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Sn-Li exchange equilibria have allowed the quantification of the stabilizing effect of cation-pi interactions in organolithium chemistry. Stabilization energy data on the effect of Li-pi complexation of an aromatic ring or a CC double bond in organolithium compounds are presented. The amount of stabilization gained by complexation of the Li atom with a pi system in alpha-oxy-organolithium compounds is quite comparable to the one observed in systems containing Li-N or Li-O interactions.

A ΔGeq stability scale of secondary a-oxy-organolithium compounds was established from measurements of tin-lithium exchange equilibria in THF, and the quantitative effects of substituents at the anionic center on carbanion stability are presented. A new lead-lithium exchange equilibrium reaction was also investigated and shown to be a very useful alternative for the determination of the relative stability of the more sterically hindered organolithium compounds. Alkyl groups adversely affect the stability of organolithium compounds when attached to the carbon bearing the negative charge, but the extent of this effect is highly dependent on the nature of the rest of the substituents attached to the anionic center. Quantitative data on the stabilization imparted to organolithium compounds by Li-O and Li-N chelation have been determined for a variety of systems. The for mation of four- and five-membered chelate rings leads to a considerable stabilization of the organolithium compound, while chelation through the formation of six-membered rings affords no extra stabilization to this type of organometallics. Multinuclear NMR experiments carried out on several α-oxyorganolithium compounds to determine their aggregation state are supportive of these species being monomers in THF solution. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

Quantitative thermodynamic stability scales of organolithium compounds can be derived from measurements of tin-lithium exchange equilibria. A DeltaG(eq) scale of alpha-oxy- and alpha-aminoorganolithium compounds was established, and quantitative stabilization effects of O-alkyl, O-alkoxyalkyl, O-carbamoyl, N-carbamoyl, and O-carbonyl groups of the alpha-carbanion are presented. It has been found that an alpha-oxycarbanion is far better stabilized by a carbonyl group as the O-substituent than by an alkyl or alkoxyalkyl group, while the anion-stabilizing effects of the different O-carbonyl substituents are comparable. An N-carbamoyl group was found to have a somewhat higher stabilizing effect than its O-carbamoyl counterpart. NMR data are presented that show that benzylic N- or O-substituted carbanions have highly planarized structures where the negative charge is highly delocalized. The stability data obtained from the tin-lithium exchanges can be easily converted into "effective pK" data that are useful for predicting the acid-base behavior of this type of organolithium species.

Enantiomerically pure N-(9-phenylfluoren-9-yl)-α-amino ketones were prepared in excellent yields by acylation of organolithium reagents with N-(9-phenylfluoren-9-yl)-α-amino acid-derived oxazolidinones. The method is not applicable for the acylation of Grignard reagents as they attack the methylenic carbon of the oxazolidinone to give the corresponding N-alkylated amino acids 13 in excellent yields. The resulting N-(9-phenylfluoren-9-yl)-α-amino ketones 8 could be stereoselectively reduced to the corresponding syn- or anti-β-amino alcohols depending upon the nature of the reducing agent.

The reaction of methyl N-(9-phenylfluoren-9-yl)pyroglutamate with several organolithium reagents afforded the corresponding ketones in excellent yields and with complete retention of enantiomeric purity. The success of this transformation is due to the unusual stability of the tetrahedral intermediates 5, which stems from two factors: the electron-withdrawing effect of the amide nitrogen and the lithium complexing ability of the fluorenyl ring of the 9-phenylfluoren-9-yl group. This ester-to-ketone transformation was also successfully applied to oxazolidinone 9 and imidazolidinone 20 and provided a ketone (21b) that was ultimately transformed into the urea-lactam 26 which incorporates the bicyclic core of streptolidine lactam, a component of the streptrothricin antibiotics.

A highly efficient synthesis of bridged bicyclic compounds based on reductive alkyltin of anthracene dicarboxylate (I) is presented.

An efficient (21% overall yield), enantio- and diastereoseleclive, 11-step synthesis of (1 S ,2 R )-imidazoleglycerol has been developed. The key steps are the stereoselective hydroxylation of an acyloxazolidinone enolate, the alkylation of a thioester with (MOMOCH 2 ) 2 CuLi and the stereodivergent reduction of the resulting ketone. The scope of the reaction of the enolate derived from 10 with heteroatom electrophiles has been studied.

[reaction: see text] The reaction of R(3)SnLi with carboxylic acid derivatives proceeds through a novel, very fast stanna-Brook rearrangement that generates alpha-alkoxyorganolithium compounds as intermediates. The outcome of these reactions depends on the nature of the carboxyl derivatives. Reaction of R(3)SnLi with ester derivatives gives rise to coupled products through a novel C-C bond formation reaction. Experimental evidence of the detailed reaction mechanism is provided.

Kinetic study of the α-lithiation of benzyl methyl ether (BME) by nBuLi has revealed that increasing the concentration of the organolithium compound does not necessarily increase the reactivity, and this is a consequence of the reactivities of the different nBuLi aggregates present in solution. We propose a dimer-based mechanism, in which a pre-complexation step is a key process for substrates bearing a donor oxygen atom that can interact with the lithium cation to form mixed dimers. For these studies, we have developed a system based on UV/Vis spectroscopy that allows kinetic measurements to be conducted at -80 °C under argon.

The process is applied to the synthesis of the tricyclic benzofuran derivative (XIX), a valuable intermediate in the synthesis of galantamine alkaloid acting as a selective acetylcholinesterase inhibitor.

We report here a short and efficient synthesis (four steps, 78% overall yield) of dimethyl (25)-N-(9-phenylfluoren-9-yl)-3,4-didehydroglutamate (1) in enantiomerically pure form from L-glutamic acid. The C-C double bond in 1 was introduced by selective selenenylation-oxidation of dimethyl N-(9-phenylfluoren-9-yl)glutamate. 1 did not undergo loss of enantiomeric purity when stored for several weeks at room temperature, thus demonstrating the ability of the 9-phenylfluoren-9-yl (Pf) group to protect the highly acidic chiral center of 1 from racemization. The α,β-unsaturated carboxyl group of 1 was selectively reduced with DIBAL. Carbamoylation of the resulting alcohol, followed by OsO 4 -mediated hydroxylation of the double bond and deprotection afforded (+)-5-O-carbamoylpolyoxamic acid, a component of the polyoxin family of antifungal antibiotics ( >40% overall yield from glutamic acid). 1 underwent lithium dimethyl cuprate addition with complete retention of chiral integrity, albeit in a nonstereoselective fashion, to give 3-methylglutamates and 3-methylpyroglutamates

A chirospecific and stereodivergent synthesis of N-(9-phenylfluorenyl)-3-hydroxyaspartates by hydroxylation of aspartate enolates is described. The stereochemistry of the newly created chiral center is controlled by the nature of the enolate counterion and the ligand coordinating ability of the solvents.

In our quest for HIV-1 protease inhibitors that are not affected by the V82A resistance mutation, we have synthesized and tested a second generation set of C2-symmetric HIV-1 protease inhibitors that contain a cyclohexane group at P1 and/or P1'. The binding affinity results indicate that these compounds have an improved response to the appearance of the V82A mutation than the parent compound. The X-ray structure of one of these compounds with the V82A HIV-1 PR variant provides the structural rationale for the better resistance profile of these compounds. Moreover, scrutiny of the X-ray structure suggests that the ring of the Cha side chain might be in a boat rather than in the chair conformation, a result supported by molecular dynamics simulations.