Meerwein—Ponndorf reduction

In boiling ethanol, under nitrogen and in the presence of palladized charcoal, 2-acetyl-l,4-dihydro-3-methylquinoxaline (30) undergoes dismutation to give a mixture of 2-acetyl-3-methylquinoxaline, 2-acetyl-l,2,3,4-tetrahydro-3-methylquinoxaline (33), and 2-l -hydroxy-ethyl-3-methylquinoxaline (34), The latter compound is the product of sodium borohydride or Meerwein-Ponndorf reduction of 2-acetyl-3-methylquinoxaline. 

Dining distillation of 2-propanol recovered from the reduction of crotonaldehyde with isopropanol/aluminium isopropoxide, a violent explosion occurred. This was attributed to peroxidised diisopropyl ether (a possible by-product) or to peroxidised crotonaldehyde. An alternative or additional possibility is that the isopropanol may have contained traces of a higher secondary alcohol (e.g. 2-butanol) which would be oxidised during the Meerwein-Ponndorf reduction procedure to 2-butanone. The latter would then effectively sensitise the isopropanol or other peroxidisable species to peroxidation. 

Yohimbol (72) and 17-epiyohimbol (73) could be prepared from yohimbone (305) either with sodium borohydride or by the Meerwein-Ponndorf reduction. On prolonged reduction of yohimbone (305) under Meerwein-Ponndorf conditions, an equilibrium is established between yohimbol (72) and 17-epiyohimbol (73) in favor of the latter (209). 

Iridium tetrachloride, in modified Meerwein-Ponndorf reduction, 50, 13... 

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. 

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]. 

In the course of some stereoselective reactions the inducing chiral unit is destroyed. A classical example is the enantioselective Meerwein Ponndorf reduction of a ketone with a chiral Grig-nard compound33 ... 

The highly enantioselective production of ( + )-(S )-benzenemethan-a-configurational determination of 8 rests on an asymmetric Meerwein-Ponndorf reduction (with isobornyloxymagnesium bromide). On mechanistic grounds the reduction of benzaldehyde-rf was assumed to produce preferentially the 7 -isomer225. 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Chromenes are easily obtained by dehydration of 4-chromanols (24), in their turn readily available by reduction of 4-chromanones (23). The parent compound chrom-3-ene has been prepared by this method92 in 75-80% yield, via Meerwein-Ponndorf reduction and dehydration by azeotropic distillation over CuS04. Other reducing agents are metal... 

Reduction of ketimines.1 Reduction of N-cyclohexylideneaniline (1) with aluminum isopropoxide and isopropyl alcohol (Meerwein-Ponndorf reduction, 1, 35-36) results in N-isopropylaniline as the major product (equation I). However, if... 

The carbonyl group is a more potent acceptor than A1H3 or BH3, so the Meerwein-Ponndorf reduction is reversible but metal-hydride reductions are not. 

Di(epoxyethyl)diphenyl was obtained from diphenyl by Friedel-Crafts reaction with chloracetyl chloride. Meerwein-Ponndorf reduction gave the dichlorohydrin, and treatment with alcoholic potassium hydroxide gave 97% of the theoretical yield of diepoxide, (f.p., 161 °C.). The stabilizing effect was about one-fourth that of dibutyltin laurate. 

Alternative routes to unimolecular reduction include hydrogen abstraction by ketyls (see (ii)), Meerwein-Ponndorf reduction by alkoxides [A]f and reduction by a reactive form of magnesium hydride... 

The Meerwein-Ponndorf reduction of ketones involves the transfer of... 

Meerwein-Ponndorf reduction (I, 35-36), A recent study of the Mcerwein-Ponndorf reduction of mono- and bicyclic ketones shows that, contrary to commonly held views, the reduction of such ketones proceeds at a relatively high rate. The reduction of cyclohexanone and of 2-methylcyclohexanone is immeasurably rapid. Even... 

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