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Chemoselective reactions reduction

These reaction conditions also permit the chemoselective quantitative reduction of benzaldehyde to benzyl alcohol without any concomitant reduction of either acetophenone or 3,3-dimethylbutan-2-one present in the same reaction mixture.83 Additionally, this useful method permits the reduction of aldehyde functions in polyfunctional compounds without affecting amide, anhydride, eth-ylenic, bromo, chloro, or nitro groups.79,80,319... [Pg.60]

Bennasar et al. reported a new radical-based route for the synthesis of calothrixin B (378) (869). This synthesis starts from the 2,3-disubstituted N-Boc indole 1558 and uses a regioselective intramolecular acylation of a quinoline ring as the key step for the construction of the calothrixin pentacyclic framework. Chemoselective reaction of in s/fM-generated 3-lithio-2-bromoquinoline [from 2-bromoquinoline 1559 with LDA] with the 3-formylindole 1558 followed by triethylsilane reduction of the... [Pg.379]

Chemoselective catalytic reduction of a,/3 unsaturated ketones to allylic alcohols is a challenging problem since, but a few exceptions [1-3], this reaction generally proceeds with formation of saturated ketones or saturated alcohols [4]. This reduction indeed is best carried out with stoicheiometric hydrides [4] but even in this case overreduction products are often obtained [5]. Recently, we reported in a preliminary communication [6] the unprecedented observation that a,/3 unsaturated ketones are reduced to the corresponding allylic alcohols by hydrogen transfer from propan-2-ol over MgO as catalyst according to the following scheme ... [Pg.253]

We hope that our survey of the important methods for reduction has shown you that, by choosing the right reagent, you can often react the functional group you want. The chemoselectivity you obtain is kinetic chemoselectivity—reaction at one functional group is simply faster than at another. Now look at the acylation of an amino alcohol (which is, in fact, a synthesis of the painkiller isobu-caine) using benzoyl chloride under acid conditions. The hydroxyl group is acylated to form an ester. Yet under basic conditions, the selectivity is quite different, and an amide is formed. [Pg.630]

Many other examples of chemoselective enone reduction in the presence of other reducible functionalities have been reported. For instance, the C—S bonds of many sulfides and thioketals are readily cleaved by dissolving metals. " Yet, there are examples of conjugate reduction of enones in the presence of a thioalkyl ether group." " Selective enone reduction in the presence of a reducible nitrile group was illustrated with another steroidal enone. While carboxylic acids, because of salt formation, are not reduced by dissolving metals, esters" and amides are easily reduced to saturated alcohols and aldehydes or alcohols, respectively. However, metal-ammonia reduction of enones is faster than that of either esters or amides. This allows selective enone reduction in the presence of esters"" and amides - -" using short reaction times and limited amounts of lithium in ammonia. [Pg.531]

The enantioselection in certain catalytic hydrogenations is very dependent on the pressure of H2 which, in turn, determines the equilibrium concentration, [H2]sat, of H2 in the solution. A thorough recent analysis [2] of solubility and mass-transfer processes has shown, that it is the availability of dissolved H2 in the solution, rather than [ H2]sal, which influences the enantioselectivity, and the former is determined by the relation between the chemical and the mass-transfer rates. The solubility of hydrogen in water is 8 x 10 4 M, which is about 20% of the solubility in MeOH, 3.8 x 10 1 M (both at 20°C, 1 bar total pressure [102]). It follows that working in an aqueous solution is equivalent to using methanolic reaction mixtures under reduced pressure of H2, and that reactions in which the enantioselectivity is known to be pressure-sensitive (such as those described in [92] and in [97]) cannot be strictly compared at the same partial pressure of dihydrogen. The limited solubility of H2 may influence the chemoselectivity, too (reduction vs. isomerization ofalkenes) [103]. [Pg.450]

Baeyer-Villiger oxidation (p. 853) catalytic hydrogenation (p. 844) chemoselective reaction (p. 848) dissolving-metal reduction (p. 846) enantioselective reaction (p. 857) epoxidation (p. 855) functional group interconversion (p. glycol (p. 858)... [Pg.875]

