Ammonia deblocking with

The 9-a-L-rhamnopyranosyl derivative 246 has been synthesized by the condensation of the 2,3,4-tri-O-benzoyl-L-rhamnopyranosylbromide (245) with chloromercuric complex 244. Deblocking of 246 with methanolic ammonia gave the 8-azaadenosine 247 [84IJC(B)369] (Scheme 52). 

The simpler thiophene derivative (280 R = R1 = H, R2 = CHO) is converted to a C-nucleoside (281 R1 = tribenzoylribose). Reaction with methanolic ammonia at 100°C cyclized the pyrimidine ring affording no substituent at C-2, and deblocked the nucleoside <88TL3537>. 

Whereas ammonia is an unsuitable nucleophile towards 7r-allylpalladium complexes, primary amines often participate well. Benzylamine and 4,4 -dimethoxybenzhydrylamine are especially useful, since subsequent removal of the benzylic substituents permits their use as ammonia equivalents. Such a deblocking procedure was used in a short synthesis of the enzyme inhibitor gabaculine from amino ester 1118. An allyl carbonate can be successfully employed as a substrate for palladium(0)-catalyzed animation to give 1219,20, but experimental difficulties have been experienced with a structurally similar carbonate9. As shown by the formation of 1016 and by the partial production of 13 prior to basic hydrolysis16, additional attack on an ester function may occur. Amino alcohols 13, formed with complete regioselectivity due to steric reasons, have been efficiently converted into isoquinuclidines16. 

Zemplen deacylation is the most commonly used deblocking reaction for the removal of ester protecting groups [220]. Using this transesterification reaction, OH-functions can be regenerated under mild conditions, in methanol with a catalytic amount of sodium methoxide at room temperature. The difference in the rate of benzoate and acetate solvolysis is sufficient to enable removal of acetates in the presence of benzoates. Typical conditions for this selective cleavage include ammonia in MeOH. 

Treatment of the silver salt of methyl imidazole-4,5-dicarboxylate (138) with acylglycopyranosyl halides gave the imidazole nucleosides (139) which, on reaction with ammonia, were deblocked and converted to the diamides (140). Reaction of the diamides (140) with alkaline hypobromite (the Hofmann reaction) gave mixtures of the 7- and 9-nucleosides (141 and 142), with the 9-isomer predominating. In fact, in the early investiga-tions- of this reaction, only the 9-isomer w as isolated but, in the preparation of 8-azapurine nucleosides by the same method,both... 

The recovery of the deblocked polysaccharide may be most conveniently achieved by sodium in liquid ammonia. Usually, deblocking is accompanied by chain scission, but this is negligible under proper conditions. For example, debenzylation of a polymer with DPn close to 900, which requires the removing of 2700 benzyl ether functions, causes a three- to four-fold decrease in DPn 20,21 Sterically pure linear polysaccharides with a number average degree of polymerization of 150-300 can be prepared by cationic polymerization of ether derivatives of 1,6-anhydro sugars followed by deblocking. 

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