Protection, blocking, masking amino

Aliphatic aldehydes typically provide only moderate yields in the Biginelli reaction unless special reaction conditions are employed, such as Lewis-acid catalysts or solvent-free methods, or the aldehydes are used in protected form [96]. The C4-unsubstituted DHPM can be prepared in a similar manner employing suitable formaldehyde synthons [96]. Of particular interest are reactions where the aldehyde component is derived from a carbohydrate. In such transformations, DHPMs having a sugar-like moiety in position 4 (C-nucleoside analogues) are obtained (see Section 4.7) [97-106]. Also of interest is the use of masked amino acids as building blocks [107, 108]. In a few cases, bisaldehydes have been used as synthons in Biginelli reactions [89, 109, 110]. 

Amino protecting groups fall into four broad categories nitrobenzyl (e.g., nitrobenzyloxy-carbonyl and 4,5-dimethoxy-2-nitrobenzyloxycarbonyl), phenacyl (e.g., 4-methoxyphen-acyloxycarbonyl), benzyloxycarbonyl (e.g., a,a-dimethyl-3,5-dimethoxybenzyloxycarbonyl), and arylsulfonamides (e.g., tosyl). All but the sulfonamides mask the amino group as a carbamate. Removal of the blocking moiety releases an unstable carbamic acid, which spontaneously decarboxylates to give the unprotected amine. 

Another masking strategy, binary synthesis, utilizing four photoprotected amino acids A, B, C, and D, is presented in Figure 3. In this approach, each building block is added after one half of the synthesis area is photolyzed. The left half is illuminated through mask 1 and is then derivatized with building block A, while the other half bears only the protected amino... 

In nucleotide chemistry amide-type protecting functions have proven to be very useful and are widely used. The amino group in guanine is usually blocked as the isobutyramide (20), adenine is masked as the... 

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