Amino-imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system

The amino-imino tautomerization of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system through dual hydrogen bonding in n-hexane at room temperature was investigated using UV absorption and fluorescence spectroscopies, fluorescence time-profile measurements, and molecular orbital calculations, with those of the imino-model compound of 3-methyl-4(1H)-pyrimidinimine (3M4PMI). From the experimental results, it was confirmed that the imino-tautomer was formed in the first excited singlet state (S1) state through the double-proton transfer of the dual hydrogen-bonded complex of 4APM with AcOH. The fluorescences of the free 4APM monomer (band maximum at 353nm), imino-tautomer (at 414nm), and 3M4PMI monomer (at 437nm) exhibit the single-exponential decays of 98, 73, and 19ps time constants, respectively. The shorter decay time of the imino-tautomer fluorescence compared with the free monomer one is suggestive of the low activation energy process in the S1 state. From the result of the shortest decay time of the 3M4PMI fluorescence, it can be deduced that 3M4PMI has a non-planar structure in the S1 state. The theoretical calculations on the S0 and S1 state double-proton transfer for the 4APM/AcOH dual hydrogen-bonded system were performed with the use of formic acid (FoOH) in place of AcOH for the sake of simplicity. Only one peak of transition state was resolved in the S0 and S1 states. The energy barrier for the S1 state double-proton transfer of the 4APM/FoOH complex-->3H-4(1H)-pyrimidinimine/FoOH tautomer was estimated to be approximately 2kJmol(-1) using the CIS/6-31G(d) methods. On the other hand, the energy barrier for the S0 state reverse proton transfer of the 3H-4(1H)-pyrimidinimine/FoOH tautomer-->4APM/FoOH complex was estimated to be almost zero kJmol(-1) at B3LYP/6-31G(d) level.     

Photoinduced amino-imino tautomerization reaction in 2-aminopyrimidine and its methyl derivatives with acetic acid

The electronic absorption and fluorescence spectra of 2-aminopyrimidine (2APM), 2-amino-4-methylpyrimidine (2A4MPM), and 2-amino-4,6-dimethylpyrimidine (2ADMPM) with acetic acid (AcOH) were measured in isooctane (2,2,4-trimethylpentane) at room temperature. From the absorption spectra, a hydrogen-bonded complex formation of the 2APM/AcOH, 2A4MPM/AcOH, and 2ADMPM/AcOH systems was recognized in isooctane. The enthalpy changes (-DeltaH) for the complex formation were estimated to be ca. 41.2-45.1 kJ mol-1 and increased in proportion to the numbers of the methyl group introduced into the 2APM. The -DeltaH values refer to the formation of the hydrogen-bonded 1:1 complex between the ring nitrogen atom and NH2 group of the aminopyrimidine and the OH and CO groups of AcOH, respectively. In the 2A4MPM/AcOH double hydrogen-bonded complex the OH group of AcOH is thought to be linked to the ring nitrogen at the 1-postion of 2A4MPM. The fluorescence spectral results indicate that the double proton transfer reaction takes place during the excited state, and gives rise to an imino-tautomer vibration emission, from analogy with the fluorescences of 1-methyl-2(1H)-pyrimidinimine (MPMI), 1,4-dimethyl-2(1H)-pyrimidinimine (DMPMI), and 1,4,6-trimethyl-2(1H)-pyrimidinimine (TMPMI). The fluorescence quantum yields of the imino-tautomers also increased in proportion to the numbers of the methyl group introduced into the 2APM.     

Synthesis,crystal structure,magnetic properties and second harmonic generation of a new noncentrosymmetric coordination compound:Triaqua(4-amino-6-methoxypyrimidine) cuprate(Ⅱ)sulfate

