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Direct Generation (direct + generation)

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Chemistry	100%

Selected Abstracts

Metal enolates of , -CF3 ketones: theoretical guideline, direct generation, and synthetic use

THE CHEMICAL RECORD, Issue 1 2006
Koichi Mikami
Abstract The difficulty as well as the significance of the direct generation of metal enolates of ,-CF3 ketones cannot be easily understood by chemists unfamiliar with F. In sharp contrast to the sunny side of non-F, hydrocarbon chemistry, F chemistry has long been overshadowed. Metal enolates of carbonyl compounds are synthetically important in CC bond-forming reactions. However, the metal enolates of fluorinated carbonyl compounds have been severely limited to ,-F metal enolates. ,-CF3 metal enolates have generally been recognized as unstable and difficult to prepare because of the facile ,-M,F elimination. However, we have developed a direct generation and synthetic application of ,-CF3 metal enolates. Therefore, the present results regarding the direct generation and synthetic use of metal enolates of ,-CF3 ketones might be recognized as a real breakthrough for the general use of metal enolates in F chemistry. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 1,11; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20066 [source]

A Time-Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3,,5,-Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2010
Chensheng Ma Dr.
Abstract 3,,5,-Dimethoxybenzoin (DMB) is a bichromophoric system that has widespread application as a highly efficient photoremovable protecting group (PRPG) for the release of diverse functional groups. The photodeprotection of DMB phototriggers is remarkably clean, and is accompanied by the formation of a biologically benign cyclization product, 3,,5,-dimethoxybenzofuran (DMBF). The underlying mechanism of the DMB deprotection and cyclization has, however, until now remained unclear. Femtosecond transient absorption (fs-TA) spectroscopy and nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy were employed to detect the transient species directly, and examine the dynamic transformations involved in the primary photoreactions for DMB diethyl phosphate (DMBDP) in acetonitrile (CH3CN). To assess the electronic character and the role played by the individual sub-chromophore, that is, the benzoyl, and the di- meta -methoxybenzylic moieties, for the DMBDP deprotection, comparative fs-TA measurements were also carried out for the reference compounds diethyl phosphate acetophenone (DPAP), and 3,,5,-dimethoxybenzylic diethyl phosphate (DMBnDP) in the same solvent. Comparison of the fs-TA spectra reveals that the photoexcited DMBDP exhibits distinctly different spectral character and dynamic evolution from those of the reference compounds. This fact, combined with the related steady-state spectral and density functional theoretical results, strongly suggests the presence in DMBDP of a significant interaction between the two sub-chromophores, and that this interaction plays a governing role in determining the nature of the photoexcitation and the reaction channel of the subsequent photophysical and photochemical transformations. The ns-TR3 results and their correlation with the fs-TA spectra and dynamics provide evidence for a novel concerted deprotection,cyclization mechanism for DMBDP in CH3CN. By monitoring the direct generation of the transient DMBF product, the cyclization time constant was determined unequivocally to be ,1,ns. This indicates that there is little relevance for the long-lived intermediates (>10,ns) in giving the DMBF product, and excludes the stepwise mechanism proposed in the literature as the major pathway for the DMB cyclization reaction. This work provides important new insights into the origin of the 3,,5,-dimethoxy substitution effect for the DMB photodeprotection. It also helps to clarify the many different views presented in previous mechanistic studies of the DMB PRPGs. In addition to this, our fs-TA results on the reference compound DMBnDP in CH3CN provide the first direct observation (to the best of our knowledge) showing the predominance of a prompt (,2,ps) heterolytic bond cleavage after photoexcitation of meta -methoxybenzylic compounds. This provides insight into the long-term controversies about the photoinitiated dissociation mode of related substituted benzylic compounds. [source]

Dynamic Kinetic Resolution and Desymmetrization Processes: A Straightforward Methodology for the Enantioselective Synthesis of Piperidines

CHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2006
Mercedes Amat Prof.
Abstract A straightforward procedure for the synthesis of enantiopure polysubstituted piperidines is reported. It involves the direct generation of chiral non-racemic oxazolo[3,2- a]piperidone lactams that already incorporate carbon substituents on the heterocyclic ring and the subsequent removal of the chiral auxiliary. The key step is a cyclocondensation reaction of (R)-phenylglycinol or other amino alcohols with racemic or prochiral ,-oxo (di)acid derivatives in highly stereoselective processes involving dynamic kinetic resolution and/or desymmetrization of diastereotopic or enantiotopic ester groups. Se describe un procedimiento directo para la síntesis enantioselectiva de piperidinas polisustituidas. Consiste en la generación directa de oxazolo[3,2-a]piperidonas quirales no racémicas que ya incorporan sustituyentes carbonados en las diferentes posiciones del heterociclo, y en la posterior eliminación del auxiliar quiral. La etapa clave es una reacción de ciclocondensación entre el (R)-fenilglicinol, u otros amino alcoholes quirales, con derivados de , -oxo ácidos racémicos o proquirales, en procesos altamente estereoselectivos que implican una resolución cinética dinámica y/o la desimetrización de grupos diastereotópicos o enantiotópicos. [source]