary, patterned after a literature appro ach in the racemic series, comprise an improved (formal) synthesis of bicuculline, egenine, and corytensine, as well as an efficient synthes is of corlumine. Preliminary NMR studies show that both 1-lithio- and 1-magnesiotetrahydroisoquinolyloxazolines are dynamic mixtures in THF solution at low temperatures. The barrier to pyramidal inversion of th e secondary Grignard reagent is in the 9.8-10.1 kcal/mol range, while an upper limit of about 8.2 kcal/mol can be assigned to the barrier to the organolithium inversion.