Substituent effects on the stereochemistry of substituted cyclohexanone dimethylhydrazone alkylations. An x-ray crystal structure of lithiated cyclohexanone dimethylhydrazone

Solid-State and Solution Structures of Glycinimine-Derived Lithium Enolates

A combination of crystallographic, spectroscopic, and computational studies was applied to study the structures of lithium enolates derived from glycinimines of benzophenone and (+)-camphor. The solvents examined included toluene and toluene containing various concentrations of tetrahydrofuran, N,N,N',N'-tetramethylethylenediamine (TMEDA), (R,R)-N,N,N',N'-tetramethylcyclohexanediamine [(R,R)-TMCDA], and (S,S)-N,N,N',N'-tetramethylcyclohexanediamine [(S,S)-TMCDA]. Crystal structures show chelated monomers, symmetric disolvated dimers, S4-symmetric tetramers, and both S6- and D3d-symmetric hexamers. (6)Li NMR spectroscopic studies in conjunction with the method of continuous variations show how these species distribute in solution. Density functional theory computations offer insights into experimentally elusive details.

Evans Enolates: Solution Structures of Lithiated Oxazolidinone-Derived Enolates

The results of a combination of (6)Li and (13)C NMR spectroscopic and computational studies of oxazolidinone-based lithium enolates-Evans enolates-in tetrahydrofuran (THF) solution revealed a mixture of dimers, tetramers, and oligomers (possibly ladders). The distribution depended on the structure of the oxazolidinone auxiliary, substituent on the enolate, and THF concentration (in THF/toluene mixtures). The unsolvated tetrameric form contained a D(2d)-symmetric core structure, whereas the dimers were determined experimentally and computationally to be trisolvates with several isomeric forms.

Enediolate-dilithium amide mixed aggregates in the enantioselective alkylation of arylacetic acids: structural studies and a stereochemical model

A combination of X-ray crystallography, (6)Li, (15)N, and (13)C NMR spectroscopies, and density functional theory computations affords insight into the structures and reactivities of intervening aggregates underlying highly selective asymmetric alkylations of carboxylic acid dianions (enediolates) mediated by the dilithium salt of a C2-symmetric chiral tetraamine. Crystallography shows a trilithiated n-butyllithium-dilithiated amide that has dimerized to a hexalithiated form. Spectroscopic studies implicate the non-dimerized trilithiated mixed aggregate. Reaction of the dilithiated amide with the dilithium enediolate derived from phenylacetic acid affords a tetralithio aggregate comprised of the two dianions in solution and the dimerized octalithio form in the solid state. Computational studies shed light on the details of the solution structures and afford a highly predictive stereochemical model.

3,4'-Linked bis(piperidines) related to the haliclonacyclamine class of marine alkaloids: synthesis using crossed-aldol chemistry and preliminary biological evaluations

Compounds 2-5, incorporating various elements of the 3,4'-bis(piperidine) core associated with the sponge-derived alkaloid haliclonacyclamine A (HA, 1), have been prepared through, inter alia, aldol-type reactions of N-substituted piperidin-4-ones and certain derivatives. Screening of these compounds in various assays, including an ecological one, reveals that compound 5 exhibits allelochemical properties similar to those associated with HA itself.

Origins of stereoselectivity in the α-alkylation of chiral hydrazones

Density functional theory calculations and experiment reveal the origin of stereoselectivity in the deprotonation-alkylation of chiral N-amino cyclic carbamate (ACC) hydrazones. When the ACC is a rigid, camphor-derived carbamate, the two conformations of the azaenolate intermediate differ in energy due to conformational effects within the oxazolidinone ring and steric interactions between the ACC and the azaenolate. An electrophile adds selectively to the less-hindered π-face of the azaenolate. Although it was earlier reported that use of ACC auxiliaries led to α-alkylated ketones with er values of 82:18 to 98:2, B3LYP calculations predict higher stereoselectivity. Direct measurement of the dr of an alkylated hydrazone prior to removal of the auxiliary confirms this prediction; the removal of the auxiliary under the reported conditions can compromise the overall stereoselectivity of the process.

Stereoselective alkylations of chiral nitro imine and nitro hydrazone dianions. Synthesis of enantiomerically enriched 3-substituted 1-nitrocyclohexenes

Dianions of chiral nitro imines (generated by a combination of LDA and s-BuLi) underwent diastereoselective alkylation with methyl, butyl, isopropyl, allyl, and methallyl iodides. In contrast to the behavior of simple metalloenamines, the most selective auxiliary contained no coordinating groups but did possess a large steric difference between the two substituents. The yield and selectivity of the alkylations were improved by the addition of HMPA or DMPU. The use of (S)-1-naphthylethylamine as the auxiliary afforded the R absolute configuration of the alkylation products. This stereochemical outcome could be rationalized by simple steric approach controlled alkylation in a conformationally fixed, internally coordinated dianion. A SAMP nitro hydrazone gave poorer yields and selectivities.

Lithium diisopropylamide: solution kinetics and implications for organic synthesis

Lithium diisopropylamide (LDA) is a prominent reagent used in organic synthesis. In this Review, rate studies of LDA-mediated reactions are placed in the broader context of organic synthesis in three distinct segments. The first section provides a tutorial on solution kinetics, emphasizing the characteristic rate behavior caused by dominant solvation and aggregation effects. The second section summarizes substrate- and solvent-dependent mechanisms that reveal basic principles of solvation and aggregation. The final section suggests how an understanding of mechanism might be combined with empirical methods to optimize yields, rates, and selectivities of organolithium reactions and applied to organic synthesis.