Studies on the role of tetrazole in the activation of phosphoramidites

The mechanism of the tetrazole-activated coupling step in the synthesis of oligonucleotides via phosphoramidites is studied with the help of model reactions: Treatment of diethoxydiisopropylaminophosphane with two equivalents of tetrazole resulted in a diethoxy-tetrazolophosphane, whose (31P)-NMR shift of 126 ppm is identical with the signal observed during internucleotide bond formation. A series of different related diethoxy-phosphorous-acid derivatives were also synthesized; their (31P)-NMR signals between 123.9 and 130.8 ppm are additional evidence for the intermediacy of a tetrazolide species. Further NMR investigations with more basic azoles showed that tetrazole is also active as a proton donor.