Meerwein synthesis

Meerwein-Ponndorf-Veriey Reduction opposite of Oppenauer oxidation Synthesis 1994, 1007 Organic Reactions 1944, 2, 178... 

This synthesis is only one example of a wide range of reactions which involve aryl (or alkyl) radical addition to electron-deficient double bonds resulting in reduction.The corresponding oxidative reaction using aryl radicals is the well known Meerwein reaction, which uses copper(II) salts. 

Meerwein-Pondorff reduction is the synthesis of alcohols by heating carbonyl compounds with aluminium isopropoxide in isopropanol and distilling off acetone by-product... 

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

A second example of the use of esters as enolates is the formation of Meerwein s ester, an intermediate in the synthesis of substituted adamantanes (6). Dimethyl... 

Sandmeyer reaction, 943 saponification. 809-810 SN1 reaction, 373-375 Sn2 reaction, 363-364 Stork enamine reaction, 897-898 transamination, 1167 Williamson ether synthesis, 655 Wittig reaction, 720-721 Wolff-Kishner reaction, 715-716 Meerwein-Ponndorf-Verley reaction, 746... 

Meerwein reactions can conveniently be used for syntheses of intermediates which can be cyclized to heterocyclic compounds, if an appropriate heteroatom substituent is present in the 2-position of the aniline derivative used for diazotization. For instance, Raucher and Koolpe (1983) described an elegant method for the synthesis of a variety of substituted indoles via the Meerwein arylation of vinyl acetate, vinyl bromide, or 2-acetoxy-l-alkenes with arenediazonium salts derived from 2-nitroani-line (Scheme 10-46). In the Meerwein reaction one obtains a mixture of the usual arylation/HCl-addition product (10.9) and the carbonyl compound 10.10, i. e., the product of hydrolysis of 10.9. For the subsequent reductive cyclization to the indole (10.11) the mixture of 10.9 and 10.10 can be treated with any of a variety of reducing agents, preferably Fe/HOAc. 

The Meerwein-Ponndorf-Verley (MPV) reduction is generally mediated by aluminum triiso-propoxide, Al(01Pr)3. In MPV reduction, reversible hydride transfer occurs via a six-membered transition state (Scheme 67). By removing acetone from the reaction system, the reversible reaction proceeds smoothly. The advantages of the reduction are the mildness of the reaction conditions, chemoselectivity, safety, operational simplicity, and its applicability to large-scale synthesis. It is reported that the addition of trifluoroacetic acid, significantly accelerates the reduction (Scheme 68) 304,305 in which case a catalytic amount of Al(0 Pr)3 is enough to complete the reaction. 

The asymmetric reduction of prochiral ketones to their corresponding enantiomerically enriched alcohols is one of the most important molecular transformations in synthetic chemistry (20,21). The products are versatile intermediates for the synthesis of pharmaceuticals, biologically active compounds and fine chemicals (22,23). The racemic reversible reduction of carbonyls to carbinols with superstoichiometric amounts of aluminium alkoxides in alcohols was independently discovered by Meerwein, Ponndorf and Verley (MPV) in 1925 (21—26). Only in the early 1990s, first successful versions of catalytic... 

Complete control of the diastereoselectivity of the synthesis of 1,3-diols has been achieved by reagent selection in a one-pot tandem aldol-reduction sequence (see Scheme l). i Anti-selective method (a) employs titanium(IV) chloride at 5°C, followed by Ti(OPr )4, whereas method (b), using the tetrachloride with a base at -78 °C followed by lithium aluminium hydride, reverses the selectivity. A non-polar solvent is required (e.g. toluene or dichloromethane, not diethyl ether or THF), and at the lower temperature the titanium alkoxide cannot bring about the reduction of the aldol. Tertiary alkoxides also fail, indicating a similarity with the mechanism of Meerwein-Ponndorf reduction. 

An improved synthesis of 2-(r-hydroxyethyl)dibenzothiophene has been reported involving Meerwein-Ponndorf reduction of 2-acetyldi-benzothiophene. An alternative synthetic route has been employed in the synthesis of 4-(l -hydroxyethyl)dibenzothiophene (101) involving treatment of 4-lithiodibenzothiophene with acetaldehyde (36%). Both of these compounds are readily dehydrated to the corresponding vinyl compounds, the polymerization of which is discussed in Section VII. 

In 1976, the synthesis of oxaziridininm salt 66 was reported by Lusinchi and coworkers (Fig. 23) [128-130]. Salt 66 was obtained by either methylation of the corresponding oxaziridine with FSO Me or oxidation of the corresponding iminium salt with peracid. Subsequently, Hanquet and coworkers prepared oxaziridininm salt 67 by methylation of the corresponding oxaziridine with Meerwein s salt (MejO BF ") or oxidation of the A -methyl isoquinoUnium fluoroborate salt with peracid [131, 132],... 

Hanson JR (1991) Wagner-Meerwein rearrangements. In Trost BM, Fleming I (eds) Comprehensive organic synthesis, vol 3. Pergamon, Oxford, p 705... 

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