Ketones, unsaturated selective

Me3SiSCH2CH2SSiMe3, Znl2, Et20, 0-25°, 12-24 h, high yields. Less hindered ketones can be selectively protected in the presence of more hin-i dered ketones. o , 3-Unsaturated ketones are selectively protected (94 1, 94 4) in the presence of saturated ketones by this reagent. ... 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Substrate Conversion [%] Saturated ketone [%] Saturated alcohol [%] Unsaturated Selectivity alcohol [%] [%] ... 

As for some of the monodentate phosphine-based catalysts, ds-[Ru(6,6 -Cl2bpy)2(0H2)2][CF3S03]2 was found to require water for the best catalytic activity in the reduction of aldehydes and ketones [57]. Aldehydes and ketones were found to be hydrogenated, with reasonable yields. Unsaturated aldehydes were reduced with selectivity towards the unsaturated alcohol, whereas unsaturated ketones showed selectivity towards the saturated ketones. 

Further selectivity is needed if the enol component is an unsymmetrical ketone. Some selectivity can be achieved by choice of acid, favouring the more substituted enol, or base, favouring kinetic enolate formation on the less substituted side. The acid 32 was used at a very early stage of Woodward and Eschenmoser s synthesis5 of vitamin Bi2. Standard a -unsaturated carbonyl disconnection revealed unsymmetrical ketone 33 and unenolisable but very electrophilic glyoxylic acid 34 available as its hydrate. In acid solution reaction occurred very selectively indeed. 

Figure 13-2 Gaschromatogram (a) and SlM-speclrum (b) of two isomers of an unsaturated ketone after selective enrichment, m/e = relative molecular or fragmental mass t = time.

The Baeyer-Villiger rearrangement of cyclohexanone and acetophenone with TS-I/H2O2 proved to be poorly selective [117]. Notably, Ti-P and Sn-P have different chemoselectivities in the oxidation of unsaturated ketones, leading selectively to corresponding epoxides and lactones, respectively [118]. The different oxidation pathways were attributed to the preferential adsorption of hydrogen peroxide on Ti-sites and of the carbonyl group on Sn-sites. 

P,P-Bis(alkylthio)-a,P-unsaturated ketones are selectively reduced in a 1,2-manner to the corresponding allylic alcohols by tritiated NaBH4 in ethanol. ° Enaminones are similarly reduced by tritiated... 

The ate complex LiAlHBu"Bu 2 is prepared from DIBAL-H and n-butyllithium in either THF or toluene-hexane. This reagent is more effective for selective 1,2-reduction of enones to the corresponding allylic alcohol than is DIBAL-H alone. The reagent also reduces esters, lactones and acid chlorides to the corresponding alcohols, and epoxides to the respective alcohols. a,p-Unsaturated ketones derived from dehydration of aldol products from I-(arylthio)cyclopropanecarbaldehydes and ketones were selectively reduced by this ate complex or by DIBAL-H itself, yielding the allylic alcohols with minor... 

The a,p-unsaturated ketone is selectively reduced to afford the ally lie alcohol in excellent diastereoselectivity (98 % de). Owing to the use of enantiopure 20, one single enantiopure diastereomer is obtained. 

Seebach and cowotkers reported that the doubly lithiated 2-propenethiol (13) reacts preferentially in the y-position with a variety of electrophiles, including aldehydes and ketones. The selectivity is 67-15%P The magnesium derivative (14), readily accessible by addition of MgBr2 (1 equiv.), reacts with aldehydes and ketones to give a-adducts almost exclusively (more than 90%). Successive addition of a saturated or a,p-unsaturated aldehyde or ketone followed by methyl iodide to a solution of the magnesium derivative at -80 °C affords very high yields of the a-adducts, which are synthetic precursors for vinyloxiranes (Scheme 5). ... 

P.Y-Unsaturated ketones arc selectively rearranged to (E) a.P UQsaiurated ketones by neutral AI Ov This mild method hi particularly usehil for the rearrangement of trisubstituicd alkenes. 

Dipolar cycloaddition reactions are interesting processes for the preparation of several heterocyclic systems due to the generally mild reaction conditions and the simultaneous formation of several bonds in a single operation. The preparation of pyrrolidines via cycloaddition reactions of azomethine ylides is a well known process and has been extensively studied. A strategy based on the reaction of azomethine ylides with a,P-unsaturated ketones was selected by Hollinshead... 

Hara et al. found that, in the presence of cyanuric fluoride, alkenylboronic acids 310 reacted with a,/3-unsaturated ketones 311 through 1,4-addition to give 7,5-unsaturated ketones 312, selectively. The reaction proceeded under mild conditions, and a variety of functional groups could be introduced into the products (Equation 59) <1996SL993>. 

The procedure described here illustrates the preparation of mixed lithium arylhetero(alkyl)cuprate reagents and their reactions with carboxylic acid chlorides. These mixed cuprate reagents also react with a,a -dihromoketonea, primary alkyl halides, and a, 8-unsaturated ketones, with selective transfer of only the alkyl group. 

Reduction of Other Jt-Bonded Functional Groups. In acidic media (i.e. pH < 4), aldehydes and ketones are selectively reduced to alcohols (eq 21). a, 8-Unsaturated ketones are reduced primarily to allylic alcohols (eq 22) except cyclohexenones, which give mixtures of allylic and saturated alcohols. Allylic ethers are... 

Recently the addition of terminal alkynes (109) to acid chlorides (117) was reported using nano-ZnO under solvent-free conditions at room temperature to afford the corresponding (Z)-p-Cl-a,p-unsaturated ketones (118) selectively (Scheme 9.36) (Hosseini-Sarvari and Mardaneh 2011a). An important aspect of this method was... 

The selective reduction of polyfunctional molecules is of continuous interest in organic synthesis, especially in the field of natural products. Tetracyclic systans like styrenoid ketones have been found to be attractive precursors for the synthesis of cardiac glycosides. When the styrenoid bond in the enone was hydrogenated over 10% Pd/C in ethanol, at 1 atm of hydrogen and room tanperature, the double hydrogenated saturated ketone was formed (Scheme 11). However, the conjugated double bond in the unsaturated ketone was selectively reduced when the reaction was performed in piperidine (Scheme 11). 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

ZnCl2 is essential. The reaction was utilized in the synthesis of strophanthidin. Only the q, /3-alkene in the a, fi- and 7, (5-unsaturated ketone 51 is reduced selectively[47], Triethoxysilane is another reducing agent of the enone 52 and simple alkenes[48]. 

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