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Reduction of a-hydroxy ketones

The reduction of a-hydroxy ketones 43 to 1,2-diols 44 was achieved in good yield and high diastereoselectivity in favour of the anti product by using ClsSiH and photolytic conditions [56]. In the proposed mechanism of Scheme 5.8, the addition proceeded via the intermediate adduct 45, which participated in the stereo-controlled hydrogen donation, and via the a-silylether 46, affording 1,2-diols in a preferential anti conformation. [Pg.105]

Scheme 5.8 Reduction of a-hydroxy ketones using CbSiH... Scheme 5.8 Reduction of a-hydroxy ketones using CbSiH...
Hydrogenation and reduction of C=0 bond. Three metals make up the major classes of homogeneous hydrogenation catalysts for the reduction of the carhonyl group. A Cp Ir complex 3 with a hidentate ligand of monosulfonylated diphenylethanediamine is able to promote reduction of a-hydroxy ketones and with the same catalyst (or enantiomer) to rapidly transfer hydrogenate aryl ketones even in the air. For the latter purpose two other useful Ir complexes are that which contain the hgands 174 and 175. ... [Pg.150]

Table 3. anti-, 2-Diols by Selective Reduction of a-Hydroxy Ketones with Zinc Borohydride (Method A)15 16 or Lithium Aluminum Hydride (Method B)13 14... [Pg.693]

The following synthesis contains almost everything that makes academic research nice and expensive. [283] Key steps are atitanium enolate anfi-aldolreac-tion, a nitroaldol reaction and the diastereoselective reduction of a ) -hydroxy-ketone. [Pg.378]

Good results were also obtained [65] for the reduction of aliphatic ketones this subject had already been covered in several reviews [21,47,66-68]. The stereochemical course of these reductions has been investigated by several deuteration studies [69-71]. Thus, the reduction of a-hydroxy ketones 49 gave die corresponding 1,2-diols 50 (Fig. 14) [72-78]. From 1-methylsulfonylated ketones the corresponding alcohols were obtained with e.e. values up to 87% [79]. [Pg.533]

Borohydrides reduce a-substituted ketones to the corresponding a-substituted alcohols, and such products can be further reduced to olefins (see section VIII). Other reagents serve, through participation of the carbonyl group, to remove the substituent while leaving the ketone intact. The zinc or chromous ion reduction of a-halo ketones is an example of this second type, which is not normally useful for double bond introduction. However, when the derivative being reduced is an a,jS-epoxy ketone, the primary product is a -hydroxy ketone which readily dehydrates to the a,jS-unsaturated ketone. Since... [Pg.348]

A third type of reduction of a-substituted ketones is typified by the expulsion of the substituent and the reduction of the keto function to form an olefin. Wolff-Kishner reductions of a-hydroxy, a-acetoxy, " a-halo, °° and a-epoxy (see below) ketones are the most frequently encountered steroid examples of this general class. ... [Pg.349]

In theory, the chiral center can be anywhere in the molecule, but in practice, reasonable diastereoselectivity is most often achieved when it is in the a position. For examples of high diastereoselectivity when the chiral center is further away, especially in reduction of P-hydroxy ketones, see Narasaka, K. Pai, F. Tetrahedron, 1984, 40, 2233 Hassine, B.B. Gorsane, M. Pecher, J. Martin, R.H. Bull. Soc. Chim. Belg., 1985, 94, 597 Bloch, R. Gilbert, L. Girard, C. Tetrahedron Lett., 1988, 53, 1021 Evans, D.A. Chapman, K.T. Carreira, E.M. J. Am. Chem. Soc., 1988, 110, 3560. [Pg.1268]

Reduction of (3-hydroxy ketones through chelated TSs favors. yy -l,3-diols. Boron chelates have been exploited to achieve this stereoselectivity.129 One procedure involves in situ generation of diethylmethoxyboron, which then forms a chelate with the (3-hydroxyketone. Reduction with NaBH4 leads to the vyn-diol.130... [Pg.412]

The regiospecificity of the exclusive O-acylation [8] and O-phosphorylation [9] of P-dicarbonyl compounds (Chapter 3) also illustrates the effect of phase-transfer catalysts on the stereochemical course of reactions. Similarly, directed reduction of P-hydroxy ketones using tetramethylammonium trisacetoxyborohydride leads to the preferential formation of the anti dihydroxy system in high yield with a stereoselectivity >95% [10] (Section 11.4). [Pg.516]

