Alcohols, unsaturated reduction

The classification is unaffected by allylic, vinylic, or acetylenic unsaturation appearing in both starting material and product, or by increases or decreases in the length of carbon chains for example, the reactions f-BuOH f-BuCOOH, PhCHgOH - PhCOOH, and PhCH=CHCH20H -PhCH=CHCOOH would all be considered as preparations of carboxylic acids from alcohols. Conjugate reduction and alkylation of unsaturated... 

The alcohol-unsaturate couplings developed in our laboratory provide products of carbonyl addition. In contrast, related hydrogen auto-transfer processes provide products of alcohol substitution via pathways involving oxidation-condensation-reduction and the use of preactivated nucleophiles. For recent reviews, see [22-25]. 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

Acrolein (CHj=CHCHO, also known as 2-propenal) is a a,P-unsaturated aldehyde that can be transformed reducfively to saturated or unsaturated alcohols by reduction of the C = 0 or C = C double bonds (Claus 1998). In addition, a,P-unsaturated aldehydes may undergo hydration reactions in aqueous solutions. It was observed that, under acidic (pH12) conditions, acrolein is hydrated to 3-hydroxypropanal (Jensen and Hashtroudi 1976). In a natural subsurface environment, where pH may range from 6.5 to 8.5, the hydration rate of acrolein increases with the pH and its half-life decreases. Based on an experiment to analyze effects of iron on acrolein transformation, Oh et al. (2006) note that, under acidic conditions (e.g., pH = 4.4), acrolein disappears rapidly from solution in the presence of elemental iron (Fig. 16.1). Moreover, the formation of... 

Complex hydrides can be used for the selective reduction of the carbonyl group although some of them, especially lithium aluminum hydride, may reduce the a, -conjugated double bond as well. Crotonaldehyde was converted to crotyl alcohol by reduction with lithium aluminum hydride [55], magnesium aluminum hydride [577], lithium borohydride [750], sodium boro-hydride [751], sodium trimethoxyborohydride [99], diphenylstarmane [114] and 9-borabicyclo[3,3,l]nonane [764]. A dependable way to convert a, -un-saturated aldehydes to unsaturated alcohols is the Meerwein-Ponndorf reduction [765]. 

Esters are completely reduced to alcohols whde unsaturated esters are converted to unsaturated alcohols. Other reduction reactions include conversion of phenyl isocyanate to N-methylanihne ... 

Reduction Carbonyl groups. The carbonyl group (-(C=0)-) is reduced through a reaction that is catalyzed by an aldo-keto reductase requiring NADH as a cofactor. A large number of aromatic and aliphatic ketones are reduced to the corresponding alcohols these reductions are frequently stereospecific. a,P-Unsaturated ketones are typically metabolized to saturated alcohols. 

Alcohols, such as methanol and ethanol, lead to the sole formation of saturated alcohols from unsaturated ketones when the former are present in excess during the reduction. Mixtures of ketones and alcohols are generally formed when 1 equiv. of these proton donors is employed. These alcohols have an acidity comparable to that of saturated ketones, and when they are present, an equilibrium can be established between the initially formed metal enolate and the saturated ketone. The latter is then reduced to the saturated alcohol. Such reductions generally do not occur to a very significant extent when 1 equiv. of r-butyl alcohoP or some less acidic proton donor, such as triphenylcarbinol, is employed. The acidity of the ketone involved, as well as the solubility of the metal enolate in the reaction medium, are important in determining whether alcohols are formed. 

Although diborane (BH3)2 reduces epoxides, it usually gives a mixture of products in addition to the usual alcohols. The reduction of styrene oxide derivatives with diborane has been studied in order to develop a new route to 1,3-diols from epoxides (equation 19).4 a,3-Unsaturated epoxides undergo... 

C. Alcohols from reduction of o ,)6-unsaturated carbonyl compounds References... 

C. Alcohols From Reduction of a. -Unsaturated Carbonyl Compounds... 

ADH was isolated and partially purified from orange juice vesicles and examined for substrate specificity, maximum relative velocity (Vr) and affinity (1/Km) (12) Ethanol is the preferred saturated alcohol for reduction to the aldehyde based on Vr and 1/Km. Unsaturated alcohols, 2-propenol, 2-butenol and 2-hexenol, had comparable to or higher Vr s and l/Km s than ethanol. ADH had 5- to 30-fold greater affinity for saturated aldehydes than the corresponding saturated alcohols, whereas affinities of the unsaturated alcohols and aldehydes were similar. The apparent equilibrium constants (Kapp = 0.003 for ethanol - acetaldehyde pair) favor alcohol formation in the saturated series. Other aldehydes compete with acetaldehyde for the enzyme but the concentration of acetaldehyde is much higher than other aldehydes in juice vesicles and the 1/Km for acetaldehyde is 10 X higher than for other aldehydes found in the juice vesicles. 

Michael addition of methyl vinyl ketone to 24, followed by base-catalyzed cyclization, afforded the j8-diketone 25. Reaction of 25 with phosphorus oxychloride in dimethylformamide gave the vinylogous acid chloride 26 which afforded the corresponding chloro alcohol on reduction with lithium aluminum tri-f-butoxyhydride. Hydrolysis and concomitant dehydration of this chloro alcohol then gave the desired unsaturated ketone (23). 

Homoallylic Alcohols. The Lewis-acid-catalysed ene reaction of monosubstituted and 1,1- or 1,2-disubstituted alkenes with chloral leads to adducts (29) which have potential as precursors of y5-unsaturated (homoallylic) alcohols through reductive dehalogenation (Scheme 9). Phenylglycine can be used as a vehicle to create a... 

Several years ago, we reported the synthesis and synthetic utility of lithium aminoborohydrides (LABs) a new class of powerful, safe, and highly selective reducing agents (2, 3). These reagents performed many of the transformations for which lithium aluminum hydride is usually used. Thus, the following reduction reactions were carried out with LABs aldehydes and ketones to alcohols, esters to alcohols, oc,P>unsaturated ketones to allylic alcohols, a,P-unsaturated esters to allylic alcohols, alkyl halides to hydro-carbons, azides to amines, and epoxides to alcohols. These reduction reactions are summarized in Figure 3. 

Other Unsaturated Alcohols. An improved procedure that has been reported recently for the preparation and utilization of alkali-metal acetylides in liquid ammonia allows the ethynation of sensitive carbonyl compounds to be carried out, in good yield, to give propargylic alcohols. The reduction of ap-acetylenic ketones to (S propargyl alcohols of high enantiomeric purity can be performed with NB-Enantrane (27) [cf. (6) earlier], which is derived from nopol benzyl ether as a low-cost alternative to (—)-a-pinene. a-Allenic alcohols have been prepared by the new routes shown in Schemes... 

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