Trigonal-bipyramid structures are indicated for PF6 (1) and CH3PF4 (2) by electron diffraction data,2•3 for (CF3)2PF3 (3) by ir and Raman spectra,4 and for PC12F3 (4) by chlorine nuclear quadrupole resonance *

The sequence-specific recognition of double-helical DNA by oligonucleotide-directed triple-helix formation is limited mostly to purine tracts. Design leads that could expand the recognition code to all four Watsonxrick base pairs would provide one step toward a general solution targeting single sites in megabase size DNA. The nonnatural deoxyribonucleoside 1 -(2-deoxy-8-~-ribofuranosyl)-4-( 3-benzamidophenyl)imidazole (D3) was synthesized in four steps and incorporated by automated methods into pyrimidine oligodeoxyribonucleotides. Within a pyrimidine oligonucleotide, D3 binds pyrimidinepurine base pairs with higher affinity than it binds purinepyrimidine base pairs. From affinity-cleaving analysis, the stabilities of base triplets decrease in the order D3.TA N D3CG > D3.AT > D3.GC. Such specificity allows binding by triple-helix formation at an 18 base pair site in SV40 DNA containing all four base pairs at physiologically relevant pH and temperature. The stabilities of these novel triplets may be an example of shape-selective recognition of CG and TA Watson-Crick base pairs in the major groove.

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Reactions of phosphorus(v) compounds involving the mutual interconversion of tetra-and pentacoordinate species are discussed in a critical review emphasizing stereochemical implications of the reaction mechanism. This discussion includes the formation and decomposition of the stable oxyphosphoranes, the Michaelis-Arbusov, Perkov, and Wittig reactions, interconversions of phosphines and their oxides, and the nucleophilic displacements on phosphonium compounds. Reactions of phosphate esters and related compounds receive particular attention. All chemical arguments are derived by considering the effect of factors determining the relative stabilities of phosphorane derivatives, their rates of formation, decomposition and rearrangement by bond deformation or rupture and recombination processes, considerations which are uniformly applied on the basis of concepts developed in a preceding communication'']. It is shown that a comprehensive mechanistic interpretation of the foregoing reactions requires substantial addition to available conceptual foundations such that, in many cases, present concepts and mechanisms must be revised.