Cycloheptene reduction

It has been postulated that these cycloheptenes must be formed via a 7r-allylruthenium intermediate (Scheme 59). The cyclization is initiated by activation of the allylic C-H bond to form the 7r-allylruthenium 234. The 1-exo-dig carboruthenation of the alkynoate 234 produces the hydrido-ruthenium enolate 235. Equilibration of 235 followed by reductive elimination gives the corresponding cycloheptenes 237 and regenerates the cationic ruthenium complex. 

Few examples of total syntheses have been reported that involve an intramolecular nitrile oxide cycloaddition and ensuing reduction to an aminoalcohol. The very first example was reported by Confalone et al. (334) and involved a synthesis of the naturally occurring vitamin biotin (287). The nitro precursor 284 was easily prepared from cycloheptene. When treated with phenyl isocyanate-triethylamine, cycloaddition led to the all-cis-fused tricyclic isoxazoline 285 with high stereoselectivity (Scheme 6.102). Reduction with LiAlFLj afforded aminoalcohol 286 as a... 

Alternatively, ethyl A-(/J-chloroalkyl)carbamates can be obtained by the addition of ethyl N.N-dichlorocarbamate to alkenes, followed by a reduction step with sodium bisulfite or sodium metabisulfite (Na,S205). The same mixture of diastereomeric adducts (antijsyn 62 38) was obtained from (E)- and (Z)-2-butene under UV irradiation74, and the thermal addition by a radical mechanism to cyclohexene, cycloheptene, and cyclooctene was unsatisfactory because of the low yield and/or the low diastereoselectivity72,142. However, the addition to cyclopentene, indene, and acenaphthene in benzene as solvent was stereoselective in all cases the <7.s-adducts were formed initially at 0 C, but on heating the m-ad ducts or performing the reaction at 25 °C the tram-adducts were exclusively obtained142. 

While this methodology could be extended to more-substituted cyclooctene oxides (Scheme 56, Equation 12), examination of cycloheptene oxide (Scheme 56, Equation 13) revealed the need for considerably longer reaction times, leading to reduced yields due to competing carbenoid-insertion pathways (cycloheptanone formation and reductive alkylation) <2002AGE2376, 2003OBC4293>. 

Evidence has been obtained that the photodecarboxylation of benzannelated acetic acids in aqueous solution proceeds via carbanion intermediates 5H-dibenzo[a,d]cyclohepten-5-carboxyl-ic acid, for example, undergoes photodecarboxylation with a quantum efficiency of close to unity. The reactions of the -nitrobenzyl anion, generated by photochemically induced loss of carbon dioxide from -nitrophenylacetate anion, have been examined and a mechanism proposed to account for the formation of the corresponding bibenzyl derivative. Reductive decarboxylation of carboxylic... 

The neutral molecular sieve, aluminum phosphate (ALPO, Chapter 10) can also serve as a shape selective support for metal catalysts. These catalysts are normally prepared by the incipient wetness process followed by appropriate drying and reduction procedures. Fig. 13.19 illustrates the selectivity observed using a Ni/ALPO catalyst to hydrogenate a mixture of styrene and a methyl styrene (Eqn. 13.13). The rates of hydrogenation of cyclic alkenes over a Rh/ALPO catalyst decreased in the order cyclopentene cyclohexene > cycloheptene > cyclooctene but no selectivity was observed in a competitive hydrogenation of a mixture of cyclohexene and cyclooctene. 75... 

While solvolysis of 5-(4-bromophenylsulfonyloxy)cycloheptene in either acetic acid or tri-fluoroacetic acid after lithium aluminum hydride reduction gave cyclohept-4-enol (71%), cy-clohept-3-enol (12%) and cyclohepta-1,3- and -1,4-diene (5 and 12%, respectively), in contrast acetolysis of 4-(4-bromophenylsulfonyloxy)cycloheptene gave, after saponification with sodium hydroxide in methanol/water, a 78 22 mixture of endo- and exo-bicyclo[4.1.0]heptan-2-ol (1) in 74% yield. Solvolysis of 3-bromocycloheptene afforded, after lithium aluminum hydride reduction, cyclohept-2-enol (37%) and cyclohepta-1,3-diene (63%). ... 