The reduction of /1-hydroxy ketones with a tetramethylammonium triace-toxyborohydride-anhydrous acetic acid system yielded the anti diols via the transition state (TA). The stereochemistry of the 1,3-diols was assigned by using H NMR coupling constants after their conversion to the corresponding cyclic acetonide. [Pg.103]

The presence of bromine at C-12 does not affect the selectivity 12a-bromo-3a-hydroxy-5/J-pregnane-ll,20-dione is reduced to the 20/ -alcohol by NaBH4 in either aqueous tetrahydrofuran or 2-propanol.72 Similarly, a 17-bromo substituent has no effect.239 LiBH4 has also been used for selective reductions of a 20-ketone in the presence of an 11-ketone.92... [Pg.313]

Reduction of a-alkoxy ketones.2 a-Hydroxy ketones are reduced by Sml2 in THF/CH3OH in rather low yield ( 30%). In contrast a-acetoxy ketones are reduced in >75% yield. A simple expedient is to reduce the a-hydroxy ketone in the presence of AczO with Sml2 (equation I). a-Silyloxy, benzoyloxy, and tosyloxy... [Pg.270]

In 1999, Keck examined the reduction of (1-hydroxy ketones using Sml2 in THF, and screened a number of proton donors for use in the reaction.30 Although low concentrations of water provided a good yield and reasonable diastereoselectivity in the conversion of 6 to 7, an increase in water concentration resulted in a loss of diastereoselectivity. The use of methanol as a proton source provided excellent yields and diastereoselectivity over a range of concentrations whereas the use of tm-butanol led to the recovery of starting material (Scheme 2.9). [Pg.12]

Keck reported that the reduction of (3-hydroxy ketones with Sml2 is an excellent method for the preparation of 1,3-arth-diols.13 For example, reduction of 9 with Sml2 in THF with MeOFi gave 1,3-anfz-diol 10 in 95% yield and 98% de. The reaction proceeds by initial coordination of Sm(II) to the free hydroxyl, followed by intramolecular electron transfer to the ketone carbonyl to form ketyl radical anion 11. A second electron transfer gives alkylsamarium... [Pg.41]

Keck recently applied the diastereoselective reduction of ( S-hydroxy ketones in an approach to the anti-tumour natural products epothilone A and B. Sml2-mediated reduction of p-hydroxy ketone 15 in THF-MeOH gave anti- 1,3-diol 16 in 94% yield and as a 96 4 mixture of diastereoisomers.18 Attempts to carry out the reduction using common hydride reducing agents gave 16 with poor levels of diastereoselectivity (Scheme 4.10).18... [Pg.43]

The 1,3-anti -reduction of (3-hydroxy ketones was utilized in the total synthesis of (+)-roxaticin (14), a pentaene macrolide isolated from streptomycete X-149944 (Scheme 4.2e). The (3-hydroxy ketone 15 underwent 1,3-anti -reduction to afford the diol 16 in 99% yield and greater than 95 5 diastereoselectivity. The resulting diol was then protected as a cyclopentylidene ketal (17) by using cyclopentylidene dimethyl ketal and pyridinium p-toluene sulfonate (PPTS). [Pg.166]

A boat form can also be invoked 83). It should be noted that the direction of attack is opposite to that proposed for chelation controlled reduction of p-hydroxy ketones 104W. [Pg.30]


See other pages where Reduction of a-hydroxy ketones is mentioned: [Pg.1650]    [Pg.1274]    [Pg.46]    [Pg.178]    [Pg.1650]    [Pg.1274]    [Pg.46]    [Pg.178]    [Pg.82]    [Pg.83]    [Pg.86]    [Pg.281]    [Pg.527]    [Pg.1173]    [Pg.750]    [Pg.752]    [Pg.754]    [Pg.174]    [Pg.24]    [Pg.103]    [Pg.442]    [Pg.50]    [Pg.52]    [Pg.18]    [Pg.160]    [Pg.323]    [Pg.185]    [Pg.86]    [Pg.73]    [Pg.156]    [Pg.159]   
See also in sourсe #XX -- [ Pg.156 , Pg.277 ]

See also in sourсe #XX -- [ Pg.156 , Pg.277 ]




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