Tamao oxidation

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Leighton and coworkers [217] have also used this approach to develop efficient strategies for the synthesis of polyketide-derived natural products [218]. A main motif of these compounds is a skipped polyol structure, as in 6/2-94 this can easily be prepared by a novel Rh-catalyzed domino reaction of a diallylsilyl ether in the presence of CO, followed by a Tamao oxidation [219]. Thus, reaction of, for example, the silane 6/2-93, which is readily prepared from the corresponding ho-... 

Synthetic transformations of the products of the intramolecular bis-silylation have been examined. The five-membered ring products derived from homopropargylic alcohols were hydrogenated in a stereoselective manner (Scheme ll).90 Oxidation of the products under the Tamao oxidation conditions (H202/F /base)96 leads to the stereoselective synthesis of 1,2,4-triols. This method can be complementary to the one involving intramolecular bis-silylation of homoallylic alcohols (vide infra). 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

A chiral bis(oxazolinyl)phenylrhodium complex was found to catalyze the asymmetric hydrosilylation of styrenes with hydro(alkoxy)silanes such as HSiMe(OEt)2 (Scheme 7).47 Although the regioselectivity in forming branched product 27 is modest, the enantiomeric purity of the branched product 27 is excellent for styrene and its derivatives substituted on the phenyl group. The hydrosilylation products were readily converted into the corresponding benzylic alcohols 29 (up to 95% ee) by the Tamao oxidation. 

Metathesis of a diene containing silicon atoms gives an interesting cyclic compound, whose silicon moiety can be converted into the hydroxyl group by Tamao oxidation [Eqs. (6.14) and (6.15)] ... 

One of the important advantages of the intramolecular alkene silylformylation reaction as an aldol equivalent is that the products are masked 3,5-dihydroxyalkanals, and therefore that no manipulations are required prior to iteration of the process by aldehyde al-lylation to set up the next intramolecular silylformylation. Given that allylsilanes are well-known aldehyde allylation reagents, intramolecular silylformylation employing a diallylhydrosilane would, in principle, allow for the possibility of a tandem silylformyla-tion/allylsilylation reaction. This has been reduced to practice the diaUylsilyl ethers 60 were subjected to the previously developed silylformylation conditions and the unpuri-fied reaction mixtures were subjected to the Tamao oxidation ]36] to provide triols 61... 

Although the Tamao oxidation employs an alkoxysUane, the Fleming protocol converts a-phenylsilane to a fluoro- or carboxysilane before the actual oxidation (equation 79). ... 

A highly functionalized conjugated diene has been synthesized through sequential silicon-tethered ring-closing enyne metathesis by ruthenium-carbene catalyst Ic followed by Tamao oxidation (Equation (9)). ... 

Dixneuf used [RuCl2(/>-cymene)]2 as a catalyst for the reaction of enyne 72a in the presence of imidazolium salt and CS2CO3 and obtained the enyne metathesis product 73a in a high yield. The enyne silyl ether 72b is converted under similar reaction conditions into r/i7ra-compound 73b which after the Tamao oxidation gives diol 74 (Scheme 28). In this reaction, V-heterocyclic carbene should be generated to coordinate to the ruthenium metal, but the actual species for this reaction is not well documented. 

This reaction, depending on the details of the reaction, is known as the Fleming Oxidation, the Fleming-Tamao Oxidation, the Tamao-Fleming Oxidation, or the Tamao-Kumada Oxidation. 

In 1990, the enantioselective intramolecular hydrosilylation of allylic alcohols was successfully applied to the synthesis of chiral 1,3-diol [63] (Scheme 2.6). The reaction of 3-diarysiloxy-1,4-pentadiene (104) catalyzed by (-)-DIOP-[Rh(C2H4)Cl]2 (2 mol%), followed by Tamao oxidation, gave (2S,3R)- 1,3-diol 105 (symanti = 98 2) with 93% ee [63]. 

