7432-49-7Relevant articles and documents
Parallel and competitive pathways for substrate desaturation, hydroxylation, and radical rearrangement by the non-heme diiron hydroxylase AlkB
Cooper, Harriet L. R.,Mishra, Girish,Huang, Xiongyi,Pender-Cudlip, Marilla,Austin, Rachel N.,Shanklin, John,Groves, John T.
, p. 20365 - 20375 (2013/02/25)
A purified and highly active form of the non-heme diiron hydroxylase AlkB was investigated using the diagnostic probe substrate norcarane. The reaction afforded C2 (26%) and C3 (43%) hydroxylation and desaturation products (31%). Initial C-H cleavage at C2 led to 7% C2 hydroxylation and 19% 3-hydroxymethylcyclohexene, a rearrangement product characteristic of a radical rearrangement pathway. A deuterated substrate analogue, 3,3,4,4-norcarane-d 4, afforded drastically reduced amounts of C3 alcohol (8%) and desaturation products (5%), while the radical rearranged alcohol was now the major product (65%). This change in product ratios indicates a large kinetic hydrogen isotope effect of ～20 for both the C-H hydroxylation at C3 and the desaturation pathway, with all of the desaturation originating via hydrogen abstraction at C3 and not C2. The data indicate that AlkB reacts with norcarane via initial C-H hydrogen abstraction from C2 or C3 and that the three pathways, C3 hydroxylation, C3 desaturation, and C2 hydroxylation/radical rearrangement, are parallel and competitive. Thus, the incipient radical at C3 either reacts with the iron-oxo center to form an alcohol or proceeds along the desaturation pathway via a second H-abstraction to afford both 2-norcarene and 3-norcarene. Subsequent reactions of these norcarenes lead to detectable amounts of hydroxylation products and toluene. By contrast, the 2-norcaranyl radical intermediate leads to C2 hydroxylation and the diagnostic radical rearrangement, but this radical apparently does not afford desaturation products. The results indicate that C-H hydroxylation and desaturation follow analogous stepwise reaction channels via carbon radicals that diverge at the product-forming step.
Cage escape competes with geminate recombination during alkane hydroxylation by the diiron oxygenase AlkB
Austin, Rachel N.,Luddy, Kate,Erickson, Karla,Pender-Cudlip, Marilla,Bertrand, Erin,Deng, Dayi,Buzdygon, Ryan S.,Van Beilen, Jan B.,Groves, John T.
supporting information; experimental part, p. 5232 - 5234 (2009/04/04)
(Chemical Presented) AlkBstops the radical clock: Three structurally analogous radical-clock substrates with a large span in their rearrangement rates are hydroxylated by AlkB to afford similar amounts of rearranged (2) and unrearranged products (1). Such a result is in accord with radical rebound competing with cage escape of the geminate substrate radical. The results show that radical clocks can measure both the radical lifetime and the kinetics of cage escape.
Radical intermediates in monooxygenase reactions of Rieske dioxygenases
Chakrabarty, Sarmistha,Austin, Rachel N.,Deng, Dayi,Groves, John T.,Lipscomb, John D.
, p. 3514 - 3515 (2007/10/03)
Rieske dioxygenases catalyze the cis-dihydroxylation of a wide range of aromatic compounds to initiate their biodegradation. The archetypal Rieske dioxygenase naphthalene 1,2-dioxygenase (NDOS) catalyzes dioxygenation of naphthalene to form (+)-cis-(1R,2S
Desaturase reactions complicate the use of norcarane as a mechanistic probe. Unraveling the mixture of twenty-plus products formed in enzyme-catalyzed oxidations of norcarane
Newcomb, Martin,Chandrasena, R. Esala P.,Lansakara-P, Dharmika S. P.,Kim, Hye-Yeong,Lippard, Stephen J.,Beauvais, Laurance G.,Murray, Leslie J.,Izzo, Viviana,Hollenberg, Paul F.,Coon, Minor J.
