Total Synthesis of (+)-Discorhabdin V.

The discorhabdin natural products are a large subset of pyrroloiminoquinone alkaloids with a myriad of biological activities. Despite garnering much synthetic attention, few members have thus far been completed, particularly those featuring a bridging carbon-nitrogen bond that is found in numerous discorhabdins, including discorhabdin V. Herein we report the first total synthesis and full stereochemical assignment of (+)-discorhabdin V. To access the pyrroloiminoquinone we developed a convergent N-alkylation/oxidative aminocyclization/bromination cascade that joins two key components, which are both made on multigram scale. An intramolecular Heck reaction then forms the quaternary carbon center in an intermediate containing the carbon-nitrogen bridge, and a reductive N,O-acetal cyclization sequence introduces the final piperidine ring. Furthermore, we have established the relative configuration of (+)-discorhabdin V through experimental NOESY data and DP4 NMR probability calculations. The absolute configuration of the natural product has also been determined by circular dichroism and the use of an amino acid derived chiral starting material. Our work represents one of only two reports of a total synthesis of a nitrogen-bridged discorhabdin and paves the way for future biological evaluation of such compounds.

Going Homoleptic: Synthesis and Full Characterization of Stable Manganese Tetranitrosyl Cation Salts.

Although similar to carbon monoxide, the chemistry of homoleptic nitrogen monoxide complexes is fundamentally unexplored compared to their carbonyl analogues. Herein we report the synthesis of the first truly homoleptic transition-metal nitrosyl cation as the salt of the weakly coordinating anions (WCAs) [Al(ORF )4 ]- and [F{Al(ORF )3 }2 ]- (RF =C(CF3 )3 ). These salts are easily accessible in good yields, phase pure, and were fully characterized by IR/Raman, NMR and UV/Vis spectroscopy as well as single-crystal and powder X-ray diffraction. They may serve as unprecedented simple model systems for theoretical and experimental studies of nitrosyl complexes.

Structural and electrochemical comparison of trinuclear ruthenium oxo clusters [Ru3(OAc)6O(L)3]+and [Ru3(OAc)6O(L)2(CO)] (L = imidazole, benzimidazole, and 4-phenylpyridine).

The triruthenium oxo clusters [Ru3(OAc)6O(L)3]+ and [Ru3(OAc)6O(L)2(CO)] possess unique electronic characteristics that vary based on the ligands L. Here we report an investigation of the structural, electrochemical, and optical properties of clusters with imidazole, benzimidazole, and 4-phenylpyridine ligands. The complexes [Ru3(OAc)6O(L)3]+ [1+: L = imidazole (im); 2+: L = benzimidazole (benzim); 3+: L = 4-phenylpyridine (4PP)] and [Ru3(OAc)6O(L)2(CO)] (1-CO and 3-CO) were synthesized by reaction of either [Ru3(OAc)6O(MeOH)3]+ or [Ru3(OAc)6O(MeOH)2(CO)], respectively, with the corresponding heterocycle. We further discovered that [3]OAc could be reduced to the mixed-valence neutral state 3 by refluxing the complex under nitrogen in methanol. Single-crystal X-ray structure analysis of hexa-µ2-acetato-µ3-oxido-tris(1H-imidazole)triruthenium hexafluorophosphate acetonitrile hemisolvate, [Ru3(C2H3O2)6O(C3H4N2)3]PF6·0.5CH3CN, [1]PF6, hexa-µ2-acetato-carbonylbis(1H-imidazole)-µ3-oxido-triruthenium methanol monosolvate, [Ru3(C2H3O2)6O(C3H4N2)(CO)]·CH3OH, 1-CO, hexa-µ2-acetato-µ3-oxido-tris(4-phenylpyridine)triruthenium pentahydrate, [Ru3(C2H3O2)6O(C11H9N)3]·5H2O, 3, and hexa-µ2-acetato-carbonyl-µ3-oxido-bis(4-phenylpyridine)triruthenium methanol monosolvate, [Ru3(C2H3O2)6O(C11H9N)2(CO)]·CH3OH, 3-CO, show the expected triruthenium µ3-oxo core and N-coordination of the ligands. Cyclic voltammetry revealed quasi-reversible and irreversible redox couples in [1]PF6, 1-CO, and [2]PF6, while [3]PF6 and 3-CO exhibit reversible redox couples. The optical properties of these richly colored species were investigated using UV-Vis spectroscopy.