Amino groups, masking

An alternative base labile 5 -protecting group, 2-dansylethoxycarbonyl (111) has been proposed for RNA synthesis utilising tetrahydro-4-methoxy-2H-pyran-4-yl for 2 -protection, 2-(4-nitrophenyl)ethyl and 2-(4-nitrophenyl)ethoxycarbonyl for phosphate and amino group masking. 

Maleic acid is a linear four carbon molecule with carboxylate groups on both ends and a double bond between the central carbon atoms. The anhydride of maleic acid is a cyclic molecule containing five atoms. Although the reactivity of maleic anhydride is similar to other cyclic anhydrides, the products of maleylation are much more unstable toward hydrolysis, and the site of unsaturation lends itself to additional side reactions. Acylation products of amino groups with maleic anhydride are stable at neutral pH and above, but they readily hydrolyze at acid pH values around pH 3.5 (Butler et al., 1967). Maleylation of sulfhydryls and the phe-nolate of tyrosine are even more sensitive to hydrolysis. Thus, maleic anhydride is an excellent reversible blocker of amino groups to temporarily mask them from reactivity while another... 

Goldberger, R., and C. B. Anftnsen Reversible masking of amino-groups in ribonuclease and its possible usefulness in the sjmthcsis of the protein. Biochemistry /. 401—405 (1962). 

The masking of the normal reaetions of simple ligands, such as the nitro, cyano, and ammonia groups, by coordination to a metal is a phenomenon encountered early by a chemist. One of the first examples of masking in a chelate complex was reported, signifieantly, in biological journals. It involves the protection by copper ion of the a-amino group in ornithine and lysine ... 

From the aforementioned, it should be evident that the amino group is one of the most reactive functionalities towards dioxirane oxidation consequently, to achieve a chemo-selective oxidation of a multi-functionalized substrate that possesses an amino group, the latter must be protected. This may be accomplished by masking the amino substituent in the form of its ammonium salt ", or BF3 complex ", even better as an amide functionality (iV-phenylacetamide resists TFD oxidation at room temperature""). This will reduce sufficiently the oxidative reactivity of the amino group, such that another less reactive group may be selectively oxidized" ... 

In general, amino-containing compounds are poor partners in CM due to their tendency to coordinate to the metal catalyst, even when late-transition metal systems are employed. To preserve catalyst activity, amino groups are therefore typically masked as cyanides, carbamates, amides, or phthalimides. In substrates where iV-coordination to the catalyst is less favored, such as with hindered /V-aryl-/V-allylamines, no protecting group is required. 

The same sequence of reactions was accomplished on the bicyclic iodide 16-R (Scheme 10), the reaction of which with tetrabutylammonium azide in toluene at 60 °C afforded the azide 33-R in 76% yield. Selective deprotection of the primary hydroxyl group of compound 33-R and Jones oxidation, finally afforded compound 35-R as masked glyco-aminoacid. The azido group was maintained in these compounds as protected equivalent of the amino group. 

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