Hydroxy-ketones => alkenes

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hetni-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18], /-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19],... 

When pyrolyzed, P-hydroxy alkenes cleave to give alkenes and aldehydes or ketones. Alkenes produced this way are quite pure, since there are no side reactions. The mechanism has been shown to be pericyclic, primarily by observations... 

Alkenes have also been converted to more highly oxidized products. Examples are (1) Treatment with KMn04 in aqueous acetone containing acetic acid gives a-hydroxy ketones. (2) 1,2-Disubstituted and trisubstituted alkenes give a-chloro ketones when oxidized with chromyl chloride in acetone RCH=CR R"—> RCOCCIR R". (3) a-Iodo ketones can be prepared by treating alkenes with... 

Primary nitro compounds are good precursors for preparing nitriles and nitrile oxides (Eq. 6.31). The conversion of nitro compounds into nitrile oxides affords an important tool for the synthesis of complex natural products. Nitrile oxides are reactive 1,3-dipoles that form isoxazolines or isoxazoles by the reaction with alkenes or alky nes, respectively. The products are also important precursors for various substrates such as P-amino alcohols, P-hydroxy ketones, P-hydroxy nitriles, and P-hydroxy acids (Scheme 6.3). Many good reviews concerning nitrile oxides in organic synthesis exist some of them are listed here.50-56 Applications of organic synthesis using nitrile oxides are discussed in Section 8.2.2. 

Thus, isoxazolines are converted into y-amino alcohols and (3-hydroxy ketones stereoselec-tively. However, the intermolecular cycloaddition involving 1,2-unsymmetrically substituted alkenes such as trans-cinnamyl alcohol proceeds nonregioselectively to give a mixture of the two regioisomers (Eq. 8.63).98... 

The 1,3-dipolar addition to terminal alkenes of nitrile oxides, generated from nitromethylene derivatives of bicycloheptane, provides 9,ll-ethano-13,15-isoxazolinoprostanoids, PGH analogs, with alkyl, phenyl, or additional heterocyclic fragment in the oo-chain (461). Chemical transformations of 9,11-ethano-13,15-isoxazolinoprostanoids furnish prostanoids with bifunctional fragments of P-hydroxyketone and a-aminoalcohol in the oo-chain. The reaction of P-hydroxy ketones with methanesulfonyl chloride gives rise to prostanoids with an enone component in the oo-chain. 9,ll-Ethano-16-thiaprostanoids have been prepared, for the first time, by nucleophilic addition of thiols to the polarized double bond in the oo-chain. The 1,3-dipolar addition to terminal alkenes of nitrile oxides, generated from nitromethylene derivatives of bicycloheptane provides 9,ll-ethano-13,15-isoxazolinoprostanoids with an alkyl, phenyl, or additional heterocyclic fragment in the oo-chain (462). 

Ketone homoenolatesAddition of BuLi to the potassium alkoxide (2) of a 3-hydroxy-l-alkene (1) results in partial isomerization via a to the corresponding... 

Stable acylzirconocene chloride has been utilized by Hanzawa and coworkers [58] for the synthesis of a-hydroxy ketones. Consecutive hydrozir-conation of terminal alkenes or alkynes with Schwartz reagent and in situ carbonylation with gaseous CO at room temperature affords acylzirconocene... 

The reduction of diketones is very complex. They can be partially reduced to ketols (hydroxy ketones) or ketones, or completely reduced to hydrocarbons. Depending on the mutual distance of the two carbonyl groups and reagents used, carbon-carbon bond cleavage may occur and may be followed by recyclizations or rearrangements. Some reactions may result in the formation of alkenes. Quinones react in their own specific way. 

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

Hydroxy ketone HKET NO.-alkene adducts reacting to form OLNN (OLN) ... 

Bei der Herstellung von a-Hydroxy-ketonen durch Acyloin-Kondensation erzielt man durch Zugabe von Chlor-trimethyl-silan meistens bessere Ausbeuten (vgl. Bd. El5, S. 1294ff.). Die a-Hydroxy-ketone fallen dann als l,2-Bis-[trimethylsilyloxy]-l-alkene an. Diese konnen direkt... 

Dipolar cycloaddition reactions between nitrile oxides and alkenes produce 2-isoxazolines. Through reductive cleavage of the N—O bond of the 2-isoxazolines, the resulting heterocycles can be readily transformed into a variety of important synthetic intermediates such as p-hydroxy ketones (aldols), p-hydroxy esters, cc,p-unsaturated carbonyl compounds, y-amino alcohols, imino ketones and so forth (7-12). 

Fluorinated alkenes with one fluorine atom attached to the C = C bond are converted into a-hydroxy ketones by potassium permanganate.92 In this case, the geminal fluorohydrin grouping in 52 eliminates hydrogen fluoride to give the end product 53.92... 

Two tandem alkene metathesis-oxidation procedures using Grubb s second-generation ruthenium catalyst resulted in unique functional group transformations. Use of sodium periodate and cerium(III) chloride, in acetonitrile-water, furnished cis-diols. Oxidation with Oxone, in the presence of sodium hydrogencarbonate, yielded a-hydroxy ketones.296 Secondary alcohols are oxidized to ketones by a hydrogen... 

Oxidation of alkenes.1 A well-ground mixture of KMn04/CuS04-5H20 (2 1) suspended in CH2C12 containing a trace of f-BuOH/H20 effects oxidation of alkenes at 25° to a-diketones or a-hydroxy ketones in modest to high yield. The r-BuOH/ H20 solvent is usually crucial for successful oxidation. In some cases, epoxides are the major products. 

Epothilone A (2) is a natural product that exhibits taxoterelike anticancer activity. A new synthesis of the ketoacid 6, a common C1-C6 fragment used in the total synthesis of epothilone A, was accomplished by directed aldol reaction of acetone with the aldehyde 34 (Scheme 2.3c). The aldol reaction of acetone with the aldehyde 3 in the presence of D-proline proceeded smoothly to furnish the expected aldol product (4) in 75% yield and with greater than 99% ee. Intramolecular aldol reaction of the hydroxy ketone 4 in the presence of pyrrolidine gave the cyclohexenone 5 in good yield. Protection of the alcohol as a TBS ether followed by oxidation of the alkene then produced the desired ketoacid (6). 

W(NAr)(=CHBuO(OR)2 (R = CMe(CF3)2, 50) are highly active catalysts for the metathesis of internal alkenes (equation 16), and also effect the stereoselective olefmation of hydroxy ketones (equation 17). The reactivity of these catalysts can be tuned by varying the aUcoxide ligands for example, when R = Bn, the complex acts only upon strained cyclic alkenes and is a highly effective ring-opening metathesis polymerization (ROMP) catalyst (see Metathesis Polymerization Processes by Homogeneous Catalysis). 

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