The chemical preparation, crystal structure and infrared spectroscopic characterization of the triaqua(4-amino-6-methoxypyrimidine) cuprate(II) sulfate, [Cu(C_5H_7N_3)(H_2O)_3]SO_4, is reported. The compound crystallizes in the noncentrosymmetric orthorhombic space group P2_12_12_1 with lattice parameters a = 7.9025(3), b = 11.1189(4), c = 12.9720(4) , V = 1139.81(7) ~3 and Z = 4. The Cu(II) cation is fivecoordinated, in an early half-way between square pyramidal and trigonal bipyramidal fashion, by two nitrogen atoms of the 4-amino-6-methoxypyrimidine ligand and three water oxygen atoms. In the atomic arrangement, the organic ligands and the 5-connected Cu centers are linked with each other to give a 1-D corrugated hybrid chain running along the b-axis direction. The chains are interconnected by the SO_4~(2-) anions via O–H...O, O–H...S, C–H...O and N–H...O hydrogen bonds to form layers spreading parallel to the(011) plane. The vibrational absorption bands were identified by infrared spectroscopy. Quantitative measurements of the second harmonic generation(SHG) of a powdered sample at 1064 nm were performed and a relative efficiency of 5.2 times the KDP standard was observed. Magnetic properties were also defined to characterize the complex. Magnetic measurements revealed that this material had a onedimensional antiferromagnetic character. The magnetic parameters were g = 2.11 and 2J/k B = -36 K.     

Excited-state proton transfer via hydrogen-bonded acetic acid (AcOH) wire for 6-hydroxyquinoline

Spectroscopic studies on excited-state proton transfer (ESPT) of hydroxyquinoline (6HQ) have been performed in a previous paper. And a hydrogen-bonded network formed between 6HQ and acetic acid (AcOH) in nonpolar solvents has been characterized. In this work, a time-dependent density functional theory (TDDFT) method at the def-TZVP/B3LYP level was employed to investigate the excited-state proton transfer via hydrogen-bonded AcOH wire for 6HQ. A hydrogen-bonded wire containing three AcOH molecules at least for connecting the phenolic and quinolinic -N- group in 6HQ has been confirmed. The excited-state proton transfer via a hydrogen-bonded wire could result in a keto tautomer of 6HQ and lead to a large Stokes shift in the emission spectra. According to the results of calculated potential energy (PE) curves along different coordinates, a stepwise excited-state proton transfer has been proposed with two steps: first, an anionic hydrogen-bonded wire is generated by the protonation of -N- group in 6HQ upon excitation to the S(1) state, which increases the proton-capture ability of the AcOH wire; then, the proton of the phenolic group transfers via the anionic hydrogen-bonded wire, by an overall "concerted" process. Additionally, the formation of the anionic hydrogen-bonded wire as a preliminary step has been confirmed by the hydrogen-bonded parameters analysis of the ESPT process of 6HQ in several protic solvents. Therefore, the formation of anionic hydrogen-bonded wire due to the protonation of the -N- group is essential to strengthen the hydrogen bonding acceptance ability and capture the phenolic proton in the 6HQ chromophore.     

Studies on the syntheses of heterocyclic compounds. CDXCI pyrimidine derivatives V. Abnormal condensation products of 4-amino-6-chloro-2-methoxypyrimidine with p-nitrobenzenesulfonyl chloride

Condensation of 4-amino-6-chloro-2-methoxypyrimidine (I) with p-nitrobenzenesulfonyl chloride (II) gave, in addition to 6-chloro-2-methoxy-4-(p-nitrobenzenesulfonamido)pyrimidine (III), two abnormal by-products, the structures of which were assigned as 1 -[2-methoxy-4-(p-nitrobenzenesulfonamido)pyrimidine-6-yl]pyridinium N,N-betaine (IV) and N-(p-nitrobenzene-sulfonyl)-β-ureido-β-pyridinium acrylamide N,N-betaine (V).     

Proton translocation and electronic relaxation along a hydrogen-bonded molecular wire in a 6-hydroxyquinoline/acetic acid complex

A hydrogen-bonded network formed between 6-hydroxyquinoline (6-HQ) and acetic acid (AcOH) has been characterized using a time-resolved fluorescence technique. In the bridged hydrogen-bonded complex of cis-6-HQ and AcOH, an excited-state reaction proceeds via proton transfer along the hydrogen bond, resulting in a keto-tautomer (within approximately 200 ps) that exhibits large Stokes-shifted fluorescence. The unbridged complex also undergoes excited-state proton transfer, but the Stokes shift is rather smaller.     

