- Insertion reactions of silacyclopropanes: Evidence for a radical-based mechanism
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Silacyclopropanes reacted rapidly and selectively with p-benzoquinones to provide oxasilacyclopentanes. Ring-expansion products were observed in the absence of a catalyst, elevated temperatures, or irradiation. As substitution was increased on the silacyclopropane ring, improved stereoselectivity was observed. In some cases, the regiochemistry was controlled depending on the extent of stabilization of the reactive intermediates involved. A radical clock experiment, along with stereochemical studies, confirmed that radical intermediates were involved in the ring-expansion reaction. The scope of this radical reaction was expanded to include dienones, aryl aldehydes, and electron-deficient enones in addition to benzoquinones. In the case of aryl aldehydes and electron-deficient enones, the radical reaction can be used to generate silylenes from silacyclopropanes.
- Rotsides, Christina Z.,Woerpel
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- Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2
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Processes of direct and electrocatalytic (in the presence of electrochemically reduced Cp2TiCl2) reduction of three α-bromoketones containing the C(sp3)-Br or C(sp2)-Br bond, viz., 2-bromo-and 2,6-dibromo-4,4-dimethylcyclohexa-2,5-dien-1-ones and α-bromo-acetophenone, were studied by cyclic voltammetry and preparative electrolysis. In all cases, the dissociative electron transfer proceeds via the concerted mechanism. Preparative electrolysis of these α-bromoketones in the presence of Cp2TiCl2 affords the reductive debromination products in 40-80% yield at low cathodic potentials (-0.85 V vs. Ag/AgCl/KCl). In the case of 2,6-dibromo-4,4-dimethylcyclohexa-2,5-dien-1-one in the potentiostatic regime, only one bromine atom can be eliminated selectively.
- Nikitin,Gavrilova,Magdesieva
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- Electrochemically induced transformation of 4-halomethyl-4-methylcyclohexa- 2,5-dien-1-ones into 3,4-dimethylphenol
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Electrochemical behavior of 4-halomethyl-4-methylcyclohexa-2,5-dien-1-ones and 2,6-dibromo-4,4-dimethylcyclohexa-2,5-dien-1-one was studied. Reductive dehalogenation of cyclohexa-2,5-dien-1-ones having a halogen atom at the neopentyl-like carbon atom give
- Gavrilova,Moiseeva,Beloglazkina,Gavrilov,Zyk
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- Cross-conjugated compounds: microwave spectrum of 4,4-dimethyl-2,5-cyclohexadien-1-one
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4,4-Dimethyl-2,5-cyclohexadien-1-one has been prepared by a three-step reaction from 4,4-dimethyl-2-cyclohexen-1-one.Microwave transitions of the cross-conjugated compound have been measured over extended regions from 11 to 40 GHz.The spectrum is a classical a-type spectrum of a near-prolate top with many interspersed lines presumably originating from excited vibrational states.No internal rotation splitting could be observed.We were able to assign 108 rotational transitions with J-quantum numbers up to 40.The least-squares fit showed a standard deviation of 30 kHz and yielded the three rotational constants, A = 3332.158(13) MHz, B = 1193.07386(52) MHz, C = 1082.21280(45) MHz, as well as all five quartic centrifugal distortion constants from Watson's A-reduction: ΔJ = 0.02149(73) kHz, ΔJK = 1.1008(16) kHz, ΔK = -68.42(32) kHz, δJ = 0.00332(16) kHz, δK = -2.7513(99) kHz (representation IR used).The dipole moment was determined from the Stark shift of the M-components of two transitions and was found to be 4.4522(83) D from 93 measurements.
- Hutter, Wolfgang,Bodenseh, Hans-Karl
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- Strong and Confined Acids Control Five Stereogenic Centers in Catalytic Asymmetric Diels–Alder Reactions of Cyclohexadienones with Cyclopentadiene
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We describe a highly enantioselective Diels–Alder reaction of cross-conjugated cyclohexadienones with cyclopentadiene, in which five stereocenters are effectively controlled by a strongly acidic and confined imidodiphosphorimidate catalyst. Our approach provides tricyclic products in excellent stereoselectivity. We also report methods to convert the obtained products into useful intermediates and a computational study that aids in gaining deeper insight into the reaction mechanism and origin of stereoselectivity.
