New building blocks for fluorinated bioimidazole derivatives II: preparation of beta-fluorourocanic acids

Selective Synthesis of Monofluorinated Compounds Applying Amine/HF Reagents

For nucleophilic monofluorination, amine/HF reagents such as Et3 N⋅3HF, Pyr⋅9HF (Olah's reagent) and similar combinations belong to the most frequently used fluoride sources, whereupon the selectivity of these reagents can be very different depending of its acidity, the nucleophilicity of the fluoride equivalent, and the structure of the particular substrate. These reagents can be used safely in ordinary chemistry laboratories for nucleophilic substitution reactions by fluoride at sp3 -hybridized carbon centers. For ring opening reactions of epoxides, the regio- and stereoselectivity is very much depending of the nature of the epoxide and the acidity of the HF reagent favoring either SN 1 or SN 2 type reactions. Similarly, the outcome of halofluorination and similar reactions with sulfur or seleno electrophiles can be controlled by the particular combination of the electrophile and the fluoride source. Examples for the application of these reaction types for the synthesis of fluorine-containing analogues of natural products or biologically relevant compounds are in the focus of this personal account.

Electrochemical Fluorination of Vinyl Boronates through Donor-Stabilized Vinyl Carbocation Intermediates

The electrochemical generation of vinyl carbocations from alkenyl boronic esters and boronates is reported. Using easy-to-handle nucleophilic fluoride reagents, these intermediates are trapped to form fully substituted vinyl fluorides. Mechanistic studies support the formation of dicoordinated carbocations through sequential single-electron oxidation events. Notably, this electrochemical fluorination features fast reaction times and Lewis acid-free conditions. This transformation provides a complementary method to access vinyl fluorides with simple fluoride salts such as TBAF.

Synthesis of 7-(15)N-Oroidin and evaluation of utility for biosynthetic studies of pyrrole-imidazole alkaloids by microscale (1)H-(15)N HSQC and FTMS

Numerous marine-derived pyrrole-imidazole alkaloids (PIAs), ostensibly derived from the simple precursor oroidin, 1a, have been reported and have garnered intense synthetic interest due to their complex structures and in some cases biological activity; however very little is known regarding their biosynthesis. We describe a concise synthesis of 7-(15)N-oroidin (1d) from urocanic acid and a direct method for measurement of (15)N incorporation by pulse labeling and analysis by 1D (1)H-(15)N HSQC NMR and FTMS. Using a mock pulse labeling experiment, we estimate the limit of detection (LOD) for incorporation of newly biosynthesized PIA by 1D (1)H-(15)N HSQC to be 0.96 microg equivalent of (15)N-oroidin (2.4 nmole) in a background of 1500 microg of unlabeled oroidin (about 1 part per 1600). 7-(15)N-Oroidin will find utility in biosynthetic feeding experiments with live sponges to provide direct information to clarify the pathways leading to more complex pyrrole-imidazole alkaloids.

Preparation of (Fluoromethyl)- and (Difluoromethyl)imidazoles

2-(Fluoromethyl)- and 2-(difluoromethyl)imidazoles, and 4-(fluoromethyl)- and 4-(difluoromethyl)imidazoles have been prepared by deoxyfluorination of (hydroxymethyl)imidazole or formylimidazole precursors.