No abstract

Abstract. A detailed mechanism is given for the isomerization of right-handed DNA into . left-handed DNA.Recently the crystallization and structural analysis of a double helical fragment of DNA with the sequence d(CpGpCpGpCpG) was offered by Wang e t a l . I . The structure is found to be a left-handed DNA double helix frag-

CND0/2 calculations are reported on four caged phosphorus compounds, Y-P(OCH,CH,),X (X = N, CH; Y = 0, OH', H'). Structures with and without a transannular bond between P and X were taken into account; the influence of this bond on the stability of the compounds was examined. For X = N and Y = Hf the lowest-energy structure corresponds with the geometry found by X-ray analysis. If X = CH, formation of a transannular bond proves to be unlikely, as expected. The calculations predict the formation of a P-N bond upon protonation of the phosphate (X = N, Y = 0). The net atomic charges in the protonated phosphite were used to calculate its NMR coupling constant, lJpH, which correlates well with the value found experimentally. A . R. Gregory and M. Paddon-Row, J. Am. Chem. SOC. 98,7521 (1976). L. J. VandeGriend, J . G. Verkade, J . F. M. Pennings and H . M. Buck, J. Am. Chem. SOC. 99, 2459 (1977).

Marianne M . C. F. Castelijns et al. 1 Salt-induced conversioti.v 1 4 /'( V ) i i i i o P( I V ) in solvents of different polarity Full papers Salt-induced conversions of P(V) into P(IV) in solvents of different polarity.Abstract. Evidence is presented for the involvement of pentaco-ordinated phosphorus intermediates in the rate-determining step of the de-alkylation reactions of some phosphates and phosphonates with LiCl and LiBr. Such involvement is suggested to lead to the zwitterionic pentaco-ordinated phosphorus complexes 5a and 5b, respectively when compounds 2a and 2b are treated with LiCl and LiBr. The existence of these complexes shows unambiguously the possibility of inducing (slight) conformational changes of phosphorus in the presence of cations and anions (or nucleophilic solvents). This approach may also be useful for the description of conformational changes in phosphate residues in DNA, thus leading to different conformations of DNA. Salt-induced conversions of P ( V ) into P ( I V ) in solventsof difSerent polarity: intramolecular group transfer. Nature 247, 558 (1974). l4 H . M. Buck, Red. Trav. Chim. Pays-Bas 99, 181 (1980).

Monodesalkylierungsreaktionen der Phosphate und Phosphonate (I)‐(III) mit LiCl oder LiBr in Lösungsmitteln verschiedener Polarität werden NMR‐ spektroskopisch untersucht.