Investigations in the fie ld of substituted furanurona ldehydes steric directivity of the nitromethane condensation
Summary1. The condensation of the anomeric methyl 2,3-di-O-benzyl-D-xylofuranosiduronaldehydes with nitromethane has been studied.2. It has been shown that the position of the substituent at the glycosidic center has an influence on the steric directivity of this reaction.3. The structure of the 6-nitro-6-deoxyhexofuranosides synthesized has been confirmed by their conversion into known derivatives of methyl D-glucofuranosides.
Nitromethane-Condensation Reaction with 5-Aldehydes of Pentodialdofuranosides
Treatment of methyl 2, 3-di-O-benzyl-α-L-arabino-pentodialdo-1, 4-furanoside (1) with nitromethane in the presence of sodium methoxide afforded an epimeric mixture of methyl 2, 3-di-O-benzyl-6-deoxy-6-nitro-α-L-altro-(2c) and -β-D-galactofuranoside (3c) in a ratio of 2.5 : 1. On the other hand, the analogous treatment of 3-O-benzyl-1, 2-O-isopropylidene-α-D-xylo-pentodialdo-1, 4-furanose (7b) with nitromethane gave 3-O-benzyl-6-deoxy-1, 2-O-isopropylidene-6-nitro-α-D-glucofuranose (8b) predominantly.
Pyrite multiple-sulfur isotope evidence for rapid expansion and contraction of the early Paleoproterozoic seawater sulfate reservoir
Abstract Earthʼs oxygenation is often described in terms of two unidirectional steps at the beginning and end of the Proterozoic Eon, separated by a long-lived intermediate redox state. Recent work defines a more complicated path to oxygenation, exemplified by an apparent drop in oxidation state following the early Paleoproterozoic Lomagundi carbon isotope excursion. The timing of this proposed drop in oxidation state is not well constrained, and it is not clear how it relates to redox conditions during the remainder of the Proterozoic. Here we present a study of pyrite multiple-sulfur isotopes, supported by Fe speciation and organic carbon isotopes, from early Paleoproterozoic black shales. We find evidence for the rapid expansion of the seawater sulfate reservoir during the Great Oxidation Event at ca. 2.3 Ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca. 2.05 Ga. This scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early Paleoproterozoic. Comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early Paleoproterozoic did not return until the late Neoproterozoic.