Syn- 1,2-diols

The synthesis of the polyol glycoside subunit 7 commences with an asymmetric aldol condensation between the boron enolate derived from imide 21 and a-(benzyloxy)acetaldehyde (24) to give syn adduct 39 in 87 % yield and in greater than 99 % diastereomeric purity (see Scheme 8a). Treatment of the Weinreb amide,20 derived in one step through transamination of 39, with 2-lithiopropene furnishes enone 23 in an overall yield of 92 %. To accomplish the formation of the syn 1,3-diol, enone 23 is reduced in a chemo- and... 

Reduction of a-methyl-fi-hydroxy ketones,2 The r-butyldimethylsilyl ethers of these ketones, in which chelation is difficult, are reduced by lithium aluminum hydride with a high degree of 1,2-anri-selectivity. This reaction can therefore afford either aM ,a/iri-l,3-diols or anti,syn- 1,3-diols with high selectivity. 

Reduction of jl-hydroxyketones through chelated transitions states fovors syn-1,3-diols. Boron chelates have been exploited to achieve this stereoselectivity.86 One procedure involves in situ generation of diethylmethoxyboron, which then forms a chelate with the /1-hydroxy ketone. Reduction with NaBH4 leads to the syn diol.87... 

Hydroxy ketones also give primarily syn 1,3-diols when chelates prepared with BC13 are reduced with quaternary ammonium salts of BH4 or BH3CN-.88... 

Syn 1,3-diols also can be obtained from /i-hydroxykctoncs using LiI-LiAlH4 at low temperatures.91... 

Stereoselective reduction of -hydroxy ketones,8 Boron chelates of p-hydroxy ketones are reduced by sodium borohydride stereoselectively to syn- 1,3-diols (equation I). Even higher syn-selectivity obtains on similar reduction of syn-a-substituted-p-hydroxy... 

A stoichiometric amount of 3f catalyzed the asymmetric aldol reaction of aldehydes with enol silyl ethers and subsequent asymmetric reduction, in one pot, to afford syn 1,3-diols with high enantioselectivity (Eq. 49) [43b]. With a variety of aldehydes, 1,3-diols were obtained in moderate yields (53-70 %) with high syn diastereoselectivity. The syn 1,3-diols prepared from aliphatic aldehydes in the reaction (in EtCN as sol-... 

The reduction of the 1,3-diketones 3.122 and 3.123 to diols can also be stereoselective and can lead to either syn,syn 1,3-diols or to syruanti 1,3-diols, depending... 

In the first method, the chelating center MLn is a dialkylboryl group, introduced by reaction of the substrate with a trialkylborane in the presence of a catalytic amount of air36. The resulting complex is treated with sodium borohydride in tetrahydrofuran at low temperature to give, after oxidative/hydrolytic removal of the dialkylboryl group, the corresponding syn-1,3-diol with fair to excellent stereoselectivity. 

Diastereoselective hydrogenation of y-hydroxyvinylsilanes and -tins, Syn-1,3-diols.1 This catalyst is known to effect diastereoselective hydrogenation of chiral ally lie alcohols (12, 426-428). It can also effect diastereoselective hydrogenation of vinyltins and vinylsilancs having a y-hydroxyl group (equations I and II). 

In their first and racemic synthesis of methyl nonactate 75 (Scheme 8), Bartlett and Jernstedt (18) introduced the 1,3-relationship between C-6 and C-8 in a diastereoselective manner from the diene 68, which gave the analogous cyclic phosphate. Upon treatment with sodium methoxide, the phosphate gave the epoxide 69, which was converted into the syn-1,3-diol 70. Acetylation of the diol followed by ozonolysis gave the aldehyde 71. Aldol condensation of the aldehyde with the silyl... 

Diols. The stereoselective addinoi ketones to give predominantly syn-1,3-diols is... 

Reduction of aldehydes and ketones is possible with borane. Stereoselective reduction of ketones is possible when a heteroatom is located a- or p- to the carbonyl group. Treatment of a (3-hydroxy-ketone with catecholborane results in the selective formation of the syn 1,3-diol product (7.94). The borane reacts preferentially with the alcohol to release hydrogen gas and to form the boronic ester 107. A second equivalent of the borane then effects the reduction to give the syn diastereomer. For the preparation of the anti diastereomer, triacetoxyborohydride can be used (see Scheme 7.86). 

Evans DA, Gauchet-Prunet JA. Diastereoselective synthesis of protected syn-1, 3-diols by base-catalyzed intramolecular conjugate addition of hemiacetal-derived alkoxide nucleophiles. J. Org. Chem. 1993 58 2446 2453. 

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