A multikilogram-scale synthesis of l-lyxonolactone-2,3-O-isopropylidene is reported. It proceeds efficiently from an optimized, large-scale, aqueous bromine oxidation of d-ribose to
d-ribonolactone including a one-pot isopropylidene formation,
and subsequent conversion of the d-ribonolactone-2,3-O-isopropylidene to l-lyxonolactone-2,3-O-isopropylidene via the
derived C5-mesylate and intramolecular relactonization of the
product of aqueous potassium hydroxide cleavage of the
d-ribonolactone ring. The inversion of configuration at the C4-chiral center is understood in terms of an intermediating C4−C5-epoxide. The overall process is noteworthy for its operational
simplicity, stereochemical integrity, and use of inexpensive
chemicals.
Lead tetraacetate one-electron oxidation of nine different N-alkoxy-2,6dinitroanilines substituted with trifluoromethyl, methyl and nitro groups in position 4 yielded aminyl radicals for which hypertine couplings were measured by ENDOR and TRIPLE resonance spectroscopy. The optimum temperature range for proton ENDOR and general TRIPLE resonance measurements of aminyl radicals was 210-250 K and for nitrogen ENDOR 260 K in toluene. Further lowering for the temperature rapidly decreased the EPR intensity. The concentration of the sample and the amount of oxidant were optimized for obtaining ENDOR spectra. The relative signs of the hyperfine couplings of nitrogens, fluorines and protons were determined in the basis of general TRIPLE resonance experiments. The oxidation of N-methoxy-N-2,6-dinitrophenylamine and N-ethoxy-N-2,6-dinitrophenyla~ne with lead tetraacetate produced first the aminyl radical of the respective 2,Wnitro compound at low temperature (below 260 K) and very soon afterwards the aminyl radical of the respective 2,4,6-trinitro compound, by a route in which hydrogen has to be removed from the system, Rotational correlation times were estimated for N-methoxyand N-ethoxy-N-2,4,6-trinitrophenylaminyl radicals. Nitroxyl radicals were detected only under conditions where oxygen was present in the solvent.
Oxidation of N-alkoxy-2,6- or -2,4-dinitroanilines with various oxidizing agents yields the corresponding N-alkoxydinitrophenyl-aminyls, which then undergo spontaneous conversion into the corresponding N-alkoxy-2,4,6-trinitrophenyl-aminyls. The EPR spectra of all these persistent aminyls are presented and discussed. Such disproportionations did not occur in capto-dative diaryl-aminyls which had lower spin densities on the electron-acceptor group than the present N–alkoxy-polynitrophenyl-aminyls (these have, in addition. Ν—Ο bonds which are less strong than N—C bonds). Electronic structures of diaryl-aminyls and N-alkoxy-aryl-aminyls are discussed: both are π-radicals, but they probably possess at the nitrogen atom an approximately linear, and a kinked structure, respectively.
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