A new labelling technique attaching fluorescein via a carbamoyl linker directly to the amino groups of the nucleobases was developed. The amino groups were first converted to the phenoxycarbonyl derivatives ( 3 10, 15, 19, 58), which reacted under mild conditions with 5-aminofluorescein to give the corresponding N-[(fluorescein-5-ylamino)carbonyl] derivatives ( 3 11 ± 14, 16, 17, 20, 59, 60). The introduction of the 5-aminofluorescein residue into properly protected adenylyl-adenosine dimers ( 3 39, 40) and trimer ( 3 50) worked well, and final deprotection of these uniformly blocked precursors led on treatment with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), in one step to dimer 41 and trimer 51. Synthesis of an appropriately protected monomeric phosphoramidite building block ( 3 75) was more difficult, since introduction of the 2-(4-nitrophenyl)ethyl residue into the fluorescein moiety in 59 led mainly to trisubstitution to give 61 including the urea function. Formation of the adenylyl dimer 66 and trimer 67 proceeded in the usual manner by phosphoramidite chemistry; however, deprotection of 67 with DBU was incomplete since the O-alkyl group at the urea moiety was found to be very stable. Finally, the appropriate phosphoramidite building block 75 could be synthesized by the sequence 59 3 72 3 73 3 74 3 75. The phosphoramidite 75 was used for the synthesis of dimer 77 and trimer 79 by solution chemistry, as well as for that of various oligonucleotides by the machineaided approach on solid support carrying the fluorophore at different positions of the chain ( 3 84 ± 87). The attachment of the fluorescein fluorophor via a short carbamoyl linker onto the 6-amino group of 2'-deoxyadenosine enables such molecules to function very well in fluorescence-polarization experiments.

A new labelling technique attaching fluorescein via a carbamoyl linker directly to the amino groups of the nucleobases was developed. The amino groups were first converted to the phenoxycarbonyl derivatives (→10, 15, 19, 58), which reacted under mild conditions with 5‐aminofluorescein to give the corresponding N‐[(fluorescein‐5‐ylamino)carbonyl] derivatives (→11–14, 16, 17, 20, 59, 60). The introduction of the 5‐aminofluorescein residue into properly protected adenylyl‐adenosine dimers (→39, 40) and trimer (→50) worked well, and final deprotection of these uniformly blocked precursors led on treatment with DBU (1,8‐diazabicyclo[5.4.0]undec‐7‐ene), in one step to dimer 41 and trimer 51. Synthesis of an appropriately protected monomeric phosphoramidite building block (→75) was more difficult, since introduction of the 2‐(4‐nitrophenyl)ethyl residue into the fluorescein moiety in 59 led mainly to trisubstitution to give 61 including the urea function. Formation of the adenylyl dimer 66 and trimer 67 proceeded in the usual manner by phosphoramidite chemistry; however, deprotection of 67 with DBU was incomplete since the O‐alkyl group at the urea moiety was found to be very stable. Finally, the appropriate phosphoramidite building block 75 could be synthesized by the sequence 59→72→73→74→75. The phosphoramidite 75 was used for the synthesis of dimer 77 and trimer 79 by solution chemistry, as well as for that of various oligonucleotides by the machine‐aided approach on solid support carrying the fluorophore at different positions of the chain (→84–87). The attachment of the fluorescein fluorophor via a short carbamoyl linker onto the 6‐amino group of 2′‐deoxyadenosine enables such molecules to function very well in fluorescence‐polarization experiments.

No abstract

The 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) group, introduced 1984 as protecting group for exocyclic amino functions of nucleic-acid bases, reacts with amines under mild conditions to urea derivatives. Treatment of 2',5'-di-0-acetyl-N6-[2-(4-nitrophenyl)ethoxycarbony~]cordycepin (3) with NHJMeOH overnight at room temperature affords cordycepin (4) and N6-carbamoylcordycepin (5). Preliminary investigations towards the elucidation of the reaction mechanism indicate that the aminolysis proceeds via an addition-elimination or an isocyanate mechanism, depending on the reaction conditions. The phenoxycarbonyl (phoc) group at N6 or N4 was chosen to study the mild conversion of carbamates with aromatic amines into ureas of adenosine and cytidine, respectively.

The application of the improved phosphoramidite strategy for the synthesis of oligonucleotides using p-eliminating protecting groups to phospholipid chemistry offers the possibility to synthesize phospholipid conjugates of AZT (6) and cordycepin. The synthesis of 3'-azido-3'-deoxythymidine (61 was achieved by a new isolation procedure without chromatographic purification steps in an overall yield of 50%. Protected cordycepin ( = 3'-deoxyadenosine) derivatives, the N6,2'-bis[2-(4-nitrophenyl)ethoxycarbonyl]co~dycepin (12) and the N6,5'-bis[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (13) were prepared by known methods and direct acylation of N6-[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (9), respectively. These protected nucleosides and the 3'-azido-3'-deoxythymidine (6) reacted with newly synthesized and properly characterized lipid-phosphoramidites 21-25, catalyzed by IH-tetrazole, to the corresponding nucleoside-phospholipid conjugates 26-38 in high yield. The deprotection was accomplished via 8-elimination with 1,8-diazabicyclo[5.4.O]undec-7-ene (DBU) in aprotic solvents to give analytically pure nucleoside-phospholipid diesters 39-51 as trie1:hylammonium or sodium salts. The newly synthesized compounds were characterized by elemental analyses and 1JV and 'H-NMR spectra.