Synthesis and characterization of hydrogen-bonded assemblies of W6S8L6 clusters

Ligand-exchange reactions involving octahedral W6S8 clusters and a family of pyridine-based ligands (isonicotinic acid, isonicotinamide, 4-hydroxypyridine, 4-aminopyridine, 4-pyridineacetamide) have been explored with the goal of preparing compounds that crystallize in hydrogen-bonded arrays. Two new compounds, W6S8(4-pyridineacetamide)6.DMF.4-pyridineacetamide (1) and W6S8(4-aminopyridine)6.4DMF (2), were isolated and characterized by single-crystal X-ray diffraction. Both compounds crystallize in the P2(1)/c space group with a = 16.461(1), b = 33.08(2), c = 13.165(10) A, beta = 103.270(15) degrees for 1 and a = 13.8988(5), b = 13.2791(5), c = 15.6293(6) A, beta = 108.5410(10) degrees for 2. Each compound was further characterized by 1H NMR spectroscopy, elemental (CHN) analysis, and thermogravimetric analysis. Examination of the structures shows that 1 forms a three-dimensional hydrogen-bonded network in which each 4-pyridineacetamide ligand interacts with ligands on neighboring clusters or with the free ligand of crystallization. This is the first hydrogen-bonded network formed from W6S8 clusters. In 2, the amino groups act as hydrogen-bond donors toward DMF molecules of crystallization, but an extended array is not formed. In addition, the binding strengths of these pyridine-based ligands to the W6S8 cluster were studied through quantitative 1H NMR studies of ligand-exchange reactions. A qualitative relationship was found between ligand binding strengths and Hammett substituent constants for this group of ligands.     

Pyridine nucleosides related to 5-fluorouracil

5-Fluoro-2-methoxypyridine ( 3 ) synthesized from 5-amino-2-methoxypyridine was converted to 4-benzyloxy-5-fluoro-2-methoxypyridine ( 12 ) and 2,4-dimethoxy-5-fluoropyridine ( 13 ) by a four step procedure employing the intermediate 5-fluoro-2-methoxy-4-nitropyridine N-oxide (7). Condensation of 3 , 12 , and 13 with 2,3,5-tri-O-benzoyl-D -ribofuranosyl bromide gave, after removal of the protecting groups, 4-deoxy-5-fluoro-3-deazauridine (20), 5-fluoro-3-deazauridine (23) and 5-fluoro-4-methoxy-3-deazauridine (25). Several alkylated and dealkylated derivatives of 3 and 12 were also prepared. Structure proof and anomeric configuration were determined from the uv, nmr, and CD data.     

Pyridine nucleosides related to 5-fluorocytosine

4-Amino-5-fluoro-2-pyridone ( 4 ) [5-fluoro-3-deazacytosine] was isolated as the hydrochloride salt from the dealkylation of 4-amino-5-fluoro-2-methoxypyridine ( 2 ), which was obtained from the reduction of 5-fluoro-2-methoxy-4-nitropyridine-N-oxide ( 1 ). Acetylation of 2 gave 4-acetamido-5-fluoro-2-methoxypyridine ( 3 ), which was condensed with 2,3,5-tri-O-benzoyl-D-ribofuranosyl bromide to give the blocked nucleoside ( 8 ). Removal of the protecting groups gave 5-fluoro-3-deazacytidine. Fusion of the trimethylsilyl derivative of 4 (10), with 2-deoxy-3,5-di-O-p-toluoyl-D-erythro pentofuranosyl chloride gave a mixture of the β and α-anomers 12 and 13 , which were separated and deblocked to yield 5-fluoro-2′-deoxy-3-deazacytidine ( 14 ) and its α-anomer ( 15 ). Several alkylated and acetylated derivatives of 2 were prepared as model compounds for use in the proof of structure.     

Umlagerung von 5-Amino-5-X-pentadienalen zu 2-Aminopyrylium-Salzen

Rearrangement of 5-Amino-5-X-pentadienals to 2-Aminopyrylium Salts Various 2-aminopyrylium salts 7 (X=Cl, Br, I) are available in a simple one-pot procedure by reacting `push-pull' enynes 5 with equivalent amounts of HCl, HBr, or HI. On the other hand, reaction of HF or AcOH with `push-pull' enynes 5 is considerably slower so that an excess of HF or AcOH is needed for the reaction to 7 (X=F, AcO). The 2-aminopyrylium salts 7 are the key intermediates in the postulated rearrangement of 5-amino-5-halogeno-pentadienals 6 to 5-halogenopenta-2,4-dienamides 8 (Scheme 1, bottom), which is vinylogous to the well-known rearrangement of 3-amino-3-X-propenals 2 to 3-X-propenamides 3 (Scheme 1, top).     

