The total synthesis of nocarbenzoxazoles
F (1) and
G (2), originally obtained from the marine-derived halophilic
bacterial strain Nocardiopsis lucentensis DSM 44048,
was achieved via a simple and versatile route involving microwave-assisted
construction of a benzoxazole skeleton, followed by carbon–carbon
bond formation with the corresponding aryl bromides. Unfortunately,
the 1H and 13C NMR spectra of natural nocarbenzoxazole
G did not agree with those of the synthesized compound. In particular,
the spectra of the isolated and synthesized compounds showed considerable
differences in the signals from the protons and carbons in the aryl
group. The revised structure was validated by the total synthesis
of the actual nocarbenzoxazole G (8c) molecule, which
is a regioisomer of the compound that was reported earlier as nocarbenzoxazole
G. The synthesized derivatives showed specific cytotoxicity to the
human cervical carcinoma cell line, HeLa, but did not have any remarkable
effect on the other cell lines.
Unsymmetrical diarylalkynes were synthesized in moderate to good yields through a three‐component one‐pot procedure involving thermocontrolled sequential Sonogashira reactions with potassium ethynyltrifluoroborate and two different reactive aryl halides. The one‐pot procedure was initiated by the palladium/copper‐catalyzed Sonogashira coupling of potassium ethynyltrifluoroborate to an aryl iodide or electron‐deficient aryl bromide at 40 °C. Following a subsequent deboronative Sonogashira reaction of the in situ generated potassium (arylethynyl)trifluoroborate, a second coupling to a less‐active electron‐rich aryl bromide at 80 °C, without any additional palladium/copper catalyst or base, gave rise to unsymmetrical diarylalkynes.
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