4,4′-Dimethoxytrityl and 4,4′,4″-trimethoxytrityl as protecting groups for amino functions; selectivity for primary amino groups and application in ¹⁵N-labelling

4,4′-Dimethoxytrityl tetrafluoroborate (DMT⁺ BF₄^–) and 4,4′,4″-trimethoxytrityl tetrafluoroborate (TMT⁺ BF₄^–) are useful reagents for protecting primary and some secondary amines. Protected amines are obtained either by reaction of DMT⁺ BF₄^– or TMT⁺ BF₄^– with the amine or by alkylating DMT- or TMT-amine (available from DMT⁺ BF₄^– and TMT⁺ BF₄^– by treatment with ammonia). Alkylation of DMT- or TMT-amine stops after monoalkylation. Deprotection of the alkylated DMT- and TMT-amine is achieved by treatment with an acid of appropriate molarity (e.g. 0.1 M HCl in 1∶1 tetrahydrofuran–water for TMT-amines). The value of the methodology described is illustrated by a synthesis of (¹⁵NH₂) adenosine. X-Ray molecular structures of one DMT and two TMT derivatives are reported.