Characterization of two hydroxytrichloropicolinic acids: application of the one-bond chlorine-isotope effect in 13C NMR

The structures of 4-hydroxy-3,5,6-trichloropyridine-2-carboxylic acid (1a) and 6-hydroxy-3,4,5-trichloro-2-carboxylic acid (1b) were verified by the NMR analysis of their corresponding methylated and decarboxylated derivatives 2,3,5-trichloro-4-methoxypyridine (5) and 3,4,5-trichloro-2-methoxypyridine (8), respectively. The 6-hydroxy isomer (1a) was found to be in equilibrium with its pyridinone tautomer as evidenced by the formation of significant amounts of 3,4,5-trichloro-1-methyl-6-oxo-1,6-dihydropyridine-2-carboxylic acid methyl ester (6b) on exhaustive methylation. The one-bond chlorine-isotope effect was used and shown to be an effective tool for the identification of chlorinated carbons in (13)C NMR spectra providing an additional tool for solving structural problems in chlorinated compounds.     

Selective reduction of 2-

[formula: see text] NaBH4 reduction of 2,5-bis[(4'-(n-alkoxy)benzoyl)oxy]benzaldehydes produces primarily the rearranged phenol, which does not generate liquid crystalline phases when laterally attached to a polymer backbone. Rearrangement is prevented by quenching the intermediate benzyl alkoxide with a weak acid. For example, 2,5-bis[(4'-(methoxy)benzoyl)oxy]benzaldehyde is selectively reduced to 2,5-bis[(4'-(methoxy)benzoyl)oxy]-benzyl alcohol with less than 5% intramolecular transesterification using 2-3 equiv of NaBH4 in the presence of 20-30 equiv of acetic acid (1:10 NaBH4/AcOH).     

Novel saturated red‐emitting poly(p‐phenylenevinylene) copolymers with narrow‐band‐gap units of 2,1,3‐benzothiadiazole synthesized by a palladium‐catalyzed Stille coupling reaction

Novel poly(p‐phenylenevinylene) (PPV) copolymers derived from 1‐methoxy‐4‐octyloxyphenylene (MOP), 2,1,3‐benzothiadiazole (BT), and trans‐1,2‐bis(tributylstannyl)ethylene were first prepared by a palladium‐catalyzed Stille coupling reaction. The feed ratios of MOP to BT were 99.5:0.5, 99:1, 95:5, 85:15, 70:30, and 50:50. An efficient energy transfer from the 2‐methoxy‐5‐octyloxy‐p‐phenylenevinylene segment to the narrow‐band‐gap units was observed. The poly(2‐methoxy‐5‐octyloxy‐p‐phenylenevinylene‐2,1,3‐benzothiadiazolevinylene) copolymers emitted deep red light. The maximum electroluminescence emission of these PPV copolymers occurred at 659–724 nm and was accompanied by gradual redshifting with an increasing BT concentration. The photophysical properties were examined in comparison with those of copolymers based on BT and fluorene or N‐alkylcarbazole doped with the same BT concentration in the copolymer main chain. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2325‐2336, 2005     

Computational studies of the photophysics of hydrogen-bonded molecular systems

The role of electron- and proton-transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. Adopting indole, pyridine, and ammonia as molecular building blocks, we discuss generic mechanisms of the photophysics of isolated aromatic chromophores (indole), complexes of pi systems with solvent molecules (indole-ammonia, pyridine-ammonia), hydrogen-bonded aromatic pairs (indole-pyridine), and intramolecularly hydrogen-bonded pi systems (7-(2'-pyridyl)indole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections, and the properties of frontier orbitals. A common feature of the photochemistry of the various systems is the electron-driven proton-transfer (EDPT) mechanism: highly polar charge-transfer states of 1pipi*, 1npi*, or 1pisigma* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces and thus ultrafast internal conversion. In intramolecularly hydrogen-bonded aromatic systems, out-of-plane torsion is additionally needed for barrierless access to the S1-S0 conical intersection. The EDPT process plays an essential role in diverse photophysical phenomena, such as fluorescence quenching in protic solvents, the function of organic photostabilizers, and the photostability of biological molecules.     

