The synthesis of oligonucleotides derived from uridine and adenosine using the tert-butyldimethylsilyl protecting group and the trichloroethylphosphorodichloridite condensation procedure is described. These procedures involve rapid preparation of protected starting materials and equally rapid condensation of units to form nucleotides. Optimum yields using collidine as base in tetrahydrofuran as solvent are between 65 and 80%. The utility of the procedure is rendered complete by the capability to remove both the phosphate protecting groups and the alkylsilyl groups in a single 30 min step using tetrabutylammonium fluoride in tetrahydrofuran.

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. Can. J. Chem. 67, 109 (1989). Syntheses are described for d ( e q p~) (1) and d (~~e ? ) (2), the 04-ethylated analogues of d(TpT) (3), and mass spectrometric (FAB), 'H nuclear magnetic resonance (300 MHz), and circular dichroic data are provided for the three molecules. The [M -HI-ions as well as the aggregates [2M -HI-and, in the case of 1, [3M -HI-, are easily detected by FAB mass spectrometry operating in the negative ion mode. In contrast with the modified nucleoside, e 4 d~, the two methylene protons of the 04-ethyl groups of 1 and 2 are nonequivalent in the NMR spectra, and their chemical shift difference decreases with increasing temperature. This chemical shift difference can be related to intramolecular base stacking. The vicinal coupling constants of the sugar rings reveal an unusual shift at low temperature towards the 3'-endo conformation of the e v p unit of 1, but this shift is not noted for the pe? unit of 2.04-Alkylation of either unit of 3 effects a large reduction in the amplitude of the negative CD peak at 252 nm. The CD spectra of 1 and 2 are qualitatively similar to those of d(CpT) and d(TpC), respectively, and suggest that in some respects an 04-alkylated thymine base is "cytosine-like". The amplitudes of the positive CD peaks of 1 and 3 are reduced by 40% as the temperature is increased from 10 to 65"C, indicating the presence of some base stacking at low temperatures. However, the CD spectrum of 2 shows a much smaller reduction (

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