Benzyl acetal, protecting group

Benzyl acetal protecting groups were hydrogenolyzed over Pd/C,159 over 20% Pd(OH)2/C in AcOEt for 2 hours,160 and over 10% Pd/C in AcOEt in the presence of NaHC03 for 30 minutes.161... 

Selective removal of the benzyl ether-protecting group without hydrogenolysis of the ketal C-O bonds was performed on Ra-Ni in EtOH.154 155 A solution of benzyloxy acetal in MeOH with 5% Pd/C was stirred at room temperature with a H2 pressure from a balloon until the reaction was complete (Scheme 4.30).156... 

HBr in acetic acid (just the solvent) is used to remove the benzyl ether protecting groups in this example, which forms part of a synthesis of the alkaloid galanthamine. 

Similar C-glycosidations are available via cuprate chemistry. For example, as shown in Scheme 2.3.2, Bihovsky, etal.,24 utilized a variety of cuprates in the preparation of a-C-methylglycosides. The yields were relatively poor when acetate protecting groups were used. However, substantial improvements in the yields were observed with methyl and benzyl protecting groups. In all cases, the a anomer was favored in ratios as high as 20 1. 

In an earlier example of the applicability of cuprate chemistry to C-glycosidations, Bellosta, et ai.,26 stereospecifically prepared epoxides from protected glucals (Scheme 2.3.4). Unfortunately, as shown in the previous example, the use of acetate protecting groups instead of benzyl protecting... 

Bromination might well oxidise the reactive benzylic alcohol in (5) so an acetal protecting group is added. This could be removed to give (4), but as a protecting group was needed later in the synthesis, it was left in place. 

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