Issue in Honor of Prof. Rudolph Abramovitch ARKIVOC 2001 (vi) 16-29
Scheme 9. Preparation of an oligonucleotide containing the N2 guanine trans adduct of the (+)anti dihydrodiol epoxide of benzo[a]pyrene using the post-oligomerization approach.
While we were searching for better conditions for preparing PAH-adducted oligonucleotides, it was found that oligonucleotides containing 2-fluoro-O6-(trimethylsilylethyl)inosine could be deprotected and removed from the solid support without loss of the fluoro or trimethylsilylethyl groups by treatment with 0.1 M NaOH (8 hr, rt) (Scheme 10a).31 The triflate derivative is too labile to survive this treatment. A similar approach has been successful for preparation of fully deprotected oligonucleotides containing 6-chloropurine but not 6-fluoropurine (Scheme 10b).34 These procedures offer the advantage that the oligonucleotides containing the guanine and adenine synthons can be purified prior to reaction with nucleophiles. Other advantages are that displacement reactions can be carried out in homogeneous solution rather than on the solid support and the progress of reactions can easily be monitored by HPLC or other analytical methods. The homogeneous methods are useful for reactions with unencumbered nucleophiles and they have been used to prepare oligonucleotides containing adducts of styrene oxide, the monoepoxide of butadiene and the epoxide of 1,2-dihydroxy-3-butene.34,35 HPLC purification is carried out first at the halopurine stage and again with the adducted oligonucleotide, making it easier to obtain the high purity needed for mutagenesis studies.
The homogeneous methods have been used to advantage for synthesis of DNA containing interchain crosslinks.34,36-38 Examples include dehydroretronecine and trimethylene crosslinks between guanines in a self-complementary sequence, a trimethylene crosslink between guanines in a non-self-complementary sequence, and a tetramethylene crosslink between adenines, also in a non-self-complementary sequence (Scheme 11).
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