Enals double bond

Very few enzyme-catalysed reactions involving the reduction of alkenes have achieved any degree of recognition in synthetic organic chemistry. Indeed the only transformation of note involves the reduction of a, (3-unsaturated aldehydes and ketones. For example, bakers yeast reduction of (Z)-2-bromo-3-phenylprop-2-enal yields (S)-2-bromo-3-phenylpropanol in practically quantitative yield (99 % ee) when a resin is employed to control substrate concen-tration[50]. Similarly (Z)-3-bromo-4-phenylbut-3-en-2-one yields 2(5), 3(,S)-3-bromo-4-phenylbutan-2-ol (80% yield, >95% ee)[51]. Carbon-carbon double bond reductases can be isolated one such enzyme from bakers yeast catalyses the reduction of enones of the type Ar—CH = C(CH3)—COCH3 to the corresponding (S)-ketones in almost quantitative yields and very high enantiomeric excesses[52]. 

Selective oxidation of allylic alcohols.1 This zircononcene complex when used in catalytic amount can effect an Oppenauer-type oxidation of alcohols, including allylic ones, in the presence of a hydrogen acceptor, usually benzaldehyde or cyclohexanone. This system oxidizes primary alcohols selectively in the presence of secondary ones. Thus primary allylic alcohols are oxidized to the enals with retention of the configuration of the double bond in 75-95% yield. The method is not useful for oxidation of propargylic alcohols. 

Conjugate hydrogenation. The combination of zinc and NiCl2 (9 1) effects conjugate reduction of a,(3-enones in an aqueous alcohol in which both the enone and product are completely soluble. Ultrasound increases the rate and the yields. Presumably the salt is reduced to a low-valent form that is absorbed on the zinc. No reduction takes place with a 1 1 Zn-NiCl2 couple. The method is not applicable to a,(3-unsaturated enals. Isolated double bonds are also reduced by this method, but this hydrogenation can be inhibited by addition of ammonia or triethylamine. 

Addition to enals. s Conjugate addition of lithium dialkylcuprates is favored over 1,2-addition by use of low temperatures and a nonpolar solvent (pentane). However, 1,2-addition becomes more important if the double bond is trisubstituted. 

The condensation of a,/3-unsaturated carbonyl compounds with phenols has received much attention as a route to chromenes. Problems associated with the instability of alkenals have been largely overcome by the use of masked forms of these reagents. The derived acetals have proved particularly useful in this respect. It is also advantageous to use the acetals of hydroxyalkanals and to introduce the double bond during the course of the reaction. Thus, 4,4-dimethoxy-2-methylbutan-2-ol serves as the equivalent of the unstable 3-methylbut-2-enal. 

By far the most important compounds of this type are the a,/ -unsaturated aldehydes and ketones, of which the simplest general representation is (8) and (9) respectively. Branching may be present at either or both of the a- or p- positions as for example in 2-ethylhex-2-enal (10) and 4-methylpent-3-en-2-one (11). Compounds where the carbon-carbon double bond is terminal are acraldehyde [(8), R=H], and the alkyl (or aryl) vinyl ketones [(9), R = H, R = alkyl, aryl etc.]. 

Oxidation of ailylic CH3 groups.1 "This reaction is possible with minimal side reactions with SeC>2 supported on silica g l and r-butyl hydroperoxide (70%) in hexane or CH2C12. The main products are primary alcohols, sometimes accompanied by the corresponding a,/ -enals. The ailylic alcotiol formed has the more stable configuration of the double bond (second example). 

Conjugate reduction of a,/l-enals and -enones. Tri-n-butyltin hydride in the presence of tetrakis(triphenylphosphine)palladium effects conjugate reduction of a, /J-unsaturated aldehydes and ketones in the presence of a proton source (water, acetic acid). Yields are improved by addition of a radical scavenger.15 Double bonds bearing... 

Reduction of enones. Reduction of CH3Cu, prepared in situ from Cul and CH3Li in THF, with DIBAH in the presence of HMPA results in a form of copper hydride that effects efficient and selective 1,4-reduction of enals, enones, and enoates, and 1,6-reduction of dienones and dienoates. The reagent does not reduce isolated carbonyl groups or double bonds.2 It can also be used for regiospecific preparation of enol silyl ethers from an enone.2... 

