Synthesis and lithiation of oxazolinylaziridines: the N-substituent effect

The preparation of N-substituted oxazolinylaziridines and their deprotonation to afford the corresponding aziridinyllithiums is described. The chemical and configurational stability of the lithiated species depends on the N-substituent on the aziridine ring. The trapping with carbonyl compounds of (R*,S*) configurated oxazolinylaziridines is an interesting route for the preparation of functionalised α,β-aziridino-γ-lactones.     

Regio- and stereoselective lithiation and electrophilic substitution reactions of N-Alkyl-2,3-diphenylaziridines: solvent effect

[structure: see text]. The lithiation reaction of cis- and trans-N-alkyl-2,3-diphenylaziridines has been investigated. While cis-diphenylaziridines do not undergo any lithiation upon treatment with organolithiums, the lithiation reaction of the trans counterparts is completely alpha-regioselective and the stereochemical course of the lithiation-trapping sequence is solvent dependent: inversion of configuration in coordinating solvents (THF or toluene/crown ether) and retention in hexane, ether, or toluene. The preparation of stereodefined functionalized N-alkyl-2,3-diphenylaziridines is described.     

On the lithiation of oxazolinylaziridines

Lithiated N-sulfonyloxazolinylaziridines 6a and 7a, generated by deprotonation of the corresponding aziridines 6 and 7 with sec-BuLi/TMEDA at −98°C in THF, were found to be chemically and configurationally stable to be stereospecifically captured by electrophiles, while warming up to rt resulted in the formation of oxazolinylazirine 15. In contrast, lithiation of N-phenyloxazolinylaziridines 8 and 9 led to oxazolinylenamine 18. Tricyclic aziridines 10 and 11 resulted from an intramolecular addition of the aziridinyllithium 6a to the phenyl ring of the benzenesulfonyl group.     

Regio- and stereoselective lithiation of 2,3-diphenylaziridines: a multinuclear NMR investigation

The alpha-lithiation-trapping sequence of trans-N-alkyl-2,3-diphenylaziridines (s-BuLi or s-BuLi/TMEDA), taking place with a stereochemistry which dramatically depends on the solvent coordinating ability (inversion of configuration in THF and retention in toluene), has been carefully investigated. 1H,13C, and 7Li multinuclear NMR investigations at low temperature suggest that two differently configured lithiated aziridines (monomeric cis-1-Li in THF and dimeric trans-1-Li in toluene) are involved.     

Regio- and stereoselective hydrosilylation of immobilized terminal alkynes

Regio- and stereoselective hydrosilylation of terminal alkynes on solid support using diisopropyl hydrosilanes yielding β-(E)-vinyl silanes with excellent selectivity is reported. The hydrosilylation is catalyzed by Pt(DVDS)/P(ⁱBuNCH₂CH₂)₃N (DVDS = 1,3-divinyl-1,1,3,3-tetramethyl-disiloxane), in which the bulky proazaphosphatrane ligand plays a key role for the selectivity. The immobilized products are characterized with gel phase¹³C NMR and ¹H high resolution magic angle spinning NMR.     

Regio- and stereoselective hydrosilylation of terminal alkynes using Grubbs' first-generation olefin-metathesis catalyst

Grubbs' first-generation, Ru metathesis complex catalyses the hydrosilylation of terminal alkynes. The reaction exhibits an interesting selectivity profile that is dependent on the reaction concentration and more importantly on the silane employed.     

The stereochemistry of aziridine borane lithiation: diastereoselectivity and enantioselectivity

Lithiation of 1-methylaziridine borane, 1-(tert-butyldimethylsiloxyethyl)aziridine borane, or 1-(tert-butyldimethylsiloxyethyl)-2-methylaziridine borane occurs syn to the boron substituent, while lithiation of 1-(tert-butyldimethylsiloxyethyl)-2-trimethylstannylaziridine borane occurs anti to boron as well as silicon due to the steric effect of trimethylsilyl group (s-butyllithium was used in all cases). Kinetically controlled lithiation in the first three cases results from a combination of steric and electrostatic effects. Enantioselective lithiation occurs in the presence of (−)-sparteine, with product enantioselectivities near 70% ee.     

