N⁶‐(Carbamoylmethyl)‐2′‐deoxyadenosine, a rare DNA Constituent: Phosphoramidite Synthesis and Properties of Palindromic Dodecanucleotides

N 6 ‐(Carbamoylmethyl)‐2′‐deoxyadenosine ( 1 ), a modified nucleoside occurring in bacteriophage Mu, was synthesized by two different routes. Glycinamide was introdued by nucleophilic displacement of(2,4,6,‐triisopro‐pylphenyl)sulfonyloxy or ethylsulfinyl groups at C(6) of the purine moiety. Compound 1 was converted into the protected phosphoramidite 6b and employed in solid‐phase synthesis of the self‐complementary oligonucleotides 7–14 . Replacement of 2′‐deoxyadenosine by 1 led to a strong decrease of the T m values of the oligomers d(A‐T) 6 ( 7 ) and d(A‐T‐G‐A‐A‐G‐C‐T‐T‐C‐A‐T)( 10 ), respectively. As the oligemer 10 contains the recognition site d(A‐A‐G‐C‐T‐T) of the endodeoxyribonuclease Hind III, it was subjected to sequence‐specific hydrolysis experiments. Replacement of the first or second A d by 1 prevented enzymatic phosphodiester hydrolysis (results with 11 and 12 ). In contrast, slow hydrolysis was observed if the less bulky N 6 ‐methyl‐2′‐deoxyadenosine replaced the second A d residue (results with 14 ).