(S)-a-(TrifluormethyIsulfonyloxy)carbonsZureester (S)-

The nucleotide sequence of the gene encoding the Escherichia coli selenocysteine tRNA (tRNA(SeCys] predicts an unusually long acceptor stem of 8 base pairs (one more than other tRNAs). Here we show by in vivo experiments (Northern blots, primer extension analysis) and by in vitro RNA processing studies that E. coli tRNA(SeCys) does contain this additional basepair, and that its formation results from abnormal cleavage by RNase P.

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The key step of our reaction sequence is the enantioand diastereoselective cyclopropanation of the acrylate derivative (Q-6 with isopropylidene-triphenylphosphane 7@l. A rationalization of the high stereoselection in favor of the (1R)-isomer is presented in Scheme 2. The assump-

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