Cyclization electro

Scheme 2. Thermally allowed 8 n electron and 6 k electron electro-cyclizations (Woodward-Hoffmann rules).

The Pd-catalyzed electro-cleavage of the C—O bond of allyl aryl ether proceeds smoothly in a DMF-Bu4NBp4-(Mg)-(Stainless Steel) system, giving depro-tected products in 73 99% yield [437]. The sp-sp intermolecular coupling reaction with the Pd water-soluble catalyst prepared in situ from Pd(II) acetate and sul-fonated triphenylphosphine in an MeCN-H2O system yields diynes in 45 65% yields [438]. Similarly, the sp -sp coupling of 2-iodophenols or 2-iodoanilines and terminal alkynes followed by intramolecular cyclization gives indol and furan... 

The Use of Electro-Auxiliaries - Cyclizations Originating from Oxonium Ions to Acyl Anion Equivalents... 

When diallyl diazomalonate or allyl aryldiazoacetates are used as substrates in the rhodium-catalyzed reaction with di(tert-butyl)thioketene, 4-allyl-l,3-oxathiolan-5-one derivatives of type 158 are formed (82). After the initial 1,5-dipolar electro-cyclization, 157a undergoes a subsequent Claisen rearrangement to give the thermodynamically more stable compound 158 (Scheme 5.47). 

It can also be seen that steric hindrance alone is preventing 1,7-electro-cyclization of 60 since no formation of the common intermediate 58 can occur otherwise, 1H-2,3-benzodiazepine 59 would be obtained.To account for these experimental observations Sharp [83JCS(CC)1003 84T3095] has pos-... 

The investigation of type A systems, e.g., 142, led to a new synthetic entry to the basic structure of furanophane natural products 143. The 1,5-electro-cyclization, to give 144, was again only a minor pathway (Scheme 43) (92TL57). 

A tandem sequence of double [2,3]-sigmatropic rearrangement-six-electron electro-cyclization-4 + 2-cycloaddition has been shown to convert acyclic ene-bis(propargyl alcohols) such as (71), via the corresponding bis-sulfenic ethers such as (72), into anthracene (with an intermolecular final cycloaddition) or phenanthrene or related (with an intramolecular cycloaddition) skeletons. The sequence is illustrated in Scheme 12 for synthesis of a steroid skeleton, estra-l,3,5(10)-trien-17-one (73).81... 

Noteworthy, imine 77 is slightly cyclized into 2-phenyl-5-nitroindole 79 in the presence of a base (Scheme 23) (04T247). This is probably due to the reduced electro-philicity of the benzene ring in intermediate anion 78. No cyclization of 78 at the position para to the nitro group has been observed. 

Triazolo[4,5-6]pyridine (5a R = R1 = R2 = R3 = H) and triazolo[4,5-c]pyridine (6a R = R = R2 = R3 = H) were first reported by Chichibabin and Kirsanov (27CB766) and by Bremer <35LA(518)274>, respectively. Traditionally, these heterocycles are prepared by nitrosative electro-cyclization of diaminopyridines (the [N + N—C=C—N] protocol). This procedure continues to be widely employed and is permissive of numerous substituents on the diaminopyridines (Tables 7 and 8), even the cyclization of the pentasubstituted precursor (163) occurred with minimal difficulty (Equations (16) and (17) and Scheme 31). 

Heterocyclic hydrazonyl halides which possess an amino-hydrogen undergo ring closure via 1,3-dipolar nitrile imines which can also be written as 1,5-dipolar ions. 1,5-Electro-cyclization then gives the fused product (Scheme 13). The method has been used for the preparation of 6,7-dihydro-3-aryl-5H-imidazo[2,l-c]-s-triazoles (72JCS(P2)1887),... 

Ruthenium vinylidene 826 derived from terminal alkyne 825 with RuChl -cymenelPPhs undergoes 6rt-electro-cyclization to afford benzothiophene 827 (Scheme 125) <1996JA11319>. Pt-catalyzed double cyclization of diynes 828 gives polycyclic benzodithiophenes 829 <2005TL8153>. 

