Metal-induced cyclizations

The alkali metal-induced cyclization of the 2,4,6-trisilaheptane (96) yields the 1,2,4-trisilacyclopentane ring system (Scheme 164) (76MI12003), which ring expands on further heating to the 1,3,5-trisilacyclohexane. It also results, rather surprisingly, as the major product of decomposition of bis(trimethylsilyl)silylene (Scheme 165) (80JOM(195)C1). 

Metal-Induced Cyclization 2.21 Five-Membered Rings... 

Scheme 2.12 Alkali metal-induced cyclization to provide 5,10-(trialkylsilyl)dibenzopenta-lenes.

The diazo function in compound 4 can be regarded as a latent carbene. Transition metal catalyzed decomposition of a diazo keto ester, such as 4, could conceivably lead to the formation of an electron-deficient carbene (see intermediate 3) which could then insert into the proximal N-H bond. If successful, this attractive transition metal induced ring closure would accomplish the formation of the targeted carbapenem bicyclic nucleus. Support for this idea came from a model study12 in which the Merck group found that rhodi-um(n) acetate is particularly well suited as a catalyst for the carbe-noid-mediated cyclization of a diazo azetidinone closely related to 4. Indeed, when a solution of intermediate 4 in either benzene or toluene is heated to 80 °C in the presence of a catalytic amount of rhodium(n) acetate (substrate catalyst, ca. 1000 1), the processes... 

The synthesis of hexahydroporphyrins has been achieved using two principal routes. The first route makes use of cyclizations of linear tetrapyrroles which are alkylated at the /i-positions to block the corphin-porphyrinogcn tautomerization after cyclization has been performed. In the second route the metal-induced tautomerization of porphyrinogens is utilized to obtain conjugated hexahydroporphyrins. 

Attempts to cyclize the aldehyde 17 in the presence of acid gave only undefined products. Also copper(II)- and cobalt(II)-induced cyclizations yielded the octaethylisocorrolccarbaldc-hyde metal complexes (see Section 1.8.1.) but not the desired isoporphycenes. A breakthrough was achieved by palladium(II)-induced cyclization of 17 which gave a 1 1 mixture of the Z-... 

Beyond the numerous applications of palladium in transition metal-catalyzed domino reactions there are a lot of other metals inducing domino processes. Ihara et al. found a strategy for the enan-tioselective synthesis of (+)-equilenin catalyzed by mangan and palladium complexes,1841 Whitby et al.1851 initiated domino cyclizations on a zirconocene template and furthermore Scherf et al. generated phenantrones by a nickel-mediated one-pot domino reaction.1861... 

The related cyclization of 2-ethynylanilines 67 also represents one of the usefiil methods for the synthesis of 2-substituted indoles since the precursors are easily prepared from 2-haloanilines 66 by Pd-catalyzed cross-coupling with terminal alkynes. Althou cyclizations of such alkynes are normally effected using Cu(I) or Pd(II) species, Sakamoto showed that in the absence of such metals, base catalysis (e.g., NaOEt) alone can accomplish the same goal. This author now reports that tetrabutylammonium fluoride (TBAF) is capable of inducing cyclization to the indoles 68 without affecting functionalities such as bromo, cyano, ethoxycarbonyl, and ethynyl <99JCS(P1)529>. 

Y. Ohshiro u. T. Hirao, Hcterocycles 22, 859 — 873 (1984) . .Heterocyclic Synthesis by Metal Carbonyl Induced Cyclization Reaction". 

Convenient syntheses of the rare thiazolo[5,4-c]pyridine 279 and 4-aminopyridine-3-thiol (281) systems have been achieved (Scheme 83) [88CI(L)302]. Thus, metalation of 263 followed by quenching with di-isopropylthiocarbonyl disulfide affords, in high yield, compound 278. Base-induced cyclization affords the thiazolopyridine 279, while treatment... 

W. A. Nugent and T. V. RajanBabu, Transition metal-centered radicals in organic synthesis— Ti(lH)-induced cyclization of epoxyolefins, J. Am. Chem. Soc. 770 8561 (1988). 

Two major strategies have been considered in order to obtain metallocorrolates the first one consists of the reaction of the preformed macrocycle with metal ions, while the second involves the oxidative, base induced cycliz-ation of the open-chain tetrapyrrolic precursor dihydrobilin (1,19-dideoxybiladiene-a,c) in buffered alcoholic solution in the presence of metal salts. By proper tuning of the experimental conditions (reaction time, solvent, metal carrier) several metal complexes have been obtained and characterized. 

Here the substrates are 3-silyloxy-l,5-enynes the concept was to combine a metal-induced 6-endo-trig cyclization with a ring contraction by a sigmatropic rearrangement (Scheme 12.12).27 From the cationic intermediate, elimination to the biaryl product can be observed in addition to the aldehyde. 

Metalation of the amidine 87, followed by introduction of 2-methoxyallyl bromide, furnished the intermediate 88, which underwent acid-induced cyclization to the product 89 (Scheme 12), illustrating an interesting approach to polyfunctionalized indoles suitable for further elaboration <20020L1819>. 

A simple synthetic route to 3-arylbenzofurans relies on a halogen-metal exchange with methyllithium to induce cyclization with a tethered benzoyl group (Equation 113) <2005SL2504>. 

As in the research work of Vivet et al., new bicyclic sila-heterocycles were synthesized by Handmann et al. <2000ZNB133, 2000JOM19>. Compound rac-164 underwent a thermally induced cyclization reaction to give rac-7-ethoxy-2,2-dimethyl-2,3,5,7a-tetrahydro-177-3a,6,-sila-inden-4-one 9a (Equation 27). The crystal stmcture of 9a and kinetic studies of the cyclization reaction have also been reported. Starting from 3,6-ethoxy-2,5-dihydropyrazine 163, metallation with 1 molar equivalent of -butyllithium and subsequent treatment with 1 molar equivalent of the bis(chloromethyl)silane produced compound 164 in 50% yield. Pleating neat 164 at 120 °C gave the crystalline 9a in almost quantitative yield. 

An unprecedented lithium-induced cyclization reaction of tribenzocyclotriyne (50) to the hydrocarbon 51 has recently been reported [106]. The proposed aromatic dianion 52 cyclizes and than abstracts protons from the solvent tetrahy-drofuran (THF) and, after further reduction with lithium metal, the resulting intermediate is quenched with methanol to give 51 in 60% yield (see Scheme 23). 

Radical cyclization. The CpCr(CO)3H complex initiates H addition to C=C to form a carbon radical. A 1,6-diene is induced cyclize. The transition metal hydride is used as a catalyst while hydrogen is consumed. 

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