- Bistoni, Giovanni,Das, Sayantani,De, Chandra Kanta,Ghosh, Santanu,Leutzsch, Markus,List, Benjamin,Neese, Frank,Yepes, Diana
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supporting information
p. 12347 - 12351
(2020/03/23)
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- Lewis Acid Catalyzed Enantioselective Photochemical Rearrangements on the Singlet Potential Energy Surface
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The oxadi-methane rearrangement of 2,4-cyclohexadienones to bicyclic ketones was found to proceed with high enantioselectivity (92-97% ee) in the presence of catalytic amounts of a chiral Lewis acid (15 examples, 52-80% yield). A notable feature of the transformation is the fact that it proceeds on the singlet hypersurface and that no triplet intermediates are involved. Rapid racemic background reactions were therefore avoided, and the catalyst loading could be kept low (10 mol %). Computational studies suggest that the enantioselectivity is determined within a Lewis acid bound singlet intermediate via a conical intersection. The utility of the method was demonstrated by a concise synthesis of the natural product trans-chrysanthemic acid.
- Leverenz, Malte,Merten, Christian,Dreuw, Andreas,Bach, Thorsten
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supporting information
p. 20053 - 20057
(2019/12/30)
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- Silver(I)-catalyzed ring-contractive rearrangement: A new entry to 5-alkylidene-2-cyclopentenones
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A novel silver(I)-catalyzed ring-contractive rearrangement of 5-substituted 6-diazo-2-cyclohexenones has been developed, providing a new and efficient access to 5-alkylidene-2-cyclopentenones. The AgOTf-catalyzed reaction proceeds through metal-carbenoid formation followed by endocyclic allyl [1,2] migration with excellent stereoselectivity and broad substrate scope.
- Zhao, Liang,Wang, Jinlian,Zheng, Hongyan,Li, Yun,Yang, Ke,Cheng, Bin,Jin, Xiaojie,Yao, Xiaojun,Zhai, Hongbin
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supporting information
p. 6378 - 6381
(2015/01/09)
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- NMR-based molecular ruler for determining the depth of intercalants within the lipid bilayer. Part I. Discovering the guidelines
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The development of "molecular rulers" would allow one to quantitatively locate intercalants within the liposomal bilayer. To this end, we have attempted to correlate the 13C NMR chemical shift of a polarizable "reporter" carbon (e.g., carbonyl) of the intercalant-with the ET(30) polarity it experiences, and with its Angstrom distance from the interface. This requires families of molecules with the same two "reporter carbons" separated by a fixed distance, residing at various depths/polarities within the bilayer. The families studied included 4,4-dialkylcyclohexa-2,5-dienones 1, benzenediacetic esters 15, benzenedipropionic esters 17, 4-alkoxybenzaldehydes 19 and methyl 4-alkoxybenzoates 22. These compounds possessed the following characteristics: (1) a planar backbone; (2) polar/hydrophilic "head" groups; (3) modular hydrophobic tails; (4) large changes in the 13C NMR chemical shift (Δδ) of the reporter atoms with solvent polarity. These studies revealed a fifth requirement, namely: (5) the reporter carbons must not be strongly conjugated, lest it reflect the charge build-up at another site within the conjugated system.
- Cohen, Yael,Bodner, Efrat,Richman, Michal,Afri, Michal,Frimer, Aryeh A.
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experimental part
p. 98 - 113
(2009/12/31)
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- 4,4-Dimethylbicyclohexa-1(6),2-diene - A Highly Strained 1,3-Bridged Cyclopropene
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The photochemical rearrangement of 4,4-dimethylcyclohexa-2,5-dienylidene (3) to 4,4-dimethylbicyclohexa-1(6),2-diene (5) was investigated by means of the matrix isolation technique.Carbene 3 was generated in argon matrices at 10 K by photolysis of
- Albers, Reinhard,Sander, Wolfram,Ottosson, Carl-Henrik,Cremer, Dieter
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p. 967 - 973
(2007/10/03)
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- Competitive epoxidation and quinone formation in the dimethyldioxirane oxidation of diazoquinones as ambident nucleophiles
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In addition to the expected dimethyldioxirane (DMD) oxidation of diazoquinones 1 to their quinones 2, the corresponding epoxy quinones 3 are formed directly and not as secondary overoxidation products of the quinones 2 since the latter are persistent towards DMD under these conditions; this novel oxidation is rationalized in terms of the ambident nucleophilic nature of the diazoquinones and corroborated by MO (AMI) calculations.
- Adam, Waldemar,Hadjiarapoglou, Lazaros,Mielke, Karsten,Treiber, Alexander
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p. 5625 - 5628
(2007/10/02)
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- Macrolactonization-Transannular Aldol Condensation Approach to the Taxane AB Ring System
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Cyclohexenone 13, containing the fully elaborated taxane A ring, was prepared from 4,4-dimethyl-2-cyclohexenone (43percent for four steps); two copper-catalyzed Grignard conjugate addition reactions on crowded dienones 8 and 11 were employed as key transformations.Elaboration of 13 to the chloro keto acid 6 was achieved via an epoxidation/regioselective chloride-mediated epoxide ring opening/Jones oxidation protocol (42percent for three steps).Macrolactonization (59percent) followed by chemoselective transannular aldol condensation within the resulting 11-membered bicyclic keto lactone 4 under thermodynamic control (51percent) resulted in closure of the taxane B ring to afford the target 2(5H)-furanone 3.