The photochemical addition of ethene at 0°C in methylene chloride to the enedione (77) affords a high yield of the adduct (78). This was converted to the monochloro derivative (79) which also undergoes photoaddition of ethene to yield the Z> adduct (80). This on elimination of HCl yielded the quinol (81) which can be oxidised to the quinone (82). Cycloaddition of alkenes (cyclopentene, cyclohexene, and cycloheptene) has been carried out to the same enedione (77) to yield the adducts (83). lyoda et al. have also described a convenient synthesis of the bicyclo-octanediones(84) by a photochemical addition of alkenes to the enedione (77). The adducts (84) can be reduced by zinc in acetic acid to the desired products. Cycloaddition of ethyne to the same enedione followed by reduction affords the bicyclooctanes (85). The photoaddition of alkenes to the dibromo-enedione (86) is also effective and yields, after reduction, the adducts (87). 

Keywords Nickel catalysts, BINAP, Hydrometalation, Reduction, Cyclohexenols, Cyclohepten-ols, DIBALH, Carbon-metal bond, Organoalane... 

This reaction was first reported by Nenitzescu in 1931. It is the formation of an a,p-unsaturated ketone directly by aluminum chloride-promoted acylation of alkenes with acyl halides. Therefore, it is known as the Darzens-Nenitzescu reaction (or Nenitzescu reductive acylation), or Nenitzescu acylation. Under such reaction conditions, Nenitzescu prepared 2-butenyl methyl ketone from acetyl chloride and 1-butene and dimethylacetylcyclohex-ene from acetyl chloride and cyclooctene. However, in the presence of benzene or hexane, the saturated ketones are often resolved, as supported by the preparation of 4-phenyl cyclohexyl methyl ketone from the reaction of cyclohexene and acetyl chloride in benzene, and the synthesis of 3- or 4-methylcyclohexyl methyl ketone by refluxing the mixture of cycloheptene and acetyl chloride in cyclohexane or isopentane. This is probably caused by the intermolecular hydrogen transfer from the solvent. In addition, owing to its intrinsic strain, cyclopropyl group reacts in a manner similar to an oleflnic functionality so that it can be readily acylated. It should be pointed out that under various reaction conditions, the Darzens-Nenitzescu reaction is often complicated by the formation of -halo ketones, 3,)/-enones, or /3-acyloxy ketones. This complication can be overcome by an aluminum chloride-promoted acylation with vinyl mercuric chloride, resulting in a high purity of stereochemistry. ... 

The seven-memberedring product was obtained by tandem rhodium-catalyzed C-H bond activation/cycloisomerization of MCPs. Pyridine-directed C-H activation, intramolecular addition to the C=C bond, ring expansion by P-carbon elimination, and reductive elimination gave the 5-(2-pyridylmethylene)cycloheptene derivative 90 (Scheme 2.63) [102]. Analogous cycloisomerization employing an... 

It was proposed that photoprotonation of cyclohexenes, cycloheptenes, and cyclooctenes involves initial formation of the corresponding E-isomers, which are much more easily protonated than the starting Z-isomers because of the attendant reduction in strain. A fine balance exists between the reactivity of the E-isomers and the acidity of the alcoholic medium. The highly strained and alkyl-substituted (E)-l-methylcyclohexene [f j-lb] is readily protonated in neutral methanol. However, the higher homologs (E)-lc-d, as well as the unsubstituted analogs (Ej-5b-d, require the presence of small amounts of mineral acid (e.g., 0.3-0.6% H2SO4), conditions under which the E-isomers are protonated but the Z-isomers are stable. 

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