Zemplen conditions

Zn metal. In addition, unprotected bipyridine-Cu(I) complexes were obtained via O-deacetylation under Zemplen conditions, in contrast to the terpyridine-Zn(II) adduct, where the complex decomposed. 

Boiling the first- and second-generation glycodendrons 396 and 397 in ethanol in the presence of c .v-Ru(bipy)2Cl2 or RuC13 followed by O-deacetylation under Zemplen conditions yielded dendritic complexes 398-400 (Scheme 42) containing 6 or 18 peripheral mannopyranosides, respectively. 

Methanolysis of 8-acetoxymethyl-5,6,7,8-tetrahydro-lH,4aH-pyrido[l,2-a] pyrimidin-l-one 78 under Zemplen condition in MeOH with catalytic amount of Na yielded 8-hydroxymethyl derivative 79, which underwent ring transformation by intramolecular attack of the hydroxy group on the carbonyl group to give oxazino[3,4-a]pyridinone 80 in 70% yield (07CAR1813). 

The last step is the cleavage of all protecting groups under Zemplen conditions, using NaOMe in MeOH.34 After deprotection, GM2 (14) is obtained. 

Standard conditions for the removal of ester protecting groups include a catalytic amount of sodium methoxide in methanol (Zemplen conditions), sometimes with a cosolvent. Additionally, other basic conditions such as ammonia in MeOH may be useful. 

Zinc catalyzed reductive elimination of acetobromoglucose to give triacetyl D-glucal [64] is the most usual entry into glycal chemistry. Usefully the product can be deprotected under Zemplen conditions and the resulting trihydroxyglucal [65] can be elaborated e.g. fully protected, as its trisilylether by treatment with an excess of r r -butyldimethylsilylchloride and imidazole in DMF [66]. 

An additional example was provided with the synthesis of p-(l (-linked tetra-saccharide 113 (Scheme 23). Toward this goal, allylic intermediate 107 was used as both precursor to the active donor form 108 and latent acceptor 109. Compotmd 108 was obtained by isomerization into its vinylic state using this time, butyl lithium in refluxing THF (88% yield) while 109 was obtained by de-O-acetylated under usual Zemplen conditions (NaOMe, MeOH). It is noteworthy to mention that the 4-0-acetate in 107 resisted the harsher isomerization conditions. As described for disaccharide 106 above, the acid-catalyzed (TMSOTf or NlS/TfOH) glycosidation occurred uneventfully to provide intermediate disaccharide 110 in 78% (a/p = 1 8) and 73% (a/p = 3 1) yields, respectively. Reiteration of the process provided tetrasaccharide 113 in 83% yield using TMSOTf in acetonitrile (a/p = 1 8). 

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