Zinc enolates, cycloaddition reaction

Another annellation reaction starts from a,a -dibromo-o-xylene, an activated olefin and zinc.63 This reaction, according to the authors proceeds by a Diels-Alder cycloaddition via the o-xylylene (Scheme 15.10), and was employed in carbohydrate chemistry to prepare anthracyclinones analogues.64 No reaction occurs without sonication. Instead of a cycloaddition, another explanation can be postulated, consisting first of addition of a mono-organozinc reagent to the activated olefin, followed by alkylation of the resulting enolate by the second benzylic bromide. 

The cycloaddition of ester enolates with imines is of continuing interest and potassium rert-butoxide may be used to generate the enolate (94S805) but more usually tin, titanium or zinc enolates are employed. When enolates are generated from 2-pyridylthioesters, the stereochemistry of the tin enolate may be greater than and, in some cases, opposite to that for titanium enolates (94T5821). The effect of the lithium, zinc or titanium on the diastereoselectivity in addition reactions of the enolates of a-... 

In contrast with the photochemical cycloaddition reaction of two alkenes, the [2+2] cycloaddition of a ketene and an alkene occurs under thermal conditions. The ketene is formed typically from an acid chloride and a mild base such as EtsN, or from an a-halo-acid chloride and zinc. Cycloaddition with an alkene occurs stereospecifically, such that the geometry of the alkene is maintained in the cyclobutanone product. The regioselectivity is governed by the polarization of the alkene, with the more electron-rich end of the alkene forming a bond to the electron-deficient central carbon atom of the ketene. Thus, the product from cycloaddition of dimethylketene with the enol ether Z-171 is the cyclobutanone m-172, whereas with -171, the isomer trans-lll is formed (3.116). ... 

Another intermediate for which a cycloaddition product provides convincing evidence is the oxyallyl cation. This compound can be made from a,a -dibromoketones on treatment with zinc metal. The first step is the formation of a zinc enolate (compare the Reformatsky reaction), which can be drawn in terms of the attack of zinc on oxygen or bromine. Now the other bromine can leave as an anion. It could not do so before because it was next to an electron-withdrawing carbonyl group. Now it is next to an electron-rich enolate so the cation is stabilized by conjugation. 

Ester-Enolate Cycloaddition The ester enolate-imine cyclocondensation method (Scheme 3.13) provides fS-lactams in good yields with higher stereoselectivity. Various types of esters and imines can be utilized. Some recent reactions have been mediated by zinc [47], rhodium [48], indium [49], and diethylzinc [50]. 

A mild acidic equivalent of the condensation of acetate enolates with aldehydes to give /S-hydroxy-esters has been developed. This consists of a zinc chloride-catalysed cycloaddition reaction between the aldehyde and a keten acetal followed by acid-catalysed hydrolysis (Scheme 30).The method works... 

Hsung and co-workers described the first epoxidation of 1-amidoallenes leading to highly reactive intermediate 292 (Scheme 8.76) [159]. Formation of bicyclic products 291 occurs via iminium enolate 293, which was trapped by cyclopentadiene 290 (X = CH2) or furan 290 (X = 0). [4+3] Cycloaddition of the intermediate 293furnished 291 in good yield as a mixture of mdo-diastereoisomers (-75 25). The best dia-stereoselectivity was found when the reaction was performed in the presence of 2 equiv. of zinc chloride (>96 4). 

Reactions of ketenes with electron-rich alkenes proceed more readily than with nonactivated alkenes and in the case of enol ethers, enol sulfides and ketene acetals, the cycloaddition is regiospecific (see Table 6). With tetraalkoxyethene, cycloaddition with the relatively inert ketene can be carried out 124 however, with less activated alkenes the use of metal catalysts such as zinc(ll) chloride is required for cycloaddition of the parent ketene.115... 

With an acceptor-substituted alkene moiety tethered to the molecule, the intermediate silyl enol ether may undergo an intramolecular [2-I-2] cycloaddition.The silyl-assisted addition of hydrogen halides to cyclopropanes is not restricted to ketones with carbonyl groups as activating function or iodide as nucleophile. Esters and other acid derivatives underwent similar reactions when treated with iodotrimethylsilane alone or in the presence of an additional catalyst such as mercury(II) or zinc(II) chloride.Subsequent treatment of the y-iodo ester with potassium carbonate in tetrahydrofuran gave the respective y-butyrolactones in good yield. 

Addition of an T -allyl-Fp complex to this compound affords an T -aIlyl-Fp-substituted cycloheptatriene system. Two double bonds are involved in an (T -diene)iron complex. The remaining free double bond of the silyl enol ether attacks as a nucleophile at the cationic r -alkene-Fp moiety to form an (Tj -diene)iron complexed cyclopentane annulated cycloheptadienone. Treatment with CAN in methanol under carbon monoxide atmosphere releases the methoxycarbonyl-substituted free ligand (Scheme 4-25). Reaction of the Ti -dienyliumiron intermediate of Scheme 4-25 with an ( , Z)-isomeric mixture of ri -crotyl-Fp proceeds with high diastereoselectivity. Four new stereogenic centers are formed in the course of this formal [3+2] cycloaddition. A hetero [3+2] cycloaddition is also feasible between T -ailyl-Fp complexes and aromatic aldehydes in the presence of zinc chloride or titanium(IV) chloride to provide tetrahydrofuran derivatives (Scheme 4-26). A 1,2-shift of the iron complex fragment occurs in the course of this reaction. Employment of imines affords the corresponding pyrrolidines. ... 

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