Regioselective 4- benzyl acetate

Regioselective benzylation at the primary position of the 4,6-diol has been observed in a- (Ref. [107]) and P-D-galacto [108, 109] series (see also, Ref. [110]). Benzyl 2,3,6-tri-O-benzyl-P-D-galactopyranoside was obtained crystalline first by this method in 81 % yield [108]. The preference is lower for D-gluco series [45, 105], benzyl 2-acet-amido-3-0-benzyl-2-deoxy-P-D-glucopyranoside gave [45] the desired 3,6-di-O-ben-zyl isomer in only 43% yield, together with 31.5% of 4,6- and 14.8% of 3,4,6-tri-O-benzyl derivative. 

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. ... 

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

Regioselective Cleavage of O-Benzylidene Acetals to Benzyl Ethers... 

II. REGIOSELECTIVE REDUCTIVE CLEAVAGE OF CM3ENZYUDENE ACETALS TO BENZYL ETHERS... 

A. Liptdk, J. Imre, J. Harangi, P. NSnisi, and A. Neszmilyi, Chemo-, stereo- and regioselective hydrogenolysis of carbohydrate benzylidene acetals. Synthesis of benzyl ethers of benzyl a-D-,... 

Scheme 6.13 Benzylation of different arenes with 1-phenylethyl acetate CCyield (regioselectivity). Reaction conditions 0.5 mmol 1-phenylethyl acetate, 10mol% FeCI3, 2.0mmol arene, 5mL CH2CI2, 50°C, 20 h (a) 5 mL arene, no solvent (b) 5 mL MeN02, 120°C (c) 5 mL MeNQ2.

Regioselective reductive openings of mixed phenolic-benzylic cyclic acetals, using BH3-NMe3-AlCl3, under mild conditions to yield a benzylic ether and free the phenol group have been attributed to association of boron with the more basic oxygen (benzylic) followed by reaction with aluminium chloride.309... 

Regioselective debenzylation can be achieved by treatment with Lewis acids such as ferric chloride and S11CI4 or under acetolysis conditions with acetic anhydride and sulfuric acid, and several examples are depicted in Scheme 2.3.9 Acetolysis results in cleavage of the most acid-sensitive benzyl group. In general, primary benzyl ethers can be selectively acetolysed in the presence of secondary benzyl ethers. The regioselectivity of the reaction can be explained as follows sulfuric acid protonates acetic anhydride followed by the formation of an acetyl ion and acetic acid. The acetyl ion reacts with the sterically most accessible oxygen which is at... 

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