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Cyclization Mechanism (cyclization + mechanism)
Selected AbstractsThe Biosynthesis of 3-(trans -2-Nitrocyclopropyl)alanine, a Constituent of the Signal Metabolite HormaomycinEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 1 2005Melanie Brandl Abstract Feeding experiments with Streptomyces griseoflavus using deuterium-labeled racemic 3,3-[D2]- (6b), 4,4-[D2]- (6c), 5,5-[D2]- (6d), and 6,6-[D2]-lysine (6e), and 3-amino-5-(2-amino-1,1-dideuterioethyl)-4,5-dihydrofuran-2-one dihydrochloride (34·2HCl) were carried out in order to obtain detailed information about the hitherto unknown biosynthetic pathway from lysine to the unusual amino acid 3-(trans -2,-nitrocyclopropyl)alanine [(3-Ncp)Ala] (2), which is a building block of hormaomycin 1a. The corresponding lysine dihydrochlorides were prepared in 33, 24, 19, and 30% overall yield, respectively, along a new efficient general synthetic route applying an alkylation of the lithium enolate of O,Donnel's glycine equivalent 7 as a key step. In the attempted preparation of 5,5-[D2]-4-hydroxylysine (29), the respective ,-lactone (34·2 HCl) was obtained in five steps with 10% overall yield. The distribution of isotope labels in hormaomycins 1b,d led to the formulation of a reasonable cyclization mechanism of 2-amino-4-hydroxy-6-(hydroxyimino)hexanoic acid, an ,-oxime analogue of 4-hydroxylysine. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Aromatization and ring cyclization: A better understanding on the ring cyclization mechanism of 3-amino-6-hydrazino-1,2,4-triazin-5(2H)-one reacted with acetic acid in N,N -dimethylformamideJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2005Long-Chih Hwang In this paper we report that the title compound (3) reacts with excess N,N -dimethylformamide (DMF) containing two equivalents of acetic acid to afford 6-amino-1,2,4-triazolo[3,4- f][1,2,4]triazin-8(7H)-one (1). When 3-amino-2-benzyl-6-hydrazino-1,2,4-triazin-5(2H)-one (6), the N-2 benzylated derivative of 3, is treated under the same conditions, ring cyclization does not occur; instead, 3-amino-2-benzyl-6-(2-formyl-hydrazino)-1,2,4-triazin-5(2H)-one (7) is formed. Single-crystal X-ray analysis of a 3-ethyl derivative of compound 1 reveals the predominant tautomeric structure to be the 7H -tautomer (7H - 1). From these results, we propose a reasonable cyclization mechanism that incorporates two important points: (1) the tautomerism of the N-2 hydrogen with the C-5 oxo group aromatizes the 1,2,4-triazine ring, and (2) the DMF is proto-nated by acetic acid on the nitrogen atom, then deamination occurs where DMF is attacked by the 6-hydrazino group of 3 or 6. [source] A Time-Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3,,5,-Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction PathwayCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2010Chensheng 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] | ||||||||||