Two Stemona alkaloids from Stemona sessilifolia (Miq.) Miq.

Revised Structures of Dehydrostenines A and B: Total Syntheses of (±)‐Dehydrostenine A and Structure Assigned to Dehydrostenine B

“…In addition, we synthesized the structure originally assigned to dehydrostenine B (4). [6] Consistent with our findings and analysis of relevant and complementary synthetic studies by Booker-Milburn and coworkers, [9] we propose the revised structure of dehydrostenine B (3).…”

“…[9] As expected, the spectroscopic data for isolated dehydrostenine A is not consistent with equivalent data for structure 3 reported by Booker-Milburn and coworkers (see Supporting Information). [6,9] When these findings are taken together, it is evident that the correct structure of dehydrostenine A is molecule 5 (i. e., the C9epimer of the originally assigned structure (3). [6] Interestingly, through this process we also noted that the NMR spectroscopic data for aforementioned structure 3 reported by Booker-Milburn and coworkers conspicuously matched equivalent data reported for dehydrostenine B in the original isolation study (see Supporting Information).…”

“…[6,9] When these findings are taken together, it is evident that the correct structure of dehydrostenine A is molecule 5 (i. e., the C9epimer of the originally assigned structure (3). [6] Interestingly, through this process we also noted that the NMR spectroscopic data for aforementioned structure 3 reported by Booker-Milburn and coworkers conspicuously matched equivalent data reported for dehydrostenine B in the original isolation study (see Supporting Information). [6,9] This revealed that the structure originally assigned to dehydrostenine B (4) was also incorrect.…”

“…We were also interested in preparing the reported structure of dehydrostenine B (4), as this would allow us to corroborate the structural and stereochemical assignments for this natural product that were inferred from 1D and 2D NMR spectroscopic data. [6] We recently reported unified total syntheses of sessilifoliamides C (2), B, and D from a pyrrole substrate. [7] Because these bicyclic natural products feature structural similarities to dehydrostenine A, we anticipated that we could access structure 5 by germane chemistry.…”

“…Specifically, we proposed that pyrrole 7, which bears a pendant α,β-unsaturated lactone, could be accessed from a common (early-stage) pyrrole intermediate from which we prepared the sessilifoliamides (Figure 2). [7] Furthermore, we posited that lactone 7 could be treated with a Brønsted acid to construct the tetracyclic dehydrostenine framework (6), via an intramolecular conjugate addition. [8] This would represent an original general approach to accessing the tetracyclic core of stenine-type alkaloids.…”