Reduction of tosylates

The product composition from these reactions is influenced by the location of the functional group in the substrate. Olefin formation is the most common side reaction and in certain cases, especially with reductions of tosyl-hydrazones (section IV-B), it may become dominant so that the reaction can be used for the preparation of mono-labeled olefins. 

The cathodic cleavage of CX- to CH-bonds can be achieved with a variety of substituents X, such as Hal, NR3+, PR3+, OTos, or epoxides. Generally, good yields, a high potential selectivity, and often good stereoselectivities are encountered (see also Chapters) [117-123]. Chemical reactions for similar conversions, which are well worked out, especially with regard to yield and selectivity, are the reduction of tosylates with LiAlH4 and halides with... 

Reduction of Tosylates and Similar Compounds Hydro-de-sulfonyloxy-substitution... 

Alkanes ( ree also Alicyclic Compounds) 0-76 Reduction of alkyl halides 0-77 Reduction of tosylates and similar compounds... 

Reduction of tosyl- and trisylhydrazones.3 The reagent (1 equivalent) reduces tosylhydrazones of ketones to alkanes in yields of 60-85% (GLC). The reduction is Subject to sleric hindrance and so is not effective with the tosylhydrazone of camphor. Tosylhydrazones of aldehydes are reduced in moderate yield (about 50%). The reagent does not reduce tosylhydrazones of aromatic or a,/)-unsaturated aldehydes or ketones. 

Reduction of tosylates. Reduction of the ditosylatc 1 with Li(C2H5)3BH gives a mixture of 4 and 5 in the ratio 5.25 1. The reduction proceeds via the epoxides 2 and 3.2... 

Attempts to synthesize C-terminal peptide aldehydes using other reductive techniques are less successful. 24"29 The reduction of a-amino acid esters with sodium amalgam and lithium aluminum hydride reduction of tosylated a-aminoacyldimethylpyrazoles resulted in poor yields. 26,29 The Rosemond reduction of TV-phthaloyl amino acid chlorides is inconvenient because the aldehyde is sensitive to hydrazine hydrate that is used to remove the phthaloyl group. 27 28 jV -Z-Protected a-aminoacylimidazoles, which are reduced to the corresponding aldehydes using lithium aluminum hydride, are extremely moisture sensitive and readily decomposed. 25 The catalytic reduction of mixed carbonic/carboxylic acid anhydrides, prepared from acylated a-amino acids, leads to poor reproducibility and low yields. 24 The major problems associated with these techniques are overreduction, racemization, and poor yields. 

The large excess of sodium cyanoborohydride is recommended for the reduction of tosylates. Use of reduced molar excesses led to substantially lower yields. For example, a 3 1 cyanoborohydride to tosylate ratio afforded less than 60% yield of product at 80° for 5 hours, while a 1.5 1 excess gave only 52% yield at 70° for 8 hours. 

In general, the reduction of tosyl hydrazones with boron hydrides is a convenient alternative to the classical procedures of Wolff-Kishner, Clemmen-sen, and Raney nickel reductions when one wants to delete the carbonyl of ketones and aldehydes after using it for the very many synthetic operations it allows. 

It was found that the cleavage of the primarily formed radical anion was the ratedetermining step in an indirect reduction of tosylates in DMF when R was aliphatic, whereas with R aromatic the cleavage was so fast that the homogeneous electron transfer from the reduced catalyst became rate-determining [249],... 

Influence of solvents. The effectiveness of LiAlH4 for reduction of alkyl iodides and bromides varies considerably with the solvent. The order of solvent effect is diglyme > monoglyme > THF y> ether. In contrast the solvent effect on rate of reduction of tosylates is ether > THF > monoglyme > diglyme. Thus reduction of tosylales by LiAIH4 can be carried out in ether without reduction of a halide substituent.1... 

Primary tosylates are reduced by LiAlH4. The diacetal of D-fucose 5.39, an enantiomer of L-fucose, is thus prepared by reduction of tosylate 5.38 derived from the diacetal of D-galactose 5.11. A hydroxyl group can also be replaced by a halogen (Cl, Br, I) at any position of a protected sugar. The halide is easily reduced by a radical mechanism with tributylstannane, Bu3SnH (reaction 5.19). 

While the available literature only describes the electronic activity of organic arenesulphinates, it should be assumed that esters 1 can undergo cathodic reduction only when R and/or R possess a rather low energy level of their LUMO. In other words, when R and R are fully saturated, it is rather foreseeable that 1 is totally inactive in terms of cathodic reactivity. To the best of our knowledge, all papers devoted to sulphonic esters deal only with the behaviour of arenesulphonates4 and nearly all of them focus their interest on the reduction of tosylates 2 (R" = p-CH3). 

The indirect reduction of tosyl esters can be performed7 in non-aqueous solutions. Thus, for example, the anthracene anion radical formed by cathodic reduction in DMF/TBAB (tetrabutylammonium bromide) electrolyte may reduce tosylates in solution. Similarly, the pyrene anion radical was shown8 (Figure 1) to react also with ethyl tosylate. The redox catalysis general scheme (indirect reduction by a redox P/Q couple) where P is a reducible species and Q its stable reduced form can be written as below ... 

The indirect reduction of tosyl esters by reduced forms of organic mediators may lead15 in certain cases to chemiluminescence and then help to determine the mode of cleavage of the S—O bond and the nature of the free radical present. 

Reduction of tosyl lactate 120b with sodium borodeuteride at 100 °C (48 h) in the absence of any solvent furnishes deuteropropionate 132 [50]. 

Deoxygenation of alcohols. This hydride is superior to lithium aluminum hydride for reduction of tosylates to hydrocarbons. ... 

Alcohol 143 (Scheme 6.26), prepared from (R)-glyceraldehyde derivative, was subjected to deoxygenation and epoxidation to give the racemic epoxide 144. Kinetic resolution with (S,S)-Jacobsen catalyst gave diol 145, which on further transformations was converted into the alcohol 146. Swern oxidation of 146 followed by Wittig olefination, acetonide deprotection under acidic conditions furnished the diol 147. Primary alcohol on deoxygenation through LAH reduction of tosylate afforded the alcohol 148. 

Lithium triethylborohydride reductions of tosylate derivatives of methyl 4,6-0-benzylidene-a-D-glucopyranoside were highly regio-selective and gave good yields of deoxy-sugars via epoxide intermediates thus, the 3-mono- and 2,3-dl-O-tosylates gave the same... 

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