Synthetic strategies towards 3’-deoxy-3’-C-methylenephosphinate building blocks were explored. The key transformations involved stereoselective hydroboration of 1-[2-O-(tert-butyldimethylsilyl-5-O-(4-methoxytrityl)-3-deoxy-3-C-methylene- ß -D-erythro-pentofuranosyl]uracil to give the corresponding 3’-deoxy-3’-C- hydroxymethyl derivative with ribo-configuration, as well as the further conversion into a precursor with a suitable leaving group, e. g., triflate, for subsequent substitution with the phosphinic acid synthon bis(trimethylsilyl)hypophosphite. Improvements of these steps enabled synthesis of 2’-O-(tert-butyldimethylsilyl-5’-O-( 4-methoxytrityl)-3’-deoxy-3’-C-methylenephosphinate uridine in a respectable overall yield of 40% over 6 steps, from the corresponding 2’-O-(tert-butyldimethylsilyl- 5’-O-(4-methoxytrityl)uridine.
For the introduction of internucleosidic 3’-deoxy-3’-C-methylenephosphonate linkages into oligonucleotides, a preparatory study of the elongation steps, i. e., coupling of the phosphinate building block to the 5’-hydroxyl function of a nucleoside derivative and subsequent oxidation, was performed. Of several coupling reagents studied for the activation of the phosphinate building block prior to coupling, the most promising proved to be N,N-bis(2-oxo-3-oxazolidin-1-yl)phosphinic chloride. The oxidation of the resulting 3’-deoxy-3’-C-methylenephosphinate ester to the corresponding 3’-deoxy-3’-C-methylenephosphonate linkage was achieved using iodine in pyridine-water, in the presence of a catalyst, i. e., either a base (triethylamine) or an acid (pyridinium salt).
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