Reinhard Machinek

Activation of a dienediyne model of neocarzinostatin chromophore through an acid mediated solvolysis. Evidence for a new cyclization mode of enyne[3]cumulenes

When treated with 0.1 equiv. of triflic acid, a solution of the Z-configurated dienediyne 19 in a 2:3 mixture of tert-BuSH and CH 2Cl2 furnished the brightly yellow enyne[3]cumulene 20 through a SN-reaction. 20 reacted at room temperature through a Saito-Myers cyclization to the substituted styrenes 31 and 24 and through an unprecedented cycloisomerization to the anthracene 25.

Intramolecular charge transfer with 4-fluorofluorazene and the flexible 4-fluoro-N-phenylpyrrole

With 4-fluorofluorazene (FPP4F) and its flexible counterpart 4-fluoro-N-phenylpyrrole (PP4F) an intramolecular charge transfer (ICT) reaction occurs in the singlet excited state in sufficiently polar solvents. The ICT reaction begins to appear in tetrahydrofuran (epsilon = 7.4) for FPP4F and in the more polar 1,2-dichloroethane (epsilon = 10.4) with PP4F, showing its presence by dual fluorescence from a locally excited (LE) and an ICT state. Only LE fluorescence is observed in less polar solvents such as n-hexane. The ICT reaction is more pronounced with FPP4F than for PP4F, due to the smaller energy gap DeltaE(S1,S2) of the former molecule, in accordance with the PICT model. The occurrence of an ICT reaction is confirmed by the ICT dipole moments mu(e)(ICT) of 12 D (FPP4F) and 10 D (PP4F), clearly larger than mu(e)(LE) of approximately 4 D for FPP4F and PP4F. Isoemissive points are found in the fluorescence spectra of FPP4F and PP4F in acetonitrile (MeCN), ethyl cyanide (EtCN), and n-propyl cyanide (PrCN) as a function of temperature, confirming the two-state (LE and ICT) reaction mechanism. From plots of the logarithm of the ICT/LE fluorescence quantum yield ratio versus the reciprocal absolute temperature in these solvents, the ICT reaction enthalpies DeltaH are determined, with larger -DeltaH values for FPP4F than for PP4F: 19.2 as compared with 14.9 kJ/mol in MeCN, as an example. The picosecond fluorescence decay of PP4F at -45 degrees C becomes slower with decreasing solvent polarity, 5.1 ps (MeCN), 14 ps (EtCN), and 35 ps (PrCN), from which the LE --> ICT reaction rate constant is calculated, decreasing from 19 x 10(10) to 2.1 x 10(10) s(-1) between MeCN and PrCN. The femtosecond LE excited-state absorption spectra of FPP4F and PP4F do not undergo any time development in n-hexane (no ICT reaction), but show a fast ICT reaction in MeCN at 22 degrees C, with decay times of 1.1 ps (FPP4F) and 3.3 ps (PP4F). It is concluded that FPP4F and PP4F have a planar ICT state (PICT model), indicating that a perpendicular twist of the donor and acceptor subgroups in a donor/acceptor molecule is not a requirement for fast and efficient ICT. The molecular structures of FPP4F and PP4F obtained from X-ray crystal analysis reveal that the pyrrole group of PP4F is twisted over an angle theta = 25 degrees relative to the fluorophenyl moiety in the ground state, whereas as expected FPP4F is practically planar (theta = 2 degrees). The pyrrole-phenyl bond length of FPP4F (140.7 pm) is shorter than that for PP4F (141.8 pm).

Carboncarbon and carbonchalcogen spinspin coupling constants in chalcogen-substituted acetylenes

Abstract CarbonCarbon and carbonchalcogen spinspin coupling constants have been determined for chalcogen-substituted phenyl-, alkyl- and silyl-acetylenes. The changes in the CC couplings involving acetylenic sp carbons are explained in terms of the electronegativities of the substituent. The chalcogen-carbon coupling dependence on carbon hybridization previously noted for tellurium derivatives is confirmed.

Binding of actinomycin D to single-stranded DNA

Abstract The sequence specificity and structural aspects of the mode of interaction of the antitumor drug actinomycin D (AMD) with single-stranded DNA were studied by fluorescence, absorption and NMR spectroscopy, calorimetry, ultrasonic velocity and density measurements, and molecular modeling. The binding is length and sequence dependent, with the tetranucleotide motif TAGT showing the highest affinity. A “hemi-intercalation” model for the interaction is proposed.