19206-86-1Relevant academic research and scientific papers
CATION RADICALS IN THE BROMINATION OF BENZODIPYRAN DERIVATIVES
Dean, Francis M.,France, Steven N.,Oyman, Ulku
, p. 4857 - 4862 (1988)
Bromine attacks the free aromatic sites in 2,3,4,7,8,9-hexahydro-2,2,7,7-tetramethylbenzodipyran and its angular isomer 1,2,3,8,9,10-hexahydro-3,3,8,8-tetramethylbenzodipyran without clear evidence for the intermediacy of cation radicals.When the nuclear sites are methylated bromine affords a cation radical tribromide (8) and then monohalogenates each aromatic methyl group giving 5,10-bis-(bromomethyl)-2,3,4,7,8,9-hexahydro-2,2,7,7-tetramethylbenzo-dipyran (10).In contrast, N-bromosuccinimide dehydrogenates and brominates the two pyran rings giving 3,8-dibromo-2,7-dihydrobenzo-2,2,5,7,7,10-hexamethyldipyran (14).The angular isomer reacts with either bromine or N-bromosuccinimide at the pyran rings only, giving (21).The part played by cation radicals in these reactions is discussed, related to the 1H nmr line broadening phenomena observed earlier, and explained in terms of a slight preferential concentration of unpaired spin at position 5 of a cation-radical derived from 6-hydroxychroman.
Spirans. Part 14. Regioisomeric Quinone Methides and Spirodimers Related to Tocopherol
Dean, Francis M.,Matkin, David A.,Orabi, Mohamed O. A.
, p. 1437 - 1442 (1981)
Evidence is provided that the quinone methide (5), 3,4-dihydro-2,2,5,8-tetramethyl-7-methylene-2H-benzo-pyran-6(7H)-one, is more, not less, readily formed than its regioisomer, the corresponding 5-methylenebenzopyran-6-one (4), and consequently that 'bond fixation' and allied geometrical constraints (Mills-Nixon effects) cannot be responsible for the regioselective oxidative dimerisation of tocopherol.The quinone methide (5) obtained transitorily by treating 7-chloromethyl-3,4-dihydro-2,2,5,8-tetramethyl-2H-benzopyran-6-ol (11) with hydrogen carbonate ion rapidly forms the corresponding spirodimer (7), 3,3',4,4',8,9-hexahydro-2',2',5,5',7,7,8',10-octamethylbenzodipyran-2-spiro-7'(6'H)-benzopyran-6'-one and trimer (12), which is the main product.The spirodimer (7) reverts to quinone methide (5) near 80 deg C and is, therefore, rapidly transformed into the trimer at this temperature, whereas the regioisomeric spirodimer (6), previously recognised as the product from the oxidations, is stable.The difference in reactivity between the known quinone methide (4) and the new one (5) is thought to be associated with the benzylic methylene groups of the terminal pyran rings.
Magnetic Resonance Studies of Cation Radicals from Chromans. Part 1.-Electron Spin Resonance and ENDOR Spectroscopy of Some Tricyclic Chromans
Fairhurst, Shirley A.,Sutcliffe, Leslie H.,Taylor, Sheila M.
, p. 2743 - 2756 (1982)
The e.s.r. spectrum of the cation radical from 2,3,4,7,8,9-hexahydro-2,2,5,7,7,10-hexamethylbenzodipyran is temperature dependent and shows the alternating line-width effect from interconversion.ENDOR and TRIPLE resonance measurements provided precise values of hyperfine coupling constants and their relative signs at the slow limit.These data enabled us to simulate spectra from the slow to the fast limits, which are observed at ca. -70 and ca. 100 deg C, respectively.The activation parameters for the interconversion process were found to be +28(+/-4) kJ mol-1, +50(+/-3)kJ mol-1 and +71(+/-12)J mol-1 K-1 for the free energy, enthalpy and entropy, respectively, at 298 K.The e.s.r. spectrum of the cation radical from 2,3,4,5,6,7-hexahydro-2,2,7,7,9,10-hexamethylbenzodipyran is also temperature dependent, but the slow limit is observed at room temperature and the alternating line-width effect is masked: activation parameters for the interconversion were found to be +42(+/-6)kJ mol-1, +29(+/-3)kJ mol-1 and -46(+/-9)J mol-1 K-1 for the free energy, enthalpy and entropy, respectively, at 298 K.A third tricyclic chroman, namely 2,3,6,7-tetrahydro-2,2,4,6,6,8-hexamethylbenzodifuran, gives a cation radical which has a temperature-independent e.s.r. spectrum, owing to interconversion, which is rapid on the e.s.r. time-scale.
Vitamin E chemistry. Nitration of non-α-tocopherols: Products and mechanistic considerations
Patel, Anjan,Liebner, Falk,Netscher, Thomas,Mereiter, Kurt,Rosenau, Thomas
, p. 6504 - 6512 (2008/02/10)
(Chemical Equation Presented) In contrast to the α-form permethylated at the aromatic ring, non-α-tocopherols possess free aromatic ring positions which enable them to act as potent scavengers of electrophiles in vivo and in vitro. In preparation of enzymatic studies involving peroxynitrite and other nitrating systems, the behavior of non-α-tocopherols under nitration conditions was studied. The nitration products of β-, γ-, and δ-tocopherol were identified, comprehensively analytically characterized, and their structure was supported by X-ray crystal structure analysis on truncated model compounds. Even under more drastic nitration conditions, no erosion of the stereochemistry at 2-C occurred. The nitrosation of γ-tocopherol and δ-tocopherol was re-examined, showing the slow oxidation of the initial nitroso products to the corresponding nitro derivatives by air to be superimposed by a fast equilibrium with the tautomeric ortho-quinone monoxime, which only in the case of γ-tocopherol released hydroxyl amine at elevated temperatures to afford the stable ortho-quinone. Mononitration of δ-tocopherol selectively proceeded at position 5. This selectivity can be explained by the theory of strain-induced bond localization (SIBL) to the quinoid nitration intermediates. Bisnitration was only insignificantly disfavored by the first nitro group, so that under normal nitration conditions offering an excess of nitrating species only the bisnitration product was found.
