Synthesis of the 8,19-Epoxysteroid Eurysterol†A.

We report the first chemical synthesis of eurysterol A, a cytotoxic and antifungal marine steroidal sulfate with a unique C8-C19 oxy-bridged cholestane skeleton. After C19 hydroxylation of cholesteryl acetate, used as an inexpensive commercial starting material, the challenging oxidative functionalization of ring B was achieved by two different routes to set up a 5α-hydroxy-7-en-6-one moiety. As a key step, an intramolecular oxa-Michael addition was exploited to close the oxy-bridge (8ß,19-epoxy unit). DFT calculations show this reversible transformation being exergonic by about -30 kJ mol-1 . Along the optimized (scalable) synthetic sequence, the target natural product was obtained in only 11 steps in 5 % overall yield. In addition, an access to (isomeric) 7ß,19-epoxy steroids with a previously unknown pentacyclic ring system was discovered.

Hydrogen Peroxide Sensors Based on Fluorescence Quenching of the 2-AminobenzimidazoleFluorophore.

The fluorescence emission of the parent 2-aminobenzimidazole (ABZ, 1), the mono- and disubstituted derivatives (2, 3), 2-aminonaphthoimidazole (4), and 4-amino dinaphthodiazepine 5 (λem = 315-400 nm) is strongly quenched in the presence of aqueous hydrogen peroxide. The quenching process is dual: for diazepine 5, quenching is dynamic at lower H2O2 concentrations with linear reduction of the fluorescence lifetime from 4.3 to 2.6 ns. At higher H2O2 concentrations, a second species appears in the absorption and emission spectra with fluorescence lifetimes of 1.3 ns, indicating the formation of a new (ground-state) hydrogen-bonded ABZ-H2O2 complex (static quenching). Sensors 1 and 2 show also dual quenching that fits with a static 1:1 and 1:2 model with K1:1 = 8(11) M-1 and K1:2 = 21(147) M-1 for 1(2). The formation of a 1:2 complex (1:(H2O2)2) is also supported by density functional theory (DFT) calculations and spectra simulations.