The use of umbelliferone in the synthesis of new heterocyclic compounds

New coumarin derivatives, namely 7-[(5-amino-1,3,4-thiadiazol-2-yl)methoxy]-2H-chromen-2-one, 5-[(2-oxo-2H-chromen-7-yloxy)methyl]-1,3,4-thiadiazol-2(3H)-one, 2-[2-(2-oxo-2H-chromen-7-yloxy)acetyl]-N-phenylhydrazinecarbothioamide, 7-[(5-(phenylamino)-1,3,4-thiadiazol-2-yl)methoxy]-2H-chromen-2-one and 7-[(5-mercapto-4-phenyl-4H-1,2,4-triazol-3-yl)methoxy]-2H-chromen-2-one were prepared starting from the natural compound umbelliferone. The newly synthesized compounds were characterized by elemental analysis and spectral studies (IR, 1H-NMR and 13C-NMR).     

Competitive formation of 10- and 7-membered hydrogen-bonded rings of proline-containing model peptides

Intramolecularly hydrogen-bonded structures of proline-containing model peptides with a sequence of N-tert-butoxycarbonyl-prolyl-Xaa-NHCH3 [Xaa = Gly (glycyl), Ala (alanyl), Phe (phenylalanyl), Leu (leucyl), Ile (isoleucyl), and Val (valyl)] were studied by proton nuclear magnetic resonance and infrared spectroscopy. Variation of chemical shifts of amide protons with composition change of DMSO-d6/CDCl3 mixed solvents were found to be a good measure of intramolecular hydrogen bonding of peptides in CDCl3 solution. It has been shown that 10- and 7-membered hydrogen-bonded rings, which should have the beta- and gamma-turn like structures in proteins, respectively, form competitively with each other. It is suggested that the equilibrium between the two hydrogen-bonded rings is determined by steric hindrance due to a side chain of the Xaa residue. Free energies for formation of the 10- and 7-membered hydrogen-bonded rings, deltaG10 and deltaG7, were estimated from the solvent composition-dependent change of the chemical shifts. A good correlation between deltaG10 and the occurrence frequencies of residues Xaa at the (i + 2)th position for the beta-turns in proteins has been found.     

选择性合成手性八氢NOBIN和四氢NOBIN

通过部分氢化手性的(S)-NOBIN [(S)-2-氨基-2’-羟基-1,1’-联萘]选择性地合成了(S)-H8-NOBIN和(S)- -NOBIN. 弱极性溶剂(环己烷)和强极性溶剂(2,2,2-三氟乙醇和含乙酸的乙醇)几乎专一地得到(S)-H8-NOBIN (92%). 而中等极性溶剂(甲醇, 乙醇和异丙醇)以中等产率得到(S)- -NOBIN (66%).     