The side chain R in these enals contained functionalities such as a double bond, or an acetoxy group, which were unaffected. The reaction proceeds again through a vinyl iodonium intermediate which serves as a reactive allyl cation. In this way the umpolung of allylsilanes is achieved and, indirectly, the reactivity of allyl halides is considerably increased. 

Note that the suffixes -anal, -enal, and -dienal signify the saturated form and the forms with one and two double bonds, respectively note also that the numbering of the carbon atoms begins from the— CHO, or aldehydic group, at the end of the chain, with the double bonds occurring at the 2 and 6... 

Benzaldehyde yielded no dibenzyl ditellurium. In the reation with 3-phenylprop-2-enal the double bond was hydrogenated and bis[3-phenylpropyl ditellurium was isolated in 48% yield. Ketones reacted only sluggishly under these conditions. Cyclohexanone gave dicyclohexyl ditellurium in a yield of only 11%. 

Compounds with double bonds adjacent to a C=0 group are known as a,p-unsaturated carbonyl compounds. Many a,[3-unsaturated carbonyl compounds have trivial names, and some are shown here. Some classes of a, 3-unsaturated carbonyl compounds also have names such as enone or enal , made up of lene (for the double bond) + one (for ketone) or ene + al (for aldehyde). 

There is some interesting selectivity in this sequence. Only one of the three groups next to the alkene is oxidized and only one ( -) isomer of the enal is formed. No position next to the unsaturated ester is oxidized. All these decisions are taken in the initial cycloaddition step. The most nucleophilic double bond uses its more nucleophilic end to attack SeOi at selenium. The cycloaddition uses the HOMO (7t) of the alkene to attack the LUMO (rc of Se=0). Meanwhile the HOMO (Jt) of Se-0 attacks the LUMO (C-H a ) of the allylic system. 

Addition of acyl anion equivalents (propenal d reagents) to ketones provides general access to a -hy-droxy enones. In an application of this method to pentaimulation, the trimethylsilyl- or ethoxyethyl-pro-tected cyanohydrins of a, -enals were used." The derived tertiary acetates undergo elimination (p-TsOH/benzene) to the divinyl ketones which cyclize in the acidic reaction medium (equation 25)." In some cases the a -hydroxy or a -silyloxy enones underwent cyclization but in much lower yields. Substitution in the ring and on the double bonds is compatible. 

Compounds containing both a C-C double bond and an aldehyde are named as enals, and compounds that contain both a C-C double bond and a ketone are named as enones. The chain is numbered to give the carbonyl group the lower number. 

Oxidations. Alcohols undergo rapid oxidation when exposed to PhI(0Ac)2-Alj03 (with microwave irradiation) or PhlCOAc) with catalytic amount of TEMPO. The latter procedure is mild and selective. Primary alcohols can be converted to aldehydes without oxidizing secondary alcohols. (Z)-Allylic alcohols give (Z)-enals, 1,3-diols afford 3-ketols, and cholesterol is oxidized to the 5-en-one without migration of the double bond. Tertiary cyclopropanols fragment to release carboxylic acid and alkene moieties. ... 

The unsaturated Cg aldehydes and alcohols probably arise in wheat from cleavage of the 9—10 double bond of unsaturated C g fatty acids, linoleic and linolenic acids (Figure 2). Galliard and coworkers (12,13) have partially purified enzyme systems capable of catalyzing these transformations. Lipoxygenase initially converts linoleic and linolenic acids to 9-hydroperoxides which are subsequently cleaved by hydroperoxide lyase to volatile Cg unsaturated aldehydes and 9-oxo-nonanoic acid. The 3-enals are the primary volatile cleavage products from the fatty acids and these are transformed by an isomerase to the more stable 2—enals (14). The 3—enals are rather unstable but Hatanaka et al. (15) have confirmed their presence in plant tissue with authentic samples. The Cg... 

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