Regioselective reduction

The low regioselectivity of the enc reaction of singlet oxygen with mono-alkenes can also be improved by incorporation of trialkylsilyl groups at a vinylic position. Thus, simple vinyl-silanes afforded, after photooxygenation and reduction, regioselectively, /1-silyl allylic alcohols35. The diastereoseleclivity of the titanium-catalyzed reaction of the /1-silyl allylic hydroperoxides is described in Section 4.9.4.2. 

Reduction enanhoseledive 77 Reduction regioselective 375 Reduction selective 42,164 Reduction stereoselective 50 Reduction stereospecilic 60 Reductive ammation 42,233 Reductive coupBng 107 Reductive udmation 200... 

An interesting approach to the synthesis of highly oxygenated cyclohexane derivatives has been developed by Ichihara. Benzoquinone epoxides and epox-ycyclohexenones were obtained by the thermal retro Diels-Alder reaction of epoxide 511, obtained from quinones and dimethylfulvene without isolation of the primary adduct. Intermediate 512 (R = Ac), obtained in this way from hydroxymethyl-benzoquinone, was converted into the natural compound sene-poxide (514) as shown in Scheme 6. Reductive, regioselective cleavage of the diepoxide 513 was achieved by treatment with hydrazine hydrate. The stereoselective total synthesis of crotoepoxide (515) from cyclohexenone 512 (R = Ms), and of other natural compounds with related structure, was described. ... 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

A catalyst, usually acid, is required to promote chemoselective and regioselective reduction under mild conditions. A variety of organosilanes can be used, but triethylsilane ia the presence of trifiuoroacetic acid is the most frequendy reported. Use of this reagent enables reduction of alkenes to alkanes. Branched alkenes are reduced more readily than unbranched ones. Selective hydrogenation of branched dienes is also possible. 

Regioselective coniugale reduction and reductive silylation of a,p-unsaturated ketones, esters, and aldehydes using a stable copper (I) hydride cluster (Ph3P)CuH 6... 

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

The well-known reduction of carbonyl groups to alcohols has been refined in recent studies to render the reaction more regioselective and more stereoselective Per-fluorodiketones are reduced by lithium aluminum hydride to the corresponding diols, but the use of potassium or sodium borohydride allows isolation of the ketoalcohol Similarly, a perfluoroketo acid fluonde yields diol with lithium aluminum hydnde, but the related hydroxy acid is obtainable with potassium borohydnde [i f] (equations 46 and 47)... 

Reduction with zinc dust in aqueous sulfuric acid gives 2-methyl-4-aminothiophenol (28) Condensation with 2-chlorobenzoic acid (Ullmann reaction) leads to 29, which can close to but one thiox-anthone (25) on treatment with sulfuric acid. Although this procedure is longer than the original, the yields are good and the sequence is regioselective. ... 

Aromatic rings can be reduced without difficulty. Major problems connected with these reductions concern maintenence of other functions, control of regioselectivity in polycyclic aromatics, and control of stereochemistry. 

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