Glucofuranose 1,2-0-cyclohexylidene

Das aus (-)-Menthol hergestellte chirale Hydrid liefert in erster Linie mit zweizahnigen Substraten [fi-Amino- ketone, En-(2)-in-(4)-ole (s.S. 74)] gute optische Ausbeuten1. Als chirale vie. Diole werden Zucker-2,1 und Terpen-Derivate wie cis- und rrans-Pinandiol-(2,3)4 verwendet. Von den Zucker-Derivaten hat sich der Kom-plex mit 3-0-Benzyl-l,2-0-cyclohexyliden-a-D-glucofuranose am besten bewahrt2,5 (Vorschrift S. 74). 

Although menthyl esters, especially 19, are most often used to prepare sulfoxides, esters derived from optically active alcohols other than menthol have been prepared . Ridley and Smal prepared arenesulfmic esters of 1,2 5,6-di-O-cyclohexylidene-a-D-glucofuranose. Unfortunately, these diastereomers were oils, except for the mesityl derivative, with the major epimer having configuration R at sulfur and so they offered no advantage over the menthyl esters. Separation of the epimers by chromatography failed. 

An early approach to the formation of chiral amines by nonenzymatic asymmetric synthesis was the reduction of prochiral ketoximes and their O-tetrahydropyranyl and O-methyl derivatives with lithium aluminum hydride-3-0-benzyl-1,2-0,0-cyclohexylidene-a-D-glucofuranose complex (16)33 in ether and prochiral ketoximes... 

Azido-l,2-0-cyclohexylidene-3-deoxy-5,6-di-0-methanesulphonyl-a-D-glucofuranose was converted 49) into l,2-0-cyclohexylidene-3,6-di-... 

Hohe optische Ausbeuten an 2-Amino-alkanen [z.B. (S)-2-Amino-butan, (S)-l-Amino-1-cyclohexyl-ethan] und Amino-ary 1-alkanen [z.B. (S)-i- und (S)-2-Amino-1-phenyl-propan] erhalt man auch bei der Reduktion der entsprechenden Ketoxime mit einem aus Lithium-alanat und 3-0-Cyclohexylmethyl-l,2-0-cyclohexyliden-a-D-glucofuranose ge-bildeten Komplex in Ether3. 

The preparation of 1,2-O-cyclohexylidene-a-D-xylofuranose (22) (Expt 5.117) from 1,2-O-cyclohexylidene-a-D-glucofuranose illustrates the use of sodium metaperiodate for the cleavage of carbon-carbon bonds in a-diols (see also Section 4.2.55, p. 454). In this case C-6 is lost as formaldehyde and C-5 is converted into an aldehyde group. This aldehydic product is isolated as a dimer, which is then reduced in methanol solution with sodium borohydride to the xylofuranose derivative (22). 

Di-(1,2-0-cyclohexylidene-a-D-xj/o-pentodialdofuranose-5-hydrate)-5,5 3, 5-dianhydride. Add a solution of 14.3 g (0.061 mol) of sodium metaperiodate in 220 ml of water dropwise to a well-stirred solution of 17.4 g (0.067 mol) of 1,2-O-cyclohexylidene-a-D-glucofuranose (Expt 5.115) in 50 ml of water (1)- Stir for a further 30 minutes and remove the water at a temperature below 50 °C by evaporation under reduced pressure. Extract the solid residue with three 75 ml portions of dichloromethane and dry the combined extracts over magnesium sulphate. Filter and evaporate to give a residue which crystallises spontaneously. After recrystallisation from acetone the dimer has m.p. 182— 183 °C, the yield is 7.9 g (51%). 

The enantioselective reduction of unsymmetrical ketones to produce optically active secondary alcohols has been one of the most vibrant topics in organic synthesis.8 Perhaps Tatchell et al. were first (in 1964) to employ lithium aluminum hydride to achieve the asymmetric reduction of ketones9 (Scheme 4.IV). When pinacolone and acetophenone were treated with the chiral lithium alkoxyaluminum hydride reagent 3, generated from 1.2 equivalents of 1,2-0-cyclohexylidene-D-glucofuranose and 1 equivalent of LiAlHzt, the alcohol 4 was obtained in 5 and 14% ee, respectively. Tatchell improved the enantios-electivity in the reduction of acetophenone to 70% ee with an ethanol-modified lithium aluminum hydride-sugar complex.10... 

It was found that die stereoselectivity of reduction was increased by ethanol, and that (R)-l-phenylethanol was obtained in 70% optical yield when acetophenone (12.5 mmoles) was reduced with an ethanol-modified complex of lithium aluminum hydride with 3-O-benzyl-1,2-O-cyclohexylidene-a-D-glucofuranose prepared from the sugar derivative (26 mmoles), lithium aluminum hydride (58 mmoles), and ethanol (110 mmoles). 

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