Treatment of 2-iodothiophene 830 with Z molar equiv of diethyl acetylenedicarboxylate in the presence of Pd catalyst affords tetrasubstituted benzothiophene 834. d y -addition of intermediate 831, generated by oxidative addition of 830 to Pd(0), to the acetylene gives vinylpalladium 832. Cyclization of 832 to 833 and its subsequent reaction with the acetylene affords the final product 834 (Scheme 126) <2003JOC6836>. Intramolecular electro-cyclization of the intermediate 837, produced by the reaction of the propargyl methyl carbonate 835 with 2-thiophene boric acid 836 in the presense of Pd catalyst, affords benzothiophene 838 (Scheme 127) <2004CEJ5338>. 

The initially formed vinyl radical can also be trapped through cyclization onto a carbonyl bond. For example, reaction of electro- or photochemicaUy generated NO3" with 5-cyclodecynone (58) leads to the isomeric epoxy ketones 59 and 60 (Scheme 2.11). ... 

Self-terminating radical oxygenations are not restricted to cyclic alkynes. Electro-and photochemically generated NO3 can be used for the oxidative cyclization of cycloalkyl-clamped alkynes 67 (Scheme 2.12). This reaction leads to formation of anelated tetrahydrofurans (68 with X = or pyrrolidines (68 with X = NTs,... 

Participation by aromatic rings is also possible and there are now several examples of electro M]ic aromatic substitution involving Pummerer intermediates. Equation (20), the alkylation of benzene with dimethyl sulfoxide in trifluoroacetic anhydride, illustrates the process in its inq>lest foim. As widi al-kenes, reaction with aromatics has been more widely exploit in intramolecular versions for the construction of carbocycles and heterocycles. In many cases the sulfoxide precursor is of the keto variety, thus ensuring regiospecificity in the point of cyclization. Equation (21) (formation of a six-monber carbocycle), equation (22) (formation of a six-membered sulfur heterocycle), equation (23) (formation of a six-membered nitrogen heterocycle) and equation (24) (formation of a seven-membered nitrogen, sulfur heterocycle) provide illustrations of the versatility of diis form of intramolecular aromatic alkylation. 

Several electroi ilic selenium-induced cyclizations are known. Th include etherifications, lactoni-zations and lactamidation. Some reasons why selenium electro riiiles are used extensively in cyclizations include consistently good yields, few by-pn cts and mild reaction conditions, as well as the abili to further manipulate the seleno group in a variety of straightforward fashions. 

Heimgartner and co-workers treated a-diazoketones and a-diazoamides 64 with thiones, with and without a catalyst such as Rh(OAc)2 present (1998HCA285). The products were substituted thiiranes 65 and/or substituted 1,3-oxathioles. In all cases, a thiocarbonyl ylide intermediate, which could undergo either a 1,3- or a 1,5-electro-cyclization, was held responsible. The ylide could arise either from addition of a carbene or a carbenoid to S of the thiocarbonyl compound or by loss of N2 from a primary cycloadduct between the diazo and the thiocarbonyl compounds. In one case, such a primary adduct was isolated. The thiirane carboxamides could be desulfurized with (Me2N)3P in tetrahydrofuran (THF) at 60 °C to afford acrylamides 66 (Scheme 11). 

Reaction of thioketones 317 with an excess of diazomethane 318 gave 2,5-dihydro-l,3,4-thiadiazoles 319. Thermolysis of 319 in refluxing benzene for 2h gave thiocarbonyl 3 -methylides 320, which underwent electro-cyclization to give corresponding thiiranes 321 (Scheme 92) <2005EJ01519>. 

The availability of phosphaalkenes led to a second route to phosphiranes with diazoalkanes as cycloaddition partners compare with Scheme 14. The first formed 4,5-dihydro-3//-l,2,4-diazaphospholes 43 decompose under the influence of light and extrusion of nitrogen to form the bis(methylene) phosphoranes 44, which cyclize in a 47t-electro-cyclic closure to the phosphiranes 45 (Scheme 17) <1996CHEC-II(I)298>. For the theoretical background, see Section 1.08.2.1.1, and for analogous reactions see <1995CC25> and <1997CB779>. 

Similarly, pyrido[2,3-aryl isocyanates in a so called tandem aza-Wittig/electro cyclization strategy (90S474) (Scheme 51). 

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