Synthesis of novel photolabile glycosides from methyl 4,6-O-(o-nitro)benzylidene-α-d-glycopyranosides

Novel photolabile sugar derivatives bearing a 4- or 6- O -( o -nitro)benzyl group have been prepared from the corresponding methyl 4,6- O -( o -nitro)benzylidene α- d -glycopyranosides. Regioselective cleavage with BF 3 ·Et 2 O/Et 3 SiH led to the methyl 6- O -( o -nitro)benzyl gluco - and manno -α- d -glycopyranosides 3 and 6 . Inversion of configuration at 4-OH position of gluco and manno derivatives offered the otherwise inaccessible methyl 6- O -( o -nitro)benzyl galacto - and talo -α- d -glycopyranosides 4 , 5 , and 7 . Careful reaction with PhBCl 2 /Et 3 SiH (3 equiv of reagents, 10 min at −78 °C) led to the desired methyl 4- O -( o -nitro)benzyl gluco - and manno -α- d -glycopyranosides 8 and 9 in very good yield. However, prolonged reaction with 6 equiv of PhBCl 2 /Et 3 SiH transformed the methyl 4,6- O -( o -nitro)benzylidene α- d -glucopyranoside 11 into the reduced d -glucitol derivative 15 . Oxidative cleavage of 5,6-diol function of 15 gave the corresponding photolabile l -xylose 17 . The photolabile glucosides 3 and 8 have been further transformed into the photolabile α- C -allyl d -glucopyranosides 20 and 22 .