We hope that our survey of the important methods for reduction and oxidation has shown you that, by choosing the right reagent, you can often get reaction only at the functional group you want. The chemoselectivity you obtain is kinetic chemoselectivity—reaction at one functional group is simply faster than at another. [Pg.546]

Reduction of Chemoselective Reaction Time. Panke et al. demonstrated enhanced reaction control in the crucial step of nitration of the pharmaceutically relevant intermediate l-methyl-3-propyl-l//-pyrazole-5-carboxylic acid,... [Pg.2046]

Trost coined the term chemoselective to describe the process where one of two similar functional groups is made to react, while the other is not affected or affected to a lesser extent [1]. This discrimination usually results from nuanced changes to a reagent s behavior, e.g., by addition of a salt. A typical example is the modification of sodium borohydride reduction by addition of cerium chloride in the Luche process, whereby the double bond of an ot-enone is not saturated during the reaction [85]. Similarly, a new approach to chemoselective reaction of n-pentenyl donors was soon to emerge. [Pg.20]

Reduction of Chemoselective Reaction Time Panke et al. demonstrated enhanced reaction control in the crucial step of nitration of the pharmaceutically relevant intermediate l-methyl-3-propyl-li/-pyrazole-5-carboxylic acid, a precursor to the lifestyle drug sildenafil citrate (Viagra) [5]. Control of the reaction temperature is absolutely necessary because the decay of the product above 100 C creates excess CO2 pressure and heat buildup. A standard 70 mL CYTOS microreactor was fed with reagents using external pumps. A flow rate of 1 mL/min was maintained within each channel, with the reaction temperature maintained at 90 °C. [Pg.1199]

A key step represents the chemoselective partial reduction of acid chloride from the monoester of maleic acid. This reduction can be completed to the level of aldhyde either catalytically or by complex hydrides. In the previous example we met the reagents and reaction conditions used in the final steps. [Pg.36]

Many reagents are known to reduce the nitro group of nitro aromatics, but only a limited number of reagents give chemoselective nitro reduction and are suitable for a large-scale reaction. By using the iron-ammonium chloride... [Pg.240]

The combination of a chemoselective enzymatic reduction step with another second enzymatic reaction is another opportunity to overcome limitations, for example, in the case of the enantiosdective reduction of prochiral unsaturated aldehydes by coupling a reduction step with an isolated ene reductase (OYE 2 or OYE3) together with an oxidation step with HLADH in a cascade system, which allowed both yields and enantioselectivities to be improved [136]. [Pg.17]

The highly chemoselective reaction of diethylaluminium benzenethiolate (Et2AlSPh) with aldehydes in toluene at -78 °C has enabled in situ reduction of ketones and methyl esters in the presence of aldehydes by the sequence depicted in Scheme 41. ... [Pg.37]

Stereoselective and chemoselective semihydrogenation of the internal alkyne 208 to the ew-alkene 210 is achieved by the Pd-catalyzed reaction of some hydride sources. Tetramethyldihydrosiloxane (TMDHS) (209) i.s used in the presence of AcOH[116]. (EtO)3SiH in aqueous THF is also effective for the reduction of alkynes to di-alkenes[l 17], Semihydrogenation to the d.v-alkene 211 is possible also with triethylammonium formate with Pd on carbon[118]. Good yields and high cis selectivity are obtained by catalysis with Pd2fdba)3-Bu3P[119],... [Pg.497]


See other pages where Chemoselective reactions reduction is mentioned: [Pg.7]    [Pg.255]    [Pg.22]    [Pg.999]    [Pg.630]    [Pg.320]    [Pg.551]    [Pg.44]    [Pg.354]    [Pg.1110]    [Pg.102]    [Pg.2]    [Pg.237]    [Pg.198]    [Pg.363]    [Pg.28]    [Pg.176]    [Pg.169]    [Pg.475]    [Pg.99]    [Pg.1198]    [Pg.47]   
See also in sourсe #XX -- [ Pg.112 , Pg.113 , Pg.114 ]




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