- Chai, Ki-Byung,Sampson, Paul
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p. 6807 - 6813
(2007/10/02)
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- DIELS-ALDER REACTIONS OF 4,4-DIMETHYL-2-CYCLOHEXENONES. A DIRECT ROUTE TO THE 4,4-DIMETHYL-1-DECALONES
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Diels-Alder additions to enones 1 and 2, synthetic equivalents of the synthon 3, were studied using a variety of hydrocarbon dienes.The two enones have been shown to be effective and synthetically useful dienophiles.Spectroscopic (especially 1H and 13C magnetic resonance) and chemical techniques were used to define unambiguously the full structures of the adducts.The structures of the various reaction products were used to draw qualitative conclusions about the nature of the transition states involved and the various electronic and steric effects that play a role in the mechanistic course of the Diels-Alder reaction.
- Liu, Hsing-Jang,Browne, Eric N. C.
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p. 1262 - 1278
(2007/10/02)
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- Synthesis of a 4-Acylcyclohexa-2,5-dienone: 3,4-Dihydro-3,3,8a-trimethylnaphtalene-1,6(2H,8aH)-dione
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The synthesis is reported of 3,4-dihydro-3,3,8a-trimethylnaphtalene-1,6-(2H,8aH)-dione (11; R = Me).This is the first isolated 4-acylcyclohexa-2,5-dienone, a class of compounds postulated particularly as intermediates in the Fries and photo-Fries rearrangements.The dienone undergoes very easy acyl cleavage in the presence of bases or dilute acids, to form a phenolic acid (18).Similar cleavage, or a retro-Fries migration of the acyl group from the 4-position to the dienone ring oxygen, has prevented isolation of 4-acetyl-4-methylcyclohexa-2,5-dienone.Concentrated aqueous sulphuric acid causes rearrangement of the dienone to 3,4-dihydro-8-hydroxy-3,3,5-trimethylnaphtalen-1(2H)-one (13; R = Me), via recyclisation of 18.In trifluoroacetic acid solutions the same product (13, R = Me) is formed from the dienone by direct, formal dienone-phenol rearrangement.
- Waring, Anthony John,Zaidi, Javid Hussain
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p. 631 - 640
(2007/10/02)
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- The Synthesis of 4-Methylcyclohexa-2,5-dienones from Phenols
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4-Alkyl phenols have been converted by a known sequence into 2-aryloxyisobutyric acids and thence, by bromination at the 4-position of the aromatic ring with concomitant intramolecular lactone formation, into masked 4-alkyl-4-bromocyclohexa-2,5-dienones.Reaction with lithium dimethylcuprate, as a model for other lithium dialkylcuprates, replaces the bromine by a methyl group.Hydrolytic demasking gives 4-alkyl-4-methylcyclohexa-2,5-dienones.The method should allow the general synthesis of 4-alkylated cyclohexa-2,5-dienones from phenols via a nucleophilic alkylation step.This contrasts with an existing route which uses electrophilic alkylation of phenols or phenoxide ions by a restricted range of reactive alkylating agents.
- Islam, M. Majharul,Waring, Anthony J.
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p. 768 - 783
(2007/10/02)
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- Anti-androgen compounds
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Compounds of the formula STR1 in which each R is hydrogen, C 1 -C 4 alkyl, C 1 -C 4 alkoxy, hydroxy, cyano, or halo, and both R groups are identical and are symmetrically located; R 1 is C 1 -C 3 alkyl and R 2 is methyl, or R 1 and R 2 taken together are (CH 2 -- n in which n is an integer from 4 to 6; and (1) X c and X d are hydrogen, and the combination of X a and Y a and of X b and Y b each represents a double bond, subject to the limitation that, when R 1 is C 1 -C 3 alkyl and is other than methyl, X c, X d, and R 1 are all in an α-configuration; or (2) X a, X b, X c, X d, Y a, and Y b are hydrogen, subject to the limitation that both X c and X d are in an α-configuration, both X a and X b are in an α-configuration or in a β-configuration, and, R 1, when it is C 1 -C 3 alkyl, is in an α-configuration, are useful in inhibiting the action of androgens or are intermediates to such anti-androgen compounds.
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