Alkenes zinc-acetic acid

Other synthetic routes to benzazepines involving ring expansion of six-membered heterocycles include the action of diazomethane (77CPB321), sulfonium ylides (77H(7)37> or acyl halides (75T1991) on quaternary 3,4-dihydroisoquinolines that of sulfoxonium ylides on quaternary quinolines (74IJC(B)1238) and the zinc-acetic acid reduction of quaternary 1-acyltetrahydroisoquinolines (77BSF893). Photoaddition of acyl- or aryl-nitrenes to the exocyclic alkene bond of 2-methylene-1,2-dihydroquinolines results in ring expansion to... 

In similar fashion, (55)-1,2,3,4-tetra-0-acetyl-5-bromo-/ -D-xylopyranose (28) gave27 the alkenes 162 and 163 as major products on treatment with DBU and zinc-acetic acid, respectively (see Scheme 26). 

The situation is more complicated in the case of 5-bromohexopyranose derivatives, because products of both endo- and exo-elimination may be formed, and available evidence indicates that, with 5-bromo-/ -D-glucopyr-anose esters, base-catalyzed elimination favors the production of endo-alkenes following loss of axial hydrogen and bromine atoms. Alternatively, treatment with zinc-acetic acid gives, mainly, the products of exo-elimination.90 From the acetate 164 (R = Ac) and the benzoate 164 (R = Bz), the 4-enes (165, R = Ac, Bz) were both obtained in 65% yield following treatment with 1,5-diazabicyclo[5.4.0]undec-5-ene, whereas zinc - acetic acid afforded the 5-enes (166, R = Ac, Bz) in 59 and 67% yield. The isomeric endo products 167 (R = Ac, Bz) were isolated in 15 and 11% yield, and, from the... 

Mixed bromides 33, 34 formed from either 1 -0-acetyl-2,3,5,6-tetra-O-benzoyl-/ -D-glucose or -galactose, also undergo preferential endo-elimina-tion on treatment with DBU, and give the alkene (170). As in the pyranose series, exo-elimination is favored with use of zinc - acetic acid, and geometric isomers (171) are the main products and are formed93 together with the 3-deoxy-3-ene (172). 

An entirely different approach to pyranoid 4-enes depends on eliminations from 5-bromo compounds obtained from pyranoid hexuronic acid derivatives by photobromination. Treated with zinc-acetic acid, the bromide 249 gives the glycal -like 250 (62%), while the 4-acetoxy compound 251 is formed when DBU is used to promote elimination. Similarly, base treatment of penta-0-acetyl-5-bromo-/J-D-glucose with DBU causes the analogous loss of hydrogen bromide and formation of the 4-acetoxy-4-ene, but use of zinc-acetic acid affords mainly the 5-exo-methylene alkene by the alternative available elimination process.237... 

Low -molecular-weight ozonides are explosive and are theretore not isolated. Instead, the ozonide is immediately treated with a reducing agent such as zinc metal in acetic acid to convert it to carbonyl compounds. The net result of the ozonolysis/reduction sequence is that the C=C bond is cleaved and oxygen becomes doubly bonded to each of the original alkene carbons. If an alkene with a letrasubstituted double bond is ozonized, two ketone fragments result if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result and so on. 

Compound A, C10H 8O, undergoes reaction with dilute H2S04 at 50 °C to yield a mixture of two alkenes, QpH ir>. The major alkene product, B, gives only cyclopentanone after ozone treatment followed by reduction with zinc in acetic acid. Identify A and B, and write the reactions. 

Ketene itself and simple alkylketenes are inert towards nonactivated alkenes. F or the preparation of cyclobutanones formally derived from ketene or an alkylketene and nonactivated alkenes, the more reactive dichloroketene or alkylchloroketenes can be used. The corresponding a,a-dichloro- or oc-chlorocyclobutanones can readily be dechlorinated by treatment with zinc in acetic acid, or tributyltin hydride in near quantitative yields. F or example cycloaddition of substituted cyclohexene to dichloroketene gave dichlorocyclobutanone 1 which was dechlorinated to 2 with zinc.13,18 Likewise cycloaddition of cycloalkcnes to chloro(methyl)ketene gave 3 which was dechlorinated to 4.14... 

Alkenes from diols. Reaction of v/c-diols with two secondary hydroxyls or one primary and one secondary alcohol with chlorodiphenylphosphine (2 equiv.), imidazole (4 equiv.), and iodine (2 equiv.) results in alkenes. The reaction presumably involves a vic-iododiphenylphosphinate, which can be isolated in some cases and converted to an alkene with zinc in acetic acid. 

Two examples using azo compounds in allylic amination reactions will be presented in the following. Leblanc et al. have used the more reactive trichloro derivative 82 of DEAD, 81, and found that the ene reaction proceeds at various temperatures and without any Lewis acid catalyst present, for both cyclic and acyclic alkenes to give allyl amines in good yields. The reaction of the alkene 85 with 82 gave the allylic aminated compound 86 in 85 % yield (trans. cis = 85 15) (Eq. (21)) [53f. The allyl amine 87 was formed in good yield after treatment with a suspension of zinc powder in acetic acid solution. 

Ozone, O3 Adds to alkenes to cleave the carbon-carbon double bond and give ozonides, which can be reduced with zinc in acetic acid to yield carbonyl compounds (Section 7.9). 

Deoxygenation of eptoxides to alkenes - is occasionally an important process in the synthesis of complex molecules, and a number of reagents for this transformation have been developed. Conventionally, multistep deoxygenation using hydrogen bromide and zinc in acetic acid has been used. - How-... 

Alkenes are converted to epoxides by oxidation with peroxy acids, and thereby they are protected with regard to certain chemical transformations. Alkaline hydrogen peroxide selectively attacks enone double bonds in the presence of other alkenes. The epoxides can be transformed back to alkenes by reduction-dehydration sequences or using triphenylphosphine, chromous salts, zinc, or sodium iodide and acetic acid. A more advantageous and fairly general method consists, however, of the treatment of epoxides with dimethyl diazomalonate in the presence of catalytic amounts of binuclear rhodium(II) car-boxylate salts. This deoxygenation proceeds under neutral conditions and without isomerization or cy-clopropanation of the liberated alkene (Scheme 97). Furthermore, epoxides can be converted to alkenes with the aid of various metal carbonyl complexes. Thus, they may be nucleophilically opened with... 

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