, p. 1121 - 1127 (2007/10/03)
(Chemical Equation Presented) Norcarane, bicyclo[4.1.0]heptane, has been widely used as a mechanistic probe in studies of oxidations catalyzed by several iron-containing enzymes. We report here that, in addition to oxygenated products, norcarane is also oxidized by iron-containing enzymes in desaturase reactions that give 2-norcarene and 3-norcarene. Furthermore, secondary products from further oxidation reactions of the norcarenes are produced in yields that are comparable to those of the minor products from oxidation of the norcarane. We studied oxidations catalyzed by a representative spectrum of iron-containing enzymes including four cytochrome P450 enzymes, CYP2B1, CYPΔ2B4, CYPΔ2E1, and CYPΔ2E1 T303A, and three diiron enzymes, soluble methane monooxygenase (sMMO) from Methylococcus capsulatus (Bath), toluene monooxygenase (ToMO) from Pseudomonas stutzeri OX1, and phenol hydroxylase (PH) from Pseudomonas stutzeri OX1. 2-Norcarene and 3-norcarene and their oxidation products were found in all reaction mixtures, accounting for up to half of the oxidation products in some cases. In total, more than 20 oxidation products were identified from the enzyme-catalyzed reactions of norcarane. The putative radical-derived product from the oxidation of norcarane, 3- hydroxymethylcyclohexene (21), and the putative cation-derived product from the oxidation of norcarane, cyclohept-3-enol (22), coelute with other oxidation products on low-polarity GC columns. The yields of product 21 found in this study are smaller than those previously reported for the same or similar enzymes in studies where the products from norcarene oxidations were ignored, and therefore, the limiting values for lifetimes of radical intermediates produced in the enzyme-catalyzed oxidation reactions are shorter than those previously reported.
Evaluation of norcarane as a probe for radicals in cytochome P450- and soluble methane monooxygenase-catalyzed hydroxylation reactions
Newcomb, Martin,Shen, Runnan,Lu, Yun,Coon, Minor J.,Hollenberg, Paul F.,Kopp, Daniel A.,Lippard, Stephen J.
, p. 6879 - 6886 (2007/10/03)
Norcarane was employed as a mechanistic probe in oxidations catalyzed by hepatic cytochome P450 enzymes and by the soluble methane monooxygenase (sMMO) enzyme from Methylococcus capsulatus (Bath). In all cases, the major oxidation products (>75%) were endo- and exo-2-norcaranol. Small amounts of 3-norcaranols, 2-norcaranone, and 3-norcaranone also formed. In addition, the rearrangement products (2-cyclohexenyl)methanol and 3-cycloheptenol were detected in the reactions, the former possibly arising from a radical intermediate and the latter ascribed to a cationic intermediate. The formation of the cation-derived rearrangement product is consistent with one or more reaction pathways and is in accord with the results of previous probe studies with the same enzymes. The appearance of the putative radical-derived rearrangement product is in conflict with other mechanistic probe results with the same enzymes. The unique implication of a discrete radical intermediate in hydroxylations of norcarane may be the consequence of a minor reaction pathway for the enzymes that is not manifest in reactions with other probes. Alternatively, it might reflect a previously unappreciated reactivity of norcaranyl cationic intermediates, which can convert to (2-cyclohexenyl)methanol. We conclude that generalizations regarding the intermediacy of radicals in P450 and sMMO enzyme-catalyzed hydroxylations based on the norcarane results should be considered hypothetical until the origin of the unanticipated results can be determined.
Stereodirecting Effect of a Substrate Methoxy Substituent on the Addition of Singlet Methylene to a Double Bond
Young, Terry A.,O'Rourke, Colleen,Gray, Nathan B.,Lewis, Brian D.,Dvorak, Curt A.,et al.
, p. 6224 - 6228 (2007/10/02)
The stereodirecting effects of substrate methoxy, hydroxy, methylthio, and methyl substituents were examined in the addition of 1:CH2 to the double bonds of substrates 1a-d.The carbene, generated by photolysis of CH2N2, inserted into the C-H bonds of solvent and substrate, added to the substrate double bond to give products 2a-d, and attacked the oxygen or sulfur atom of substrates 1a-c to produce ylide intermediates which underwent 2,3-sigmatropic rearrangement to give products 3a-c.A preference for addition syn to the methoxy group of substrate 1a was observed when the reaction was run in pentane solution (syn-2a/anti-2a, 1.14 +/- 0.02), while a preference for formation of anti-2a was observed in diethyl ether solution (syn-2a/anti-2a, 0.92 +/- 0.03).A preference for 1:CH2 addition anti to the substrate substituent was observed for substrates 1b-d in both pentane and ether solution.The effect of the methoxy substituent was also examined in the addition of 1:CH2 to syn-7-methoxynorbornene (5b).Explanations for the substituent effects are offered based on both steric hindrance and interaction between 1:CH2 and the substituent, including formation and subsequent reaction of the ylide intermediates.