基于甲氧肉桂酸衍生物光控分子构建光响应蠕虫状胶束的结构性质关系

In this work, light-responsive viscoelastic wormlike micelles based on cetyltrimethylammonium hydroxide (CTAOH) and cinnamic acid derivatives, including cinnamic acid (CA), 2-methoxycinnamic acid (2-MCA), 3-methoxycinnamic acid (3-MCA), 4-methoxycinnamic acid (4-MCA), 2, 3-dimethoxycinnamic acid (2, 3-DMCA), 2, 4-dimethoxycinnamic acid (2, 4-DMCA), 2, 3, 4-trimethoxycinnamic acid (2, 3, 4-DMCA), and 3, 4, 5-trimethoxycinnamic acid (3, 4, 5-DMCA), were prepared. The effects of the CA derivative structures, especially the position and number of methoxy moieties, on the formation of wormlike micelles were systematically determined. The CA derivatives facilitated the formation of long and entangled wormlike micelles. ¹H NMR results showed that the CA derivatives participated in the formation of wormlike micelles via insertion of the aromatic moieties into the aggregates. The number of methoxy moieties had a much stronger effect on the viscosity of the wormlike micelle solution than the position of this moiety. The larger the number of methoxy moiety, the smaller was the aggregate. Substituted methoxy moieties increased the steric hindrance between the surfactants and CA molecules, thus hindering the formation of large aggregates. However, the position of the methoxy moiety had a predominant effect on the UV-light-induced transition of the wormlike micelles. Specifically, the ortho-methoxy moiety in the CA molecules dramatically enhanced the efficiency of UV-light-induced trans-cis isomerization. For example, the 2-MCA/CTAOH, 3-MCA/CTAOH, and 4-MCA/CTAOH binary systems (90 mmol·L^-1/100 mmol·L^-1) were gel-like with similar viscosities of around 20 Pa·s, but after UV light irradiation, they were transformed into a fluid with lower viscosity because of the formation of smaller aggregates. However, the irradiation time required for the transition varied significantly, as suggested by the results of viscosity measurements and UV-Vis spectroscopy. The 2-MCA/CTAOH system underwent complete phase transition within 3 h, whereas continuous transitions were observed for the 3-MCA/CTAOH and 4-MCA/CTAOH systems upon irradiation for 24 h. ¹H NMR results suggested that the change in the configuration of MCA in the micelles before and after irradiation was the major cause of the abovementioned difference in the phase transition pattern. Initially, all the aromatic moieties of the trans-2-MCA molecules were deeply inserted into the hydrophobic cores of the micelles in a vertical manner, and the ionized carboxyl moiety was located in the palisade layer because of the electrostatic interactions between CTAOH and trans-2-MCA. In contrast, cis-2-MCA was inserted into the micelles in a horizontal manner, and some of the protons in the aromatic moiety were also transferred from the micellar core to the polar palisade layer. Accordingly, the CTAOH and cis-2-MCA molecules were packed loosely in the aggregates, thereby resulting in the formation of spherical micelles. Similar UV-light-induced transitions were observed for the 3-MCA/CTAOH and 4-MCA/CTAOH systems.     

Synthesis of a new Pt(II) complex containing valganciclovir drug and calf-thymus DNA interaction study using multispectroscopic methods

A new complex, [Pt(valcyte)(DMSO)Cl]Cl, in which valcyte (trade name) served as valganciclovir hydrochloride drug ([2-[(2-amino-6-oxo-3H-purin-9-yl)methoxy]-3-hydroxypropyl](2S)-2-amino-3-methylbutanoate), was synthesized and characterized by different physicochemical methods. Binding interaction of this complex with calf-thymus DNA (ct-DNA) has been investigated by multispectroscopic techniques. The complex displays significant binding properties with ct-DNA. The results of fluorescence and UV–vis absorption spectroscopy indicated that this complex interacted with ct-DNA in a groove-binding mode, and the binding constant was 3.8 × 10⁴ M^?1. Furthermore, the complex induced detectable changes in the CD spectrum of ct-DNA and slightly changed its viscosity which verified the groove-binding mode. Finally, all results indicated that Pt(II) complex interact with DNA via groove-binding mode.     

Electrochemical Chlorination of Physcion – An Approach to Naturally Occurring Chlorinated Secondary Metabolites of Lichens

The electrochemical chlorination of physcion (=1,8‐dihydroxy‐3‐methoxy‐6‐methylanthracene‐9,10‐dione; 1 ) in AcOH and CH₂Cl₂ was investigated by cyclic voltammetry and prep.‐scale electrolysis. This approach provided access to a number of diverse biologically and pharmacologically interesting chlorinated secondary metabolites of lichen. Unlike the only previous literature report on the ‘classical’ chlorination of physcion ( 1 ), which allowed the preparation of 4‐chlorophyscion ( 2b ), 4,5‐dichlorophyscion ( 3b ), and 2,4,5‐trichlorophyscion ( 4 ), the present procedure also gave fragilin (=2‐chlorophyscion; 2a ) and 2,4‐dichlorophyscion ( 3a ), alongside the previously obtained 2b, 3b , and 4 . All of these compounds, except for 2a , were isolated by column chromatography and medium‐pressure liquid chromatography (MPLC) and characterized by spectral data. The preparative electrolysis with a 2 F?mol^?1 charge consumption in AcOH and 10 F?mol^?1 in CH₂Cl₂ may have a practical synthetic utility, since the thus obtained product mixtures can be readily fractioned by column chromatography to afford pure 2b and 4 , respectively. The regioselectivity of the reaction is explained by the resonance stabilization of the corresponding arenium cations ‐ potential products of an electrophilic attack of a ‘positive’ Cl species on the physcion molecule.