An Fe(III)/NaBH4-mediated reaction for the functionalization of unactivated alkenes is described defining the alkene substrate scope, establishing the exclusive Markovnikov addition, exploring a range of free radical traps, examining the Fe(III) salt and initiating hydride source, introducing H2O-cosolvent mixtures, and exploring catalytic variants. Its use led to the preparation of a novel, potent and previously inaccessible C20′-vinblastine analogue.
A powerful Fe(III)/NaBH4-mediated free radical hydrofluorination of unactivated alkenes is disclosed using Selectfluor as a source of fluorine and resulting in exclusive Markovnikov addition. In contrast to the traditional and unmanageable free radical hydrofluorination of alkenes, the Fe(III)/NaBH4-mediated reaction is conducted under exceptionally mild reaction conditions (0 °C, 5 min, CH3CN/H2O). The reaction can be conducted open to the air and with water as a cosolvent and demonstrates an outstanding substrate scope and functional group tolerance.
N,N-dialkyl-N-chloroamines are an effective source of electrophilic nitrogen for nickel-catalyzed coupling with diarylzinc reagents. A variety of N-chloroamines as well as organozinc reagents react smoothly under the reaction conditions. A one-pot procedure that circumvents the need to isolate the N-chloroamines is described.
A series of disubstituted C20′–urea derivatives of vinblastine were prepared from 20′-aminovinblastine that was made accessible through a unique Fe(III)/NaBH4- mediated alkene functionalization reaction of anhydrovinblastine. Three analogs were examined across a panel of 15 human tumor cell lines, displaying remarkably potent cell growth inhibition activity (avg. IC50 = 200–300 pM), being 10–200-fold more potent than vinblastine (avg. IC50 = 6.1 nM). Significantly, the analogs also display further improved activity against the vinblastine-resistant HCT116/VM46 cell line that bears the clinically relevant overexpression of Pgp, exhibiting IC50 values on par with that of vinblastine against the sensitive HCT116 cell line, 100–200-fold greater than the activity of vinblastine against the resistant HCT116/VM46 cell line, and display a reduced 10–20-fold activity differential between the matched sensitive and resistant cell lines (vs 100-fold for vinblastine).
A systematic series of previously inaccessible key C20' urea and thiourea derivatives of vinblastine were prepared from 20'-aminovinblastine that was made accessible through a unique Fe(III)/NaBH4-mediated alkene functionalization reaction of anhydrovinblastine. Their examination defined key structural features of the urea-based analogues that contribute to their properties and provided derivatives that match or exceed the potency of vinblastine by as much as 10-fold in cell-based functional assays, which is directly related to their relative tubulin binding affinity. In contrast to expectations based on apparent steric constraints of the tubulin binding site surrounding the vinblastine C20' center depicted in an x-ray co-crystal structure, remarkably large C20' urea derivatives are accommodated.
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