The 4-exo cyclization of open-chain substrates 63 proceeds in trans-fashion with moderate to excellent selectivity (trans/cis = 77/23 > 99/1 )90. The trans selectivity is dependent on the substitution pattern of R2, R3 and R4. The reactions giving trans-l-aza-2-silacyclobutanes 65 have been applied to the stereoselective syntheses of syn-amino alcohols 66 via the Tamao oxidation as exemplified by the reaction of 63d (R = HMe2Si), affording 66d (76% overall yield in 4 steps, syn/anti = > 99/1) via 65d (trans/cis = > 99/1) in equation 2690. In the case of 3-iV-disilylamino-l-cyclohexene 63f (R = HMe2Si), however, cis-l-aza-2-silacyclobutane 65f is formed exclusively, that is converted to ds-2-amino-l-cyclohexanol (66f) (equation 27)90. 

Intramolecular hydrosilylation of oj-dimethylsiloxyalkynes such as 114, readily derived from alkynol 113, provides a convenient method for the regioselective functionalization of internal alkynes in combination with the Tamao oxidation and other transformations. An example of such a process giving 117 via 115 and 116 is illustrated in Scheme 1487. 

Catalytic asymmetric intramolecular hydrosilylation of dialkyl- and diarylsilyl ethers of bis(2-propenyl)methanol (245) catalyzed by (R, R)-DIOP-Rh or (R)-binap-Rh complex, followed by Tamao oxidation, gives (2S, 3R)-2-methyl-4-pentene-l,3-diol (247) with 71-93% ee and excellent syn selectivity (syn/anti = 95/5- > 99/1) (equation 96)249. The enantioselectivity of this reaction depends on the bulkiness of the silyl moiety, i.e. the bulkier the substituent, the higher is the enantiopurity of the product, except for the case of 2-MeCgH4 R = Me, 80% ee (binap-Rh) R = Ph, 83% ee (DIOP-Rh) R = 2-McC.fiI I4, 4% ee (DIOP-Rh) R = 3-MeC6H4, 87% ee (DIOP-Rh) R = 3,5-Me2C6H3, 93% ee (DIOP-Rh). This methodology is successfully applied to the asymmetric synthesis of versatile poly oxygenated synthetic intermediate 249 (equation 97)249. 

Fraser-Reid s stereocontrolled synthesis of the Woodward reserpine precursor 195 relied upon a tandem 5-exol6-exo radical cyclization of pyranose-derived dienes [76-77]. As outlined in Scheme 36, a,P-unsaturated ester 188 was prepared by free radical coupling of iodide 187 with a tin acrylate. After hydrolysis of 188 (MeONa, MeOH, 100%) to give primary alcohol 189, the silicon tethered diene 190 was installed by silylation. Treatment of 190 with n-BujSnH led to the desired cage molecule 192 in high yield via a 5-exo-trig cyclization to intermediate 191 followed by a 6-exo cyclization. Tamao oxidation of tricycle 192 led to diol... 

Silyl groups, which are non-polar electropositive groups without lone pairs, tolerate many chemical reactions that would not be possible in presence of hydroxy groups. The Fleming-Tamao Oxidation permits silyl groups to be used as masked hydroxy groups , which has found broad application in total syntheses. In addition, enantioselective hydrosilylation of alkenes followed by Fleming-Tamao oxidation allows the preparation of chiral alcohols. 

Strained siletanes may also be used in the Tamao Oxidation instead of halosilanes these intermediates offer a comparable Lewis acidity because coordination of the fluoride ion releases angle strain. 

The series of reactions leading to the 5-silyl-l-pentene - epoxidation, ring expansion, and Peterson elimination -are all stereospecific. Therefore, epoxides with different geometry can be transformed into the corresponding (E)- or (Z)-olefinic silanols <1994BCJ1694, 1991TL4545>. Subsequent Tamao oxidation affords stereodefined pentenols. 

Tin-mediated cyclizations were then carried out on halides 49D and 49E (Bu3SnH, AIBN, heat), followed by Tamao oxidation (for 49D KF, KHCO3, H2O2) or fluoride-induced desilylation (for 49E TBAF) [121, 124]. Here,... 

As shown in Scheme 49, cyclic silyl ethers bearing the ethoxycarbonyl group anti to the 7-substituents 151 upon treatment with silica gel gave acyclic ethyl 7-hydroxy-a-[2-(hydroxydimethylsilyl)]carboxylates 287, and the subsequent reduction with diisobutylaluminium (DIBAL) followed by Tamao oxidation <1990OS96> gave the corresponding acyclic triols 288 <2004TL4329>. 

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