Knorr- Rabe partial reduction of pyrroles: Application to the synthesis of indolizidine alkaloids

• Brendon S. Gourlay,
• John H. Ryan and
• Jason A. Smith

Beilstein J. Org. Chem. 2008, 4, No. 3, doi:10.1186/1860-5397-4-3

• reaction that was reported by Knorr and Rabe [5] in 1901 and has only been reported a handful of times since [6][7][8][9]. The method employs powdered zinc in an acid media to give 3-pyrrolines, presumably by protonation of the pyrrole to give an iminium ion which is then reduced. It has been shown that

• of the carbonyl group to give a conjugated iminium ion 10 (Scheme 4). This species would undergo a two-electron reduction process, with associated protonation to give the α-hydroxy pyrrole 11. Acid-promoted dehydration of 11 would afford a second iminium ion 12 which could undergo further reduction

• and protonation to give pyrrole 13. The pyrrole could then be protonated to give a third iminium ion 14 and reduction would then give rise to the product 9. Our reaction conditions are much harsher than those previously reported, and yet we do not see pyrrolidine products and this suggests that an

The enantiospecific synthesis of (+)-monomorine I using a 5-endo- trig cyclisation strategy

• Malcolm B. Berry,
• Donald Craig,
• Philip S. Jones and
• Gareth J. Rowlands

Beilstein J. Org. Chem. 2007, 3, No. 39, doi:10.1186/1860-5397-3-39

• findings described above dictated that an alternative cyclisation strategy be investigated. It was anticipated that intramolecular reductive amination of a pendant methyl ketone would furnish the correct diastereoisomer, because the hydride source would be expected to approach the iminium ion from the less

A convenient allylsilane- N-acyliminium route toward indolizidine and quinolizidine alkaloids

• Roland Remuson

Beilstein J. Org. Chem. 2007, 3, No. 32, doi:10.1186/1860-5397-3-32

• of the side chain and reduction of the iminium ion intermediate 13 to give the indolizidine 167B 1. [20] The synthesis of (±)-indolizidine 167B has been achieved in five steps in 18% overall yield from pyrrolidin-2-one. I 2. 3,5-Disubstituted indolizidines Most of the indolizidine alkaloids are

• previously described procedure. [30][31] Next, lactam 14 was protected (n-BuLi, benzyl chloroformate) then converted to ethoxycarbamate 15, isolated as a mixture of two diastereomers according to Hiemstra's procedure. [32][33] Condensation of allylsilanes 16 onto iminium ion A generated in situ by treatment

Methods for the synthesis of polyhydroxylated piperidines by diastereoselective dihydroxylation: Exploitation in the two-directional synthesis of aza-C-linked disaccharide derivatives

• Andrew Kennedy,
• Adam Nelson and
• Alexis Perry

Beilstein J. Org. Chem. 2005, 1, No. 2, doi:10.1186/1860-5397-1-2

• ). The N,O acetal was substituted,[40] both by reduction (→ 13) and by allylation (→ 14); the allylation reaction was highly diastereoselective (>95:<5 syn:anti), presumably as a result of a strong stereoelectronic preference for pseudo-axial attack on the intermediate iminium ion (with a pseudoaxial[40