SYNTHESIS APPLICATIONS OF CATIONIC AZA-COPE REARRANGEMENTS .28. ASYMMETRIC TOTAL SYNTHESES OF (-)-STRYCHNINE AND (+)-STRYCHNINE AND THE WIELAND-GUMLICH ALDEHYDE

The first asymmetric total syntheses of (-)-strychnine, ent-strychnine, and the Wieland-Gumlich aldehyde are described with full experimental details. The total synthesis of (-)-strychnine was realized in 24 steps and 3% overall yield from (1R,4S)-(+)-4-hydroxy-2-cyclopentenyl acetate (28). This synthesis fully controls the six stereogenic centers and forms the C(20) double bond of (-)-strychnine with high diastereoselection (>20:1). In the first stage of the synthesis, the (R)-cyclopentenylstannane 8 is prepared in nine steps and 30% overall yield (40% with one recycle of 38) as summarized in Scheme 4. Palladium-catalyzed carbonylative coupling of 8 with the 2-iodoaniline derivative 7 provides enone 6, which is converted to the 2-azabicyclo[3.2.1]octane 5 in seven additional steps. This latter sequence proceeds in 36% overall yield (Scheme 6). The central step of the total synthesis is aza-Cope-Mannich rearrangement of 5 which proceeds in 98% yield to form the pentacyclic intermediate 4 (Scheme 7). In five additional steps 4 is converted to the Wieland-Gumlich aldehyde 2, which is the ultimate precursor of (-)strychnine. A slight modification of this synthesis strategy allowed ent-strychnine to be prepared and provided the first samples of this unnatural enantiomer for pharmacological studies (Scheme 8). The efficiency and conciseness of this synthesis provide an important benchmark of the power of the aza-Cope rearrangement-Mannich reaction to solve formidable problems in alkaloid construction.