Mitomycins syntheses: a recent update

• Jean-Christophe Andrez

Beilstein J. Org. Chem. 2009, 5, No. 33, doi:10.3762/bjoc.5.33

• assisting deprotonation at C9a through a seven-membered ring and thereby accelerating the formation of the desired iminium ion. The authors found that changing the carbobenzyloxy protecting group to less basic groups such as a silyloxy ether or a methoxymethyl ether gave virtually no selectivity between C9a

• desired isomer. Presumably, allylstannane 185 approached the iminium ion by a synclinal transition state 192 as shown in Scheme 52. The pro-S face of the iminium being less hindered and possessing a lower LUMO energy than the pro-R face, the approach occurred accordingly from the face opposite the alkoxy

• substituent. When the allylation reaction was done with the iminium derived from pyrrolidine 186, no diastereoselectivity for the formation of 188 was observed. The bulkiness of the benzyl carbamate now proximal to the iminium ion was responsible for this poor result. The pyrrolidine 187 was synthesized from

The role of an aromatic group in remote chiral induction during conjugate addition of α-sulfonylallylic carbanions to ethyl crotonate

• Shlomo Levinger,
• Ranjeet Nair and
• Alfred Hassner

Beilstein J. Org. Chem. 2008, 4, No. 32, doi:10.3762/bjoc.4.32

• -carbonitrile was indicated by formation of a 10-tert-butyl derivative on reaction with tert-butylmagnesium chloride [9]. However, since iminium ion intermediates are much more reactive than ketones [7], imines 8h and 8j were successfully reduced to the primary amines 6 upon treatment with sodium

Synthesis of dihydrophenanthridines by a sequence of Ugi-4CR and palladium- catalyzed intramolecular C-H functionalization

• Florence Bonnaterre,
• Michèle Bois-Choussy and
• Jieping Zhu

Beilstein J. Org. Chem. 2008, 4, No. 10, doi:10.3762/bjoc.4.10

• summarized in Table 2. 4-Nitroaniline (3c) is known to be inactive in Ugi reaction due to the reduced nucleophilicity of the nitrogen and the low basicity of the resulting imine leading consequently to a low concentration of the iminium ion. However, by performing the reaction in trifluoroethanol (TFE) [35

Facile synthesis of two diastereomeric indolizidines corresponding to the postulated structure of alkaloid 5,9E- 259B from a Bufonid toad (Melanophryniscus)

• Angela Nelson,
• H. Martin Garraffo,
• Thomas F. Spande,
• John W. Daly and
• Paul J. Stevenson

Beilstein J. Org. Chem. 2008, 4, No. 6, doi:10.1186/1860-5397-4-6

• the product 3 in 68% yield. The alternate diastereoisomer could not be detected by NMR spectroscopy in the crude reaction mixture. Product 3 formally arises by attack of trimethylallyl silane from the least hindered face of the thermodynamically less stable Z-iminium ion and the mechanistic details of

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