Model support studies toward the total synthesis of the stemona alkaloid stemocurtisine
Herein we report the results of a furan-based approach toward the synthesis of the Stemona alkaloid, stemocurtisine 1 via the linearly fused tricyclic intermediate 3, representing the key A,B,C-ring structural feature of the target molecule. A highly diastereoselective synthesis of compound 3 was achieved starting from 3-furfural 6, in a synthetic sequence that involved; (1) a rapid in situ conversion of an O-mesylate to the corresponding chloride with inversion of configuration from assistance of the neighbouring 3-furanyl group; (2) an intramolecular aza-Wittig reaction to prepare the azepine ring; and (3) a base promoted lactam ring forming step. While methods were established to oxidize the furan ring of 3 to the corresponding γ-hydroxy-α,β-unsaturated lactone we were unable to affect cyclization of the lactam ring hydroxyl group to the γ-position of the lactone to create the cyclic ether feature of the natural product. Model studies were also unsuccessful.