McMurry cyclization

The synthesis of dithienylethenes bound to heteroanalogs of cyclopentene, in particular to the 2,5-dihydrothiophene ring, also deserves notice. Diketone 135 was prepared by the replacement of the bromine atoms in two molecules of 134 by the sulfide anion followed by the McMurry cyclization to form thiacyclopentene derivative 136 (03OL1435). Diiodide 137 is also successfully used as the synthon for the subsequent functionalization this compound is involved in the Suzuki reaction to... 

The McMurry cyclizations shown in Figure 14.50 are quite interesting from a synthetic point of view. The cyclization product B of ketoester A is a 10-membered enol... 

As in the porphycene series one method to obtain an isocorroie is based on the McMurry coupling of a tetrapyrroledicarbaldehyde, e.g. 1.2 4 The reductive cyclization forms a C —C double bond bridge leading directly to the isocorroie, e.g. 2, which is at the correct oxidation level. 

The same porphyrin 20 is obtained when 3,4-diethylpyrrole-2,5-dicarbaldehyde is cyclized under McMurry conditions.8a,b Although the yield of this reaction is very low, the simplicity of this procedure and the possibility of optimizing the reaction conditions could open an attractive alternative route leading to this type of expanded porphyrins. 

A third method for the synthesis of cycl[3.2.2]azines, from iV-(aroylmethyl)pyridinium salts via indolizines, involves intramolecular (reductive) McMurry coupling of the latter. For example, 3,5-dibenzoylindolizines, obtained from 2-benzoyl-iV-phenacylpyridinium bromide as shown (Scheme 91), are cyclized using zinc and titanium(iv) chloride to give the 3,4-diphenylcyclazines 352 in high yield (>90%). The reaction cannot be applied, however, to... 

Reductive Cyclizations of Dicarbonyl Compounds (Pinacol and McMurry Couplings) 529... 

The proposed catalytic mechanism for intramolecular McMurry reaction begins with the reduction of TiCl3 by zinc metal to generate the activated titanium species A-19. Reductive cyclization of the dicarbonyl substrate forms the McMurry coupling product, along with titanium oxide complex B-15. To close the catalytic cycle, the oxide complex B-15 is converted to TiCl3 by Me3SiCl (Scheme 63).8d,8e... 

The McMurry reaction (the reductive dimerization of carbonyl compounds giving olefins after treatment with the low-valent titanium reagents TiCla/Mg and TiCla/Zn in THF at 40°C) (CR891513) is a key step in the synthesis of photochromic cyclopentene derivatives. Later, it has been shown that TiCU, which is easier to handle than TiCls, can be used. The cyclization products are formed in 50-60% yields (98S1092,03EJO155). The advantages of this method are that the reactions can be scaled and inexpensive starting materials can be used. 

Scheme 4 gives the endgame from Corey s cyclization product 4 (Scheme 1) to the marine natural product scalarenedial (20, [15]), which is a strong fish feeding deterrent. The Barton-McCombie process [16] was employed to deoxygenate at C3, followed by conversion of the ketone to the vinyl triflate 18 with PhNTf2 (McMurry s method [17]). After hydroxy-desilylation of 18, the carbonylation to 19 was catalyzed by Pd(dppp) (see above). The vicinal... 

Fiirstner reported the first McMurry-type reactions working with 5-10 mol% of titanium trichloride and stoichiometric amounts of zinc powder in the presence of chlorotrimethylsilane. The amount of TiCl3 could be reduced to 2 mol% when (ClMe2SiCH2)2 was used as a reagent [125, 131]. At the same time, Burton and coworkers reported atom transfer radical additions of perfluoroalkyl iodides 39 to alkenes 40 catalyzed by 20 mol% of a low-valent titanium compound generated from TiCLt and zinc powder affording 41 in 10-85% yield (Fig. 13). A tandem radical addition/5-exo cyclization/iodine transfer reaction with diallyl ether proceeded in 66% yield [132]. 

A two-step synthesis of cicerfuran was achieved using McMurry cross coupling of 4-benzyloxy-2-hydroxy-benzoaldehyde with 6-methoxybenzo 1 3]-dioxole-5-carbaldehyde, followed by an oxidative cyclization <07JOC10283>. 

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