Abstract
Aiming to prepare optically active pyrrolidines (2) which are usable as chiral additives for asymmetric syntheses via enamines, and whose absolute configurations can correspond to that of D-proline (D-3c), an exploitation of the novel synthetic scheme for 2 from L-α-amino acids (L-3) via optically active γ-amino acids (4) was studied. Reaction of (S) (-)-ditosylate ((S) (-)-10a) easily derivable from L-phenylalanine (L-3a), with diethyl potassiomalonate (3.0 eq.) in tetrahydrofuran, could directly give a mixture of (R)-pyrrolidine-2-one ((R)-12a) and (R) (-)-malonate ((R) (-)-13a) in 70-80% yield. Some mechanistic studies revealed that the malonate reaction proceeded through the regiospecific ring opening of (S) (+)-aziridine ((S) (+)-11a). Acidic hydrolysis of a crude mixture of (R)-12a and (R) (-)-13a afforded (R) (-)-4a without racemization in 49% yield based on (S) (-)-10a. The same synthetic route was applicable to L-valine (L-3b), giving (R) (+)-4b. However, (S)-4c could be obtained from L-proline (L-3c), by the treatment of (S) (-)-iodide ((S) (-)-16c) prepared from (S) (-)-ditosylate ((S) (-)-10c), with malonate anion, followed by acidic hydrolysis. (R) (-)-4a and (R) (+)-4b thus obtained were readily converted into (R) (-)-2a and (R) (+)-2b by dehydration and reduction. When the asymmetric synthesis of 4-methyl-4-phenyl-2-cyclohexenone (18) was examined by using (R) (-)-2a and (R) (+)-2b, (S) (-)-18, being antipodal to that provided by using L-proline-derived pyrrolidines (1) as chiral additives, was successfully obtained.
