Ammonium spirocyclic ylide

The key step in the total synthesis of (—)-epilupinine 253 involved the ring expansion of a proline-derived spirocyclic ammonium ylide to give 252 through a [1,2] Stevens rearrangement, as shown in Scheme 51 <1997T16565>. 

Aminoester 121a and isomeric indolizine 121b (Scheme 31) are the products of the [1,2] and [2,3] rearrangement, respectively, of ene-endo-spirocyclic, didehydropiperidine-derived ammonium ylide 120 that was in situ prepared from the diazoester (050L2075). Acyclic ta-iodo-... 

Mader, M. M., Norrby, P.-O. Computational investigation of the role of fluoride in Tamao oxidations. Chem.- Eur. J. 2002, 8, 5043-5048. Vanecko, J. A., West, F. G. A Novel, Stereoselective Silyl-Directed Stevens [1,2]-Shift of Ammonium Ylides. Org. Lett. 2002,4, 2813-2816. Sun, P., Sun, 0., Weinreb, S. M. Stereoselective Total Syntheses of the Racemic Form and the Natural Enantiomer of the Marine Alkaloid Lepadiformine via a Novel W-Acyliminium lon/Allylsilane Spirocyclization Strategy. J. Org. Chem. 2002, 67, 4337-4345. 

Several years later, Liu and coworkers reported another synthesis of cephalotaxine 172 that relied on a distinct [2,3]-Stevens rearrangement fScheme 1 S.4QL ° Proline derivative 173 was transformed into ammonium ylide 174 in the presence of allyl bromide and K2CO3. This zwitterion rearranged to a-allyl aminoester 175. Hydration of the olefin and reduction of the ester furnished diol 176, which was converted to aminoketone 177 via oxidation and aldol condensation. The assembly of this spirocyclic intermediate represented a formal synthesis of cephalotaxine 172. ... 

Naidu BN, West FG. A short, enantioselective synthesis of (—)-epilupinine from proline via a spirocyclic ammonium ylide. Tetrahedron 1997 53(48) 16565-16574. 

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