0-benzylidene acetals regioselective openings

The regioselectivity of the reductive openings of five-membered (oxoiane-type) benzylidene acetals has been examined. The same regioselectivity as that observed for the LiAlH AlClj openings was observed (Scheme 10) [27]. 

Reductive cleavage, using the NaCNBHj-HCl-THF system to produce 2 -pro-penylidene acetals, produced the same regioselectivity as that found in benzylidene acetals [27] (Scheme 11), and also for the p-methoxybenzylidene acetals (Scheme 12) [27]. These observations open the way to further strategies for protecting group manipulations. 

M, Ek, P. J. Garegg, H. Hultberg, and S. Oscarson, Reductive ring openings of carbohydrate benzylidene acetals using borane-trimethylamine and aluminium chloride. RegioSelectivity and solvent dependence, J. Carbokydr. Chem. 2 305 (1983). 

P. J. Garegg and H. Hultberg, A novel, reductive ring-opening of carbohydrate benzylidene acetals, with unusual regioselectivity, Carbokydr. Res. 93 00 (1981). 

Simple benzylidene acetals of carbohydrates are photochemically cleaved in a way similar to the well-used AT-bromosuccinimide process regioselective ring opening process into bromodeoxysugars. 

B. P. Roberts and T. M. Smits, Regioselectivity in the ring opening of 2-phenyl-l,3-dioxan-2-yl radicals derived from cyclic benzylidene acetals and comparison with deoxygenation of a carbohydrate diol via its cyclic thionocarbonate, Tetrahedron Lett., 42 (2001) 3663-3666. 

Other possibilities are four-step sequences comprising full protection of the acetal diols, followed by removal of the acetal and, finally, regioselective protection of the obtained diol (Scheme 3.25). For example, 2,6-protection (acetylation, benzoylation or benzylation) of a 3,4-0-isopropylidene derivative and successive acetal cleavage yields the 3,4-diol. Subsequent 3-0-protection via tin-activated silylation, alkylation or acylation, or 4-O-protection via orthoester or benzylidene (endo) formation-opening sequences is then possible. 

R. W. Binkley, G. S. Ooewey, arid 3. Johnston, Regioselective ting opening of benzylidene acetals, A pbotoebemically initiated reaction for partial deprotection of carbohydrates, 7. Org. Chan. 49 992 (1984). 

L. Jiang and T.-H. Chan, Borane/Bu2BOTf A mild reagent for the regioselective reductive ring opening of benzylidene acetals in carbohydrates, Tetrahedron Lett., 39 (1998) 355-358. 

C. C. Wang, S. Y. Luo, C. R. Shie, and S.-C. Hung, Metal trifluoromethanesulfonate-catalyzed regioselective borane-reductive ring opening of benzylidene acetals A concise synthesis of 1,4-dideoxy-l,4-imino-L-xylitol, Org. Lett., 4 (2002) 847-849. 

Regioselective reductive ring opening of a benzylidene acetal. Steric interactions determine the observed selectivity. 

Diastereoselective addition of propargyl acetate to the aldehyde 61 followed by MOM protection of carbinol, removal of the acetate, and partial reduction of the alkyne furnished allylic alcohol 62. Allylic alcohol 62 on oxidation and reaction with CBr4/PPh3 was converted into (Z,Z) bromodiene 63a, which on subsequent regioselective ring opening of benzylidene acetal in 63a afforded alcohol 64. 

The ring-opening of 2-phenyl-l,3-dioxan-2-yl radicals is also catalyzed by HSTIPS in conjunction with di-f-butyl peroxide (DTPB) as initiator,when benzoate esters are formed. The regioselectivity of the reaction is controlled by kinetic factors. A strong preference for the /3-scission of the primary C-0 bond (91 9) is observed for the frawi -fused-benzylidene acetal in eq 26. 

Protection of D-erythrono-1,4-lactone as its 2,3-0-benzylidene acetal, followed by regioselective reductive acetal opening (triethylsilane-titanium tetrachloride), then hydride reduction of the lactone function gave 2-0-benzyl-L-erythritol. In contrast, tributyltin oxide mediated benzylation of the same 1,4-lactone then hydride reduction of the lactone group gave 2-0-benzyl-D-erythritol. L-Erythrulose has been converted into 2-amino-2-deoxy-L-erythritol in six steps and 35% overall yield, the key step being a stereoselective reduction of the ketoxime orthoformate 11 with K-selectride. ... 

OH free Here, for all the three monosaccharides, a derivative with the 4-OH free can be obtained from the 4,6-O-benzylidene derivative through regioselective reductive opening. Formation of the 4,6-acetal with consecutive 2,3-protection (acetylation, benzoylation, benzylation) and reductive opening using various reagents yields the desired compounds. In mannosides, the selective formation of the 4,6-6>-acetal is not trivial, but can be accomplished with a 50 to 70% yield. 

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