Stereocontrolled total synthesis of (-)-kainic acid. Regio- and stereoselective lithiation of pyrrolidine ring with the (+)-sparteine surrogate

[reaction: see text] A stereocontrolled total synthesis of (-)-kainic acid is described. cis-3,4-Disubstituted pyrrolidine ring was constructed by [3 + 2] cycloaddition of azomethine ylide with chiral butenolide. The crucial introduction of carboxyl group at the C-2 position was executed by regio- and stereoselective lithiation of the pyrrolidine ring in the presence of a (+)-sparteine surrogate followed by trapping with carbon dioxide.     

Regio- and stereoselective reductions of gem-difluorinated vinyloxiranes

[reaction: see text] gem-Difluorinated vinyloxiranes are versatile building blocks for the synthesis of fluorinated compounds. Investigations of their reactions with nucleophiles resulted in highly regio- and stereoselective reductions. In their reactions with LiAlH4, hydride reacted at the allylic epoxide carbon to produce homoallylic alcohols exclusively. Moreover, regio- and stereoselective S(N)2' reactions were observed with DIBAL-H and BH3 x THF; the former afforded E allylic alcohols, whereas the latter furnished the corresponding Z isomers with excellent selectivities.     

Regio- and stereoselective synthesis of vicinal fluorohydrins

Vicinal fluorohydrins are useful building blocks as well as important targets for synthesis of bioactive compounds or new materials. This review gives an overview on regio- and stereoselective, particularly enantioselective, syntheses of such compounds by building block methods, reduction of α-fluoro carbonyl compounds and ring opening of epoxides with hydrofluorinating reagents. Resolution of racemic vicinal fluorohydrins using enzyme-catalyzed reactions will also be discussed in detail.     

Regio- and stereoselective synthesis of 1,4-dienes

Titanocene(II)-promoted cross-coupling between (Z)-alkenyl methyl sulfones and terminal allenes produced 1,4-dienes regioselectively via the formation of 2-alkylidenetitanacyclopentanes. Preferential formation of E,Z-dienes was observed in the reaction using aryl-, amino-, and phosphinylallenes.     

Regio- and stereoselective glycosylation: synthesis of 5-haloimidazole alpha-ribonucleosides

We describe the synthesis of novel 5-haloimidazole ribonucleosides as precursors of modified cobalamins. A regio- and stereoselective glycosylation of protected ribose with silylated 4(5)-haloimidazoles produces 5-haloimidazole ribonucleosides predominantly in the alpha-configuration (60-75%) without any 4-substituted imidazole ribonucleoside. The structure of the 5-fluoroimidazole ribonucleoside was confirmed by X-ray crystallography and 2D NMR spectroscopy.     

Regio- and stereoselective dimerization of terminal alkynes to enynes catalyzed by a palladium/imidazolium system.

A Palladium/imidazolium chloride system has been used to mediate the dimerization of terminal alkynes to enynes. The combination of 1 mol % Pd(OAc)(2) and 2 mol % IMes.HCl in the presence of Cs(2)CO(3) as base shows high activity and high regio- and steroselectivity for the dimerization of aryl and aliphatic terminal alkynes to enynes.     

Regio- and stereoselective oxetane formation via paterno-buchi photocycloaddition

Carbonyl 2 + 2 photoaddition occurs selectively to the alkene moiety of 3-(4-methyl-3-pentenoxy)pyridine. Photolysis of alkene containing pyridines in acetophenone gives rise to an oxetane which is obtained with extremely high diastereoselectivity as shown by analysis of the major 2 + 2 photoproduct. A second photoproduct, 2,3-dihydroxy-2,3-diphenylbutane, is obtained as a result of acetophenone coupling.     

Regio- and stereoselective reactions of gem-difluorinated vinyloxiranes with heteronucleophiles

[Reaction: see text]. Regio- and stereoselectivity in reactions of gem-difluorinated vinyloxiranes with heteronucleophiles were successfully controlled. Halogen atoms were introduced regioselectively at the allylic epoxide carbon with an inversion in stereochemistry using MgBr2*Et2O or Li2CuCl4 to produce anti-vic-halohydrine. The other diastereomers were obtained selectively using LiBr/AcOH or BCl3, and SN2' type products were formed selectively with excellent E preference by changing the reaction temperature. Moreover, a further investigation led us to find that a regio- and stereoselective SN2' addition of several Brnsted acids was dependent on the pKa values of the acids. Under strong acidic conditions, we exclusively obtained E allylic alcohols.     

Regio- and stereoselective synthesis of functionalized N-Boc-2-alkynylcyclopropylamines

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