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At least 451 records · Page 25Linked to original sources
PubMed2004

Stereocontrolled syntheses of the nemorensic acids using 6-diazoheptane-2,5-dione in carbonyl ylide cycloadditions.

Levulinic acid-derived 6-diazoheptane-2,5-dione (9) serves as a common precursor in a formal synthesis of frontalin 19, and in syntheses of cis-nemorensic acid 1, 4-hydroxy-cis-nemorensic acid 2, 3-hydroxy-cis-nemorensic acid 3, and nemorensic acid 4. The key step in these syntheses is the Rh(2)(OAc)(4)-catalyzed tandem carbonyl ylide formation-intermolecular 1,3-dipolar cycloadditions of diazodione 9 with formaldehyde, alkynes or allene, which occur with high regioselectivity. Subsequent oxidative cleavage of the ring originally derived from the cyclic carbonyl ylide intermediate provides a straightforward access to polysubstituted tetrahydrofurans, and in particular an efficient entry to the nemorensic acids. Enantioselective cycloadditions with diazodione 9, using chiral rhodium catalysts, gave cycloadducts in up to 51% ee.

Azo Compounds↗
PubMed1989

Distribution of delta-aminolevulinic acid biosynthetic pathways among phototrophic bacterial groups.

Two biosynthetic pathways are known for the universal tetrapyrrole precursor, delta-aminolevulinic acid (ALA). In the ALA synthase pathway which was first described in animal and some bacterial cells, the pyridoxal phosphate-dependent enzyme ALA synthase catalyzes condensation of glycine and succinyl-CoA to form ALA with the loss of C-1 of glycine as CO2. In the five-carbon pathway which was first described in plant and algal cells, the carbon skeleton of glutamate is converted intact to ALA in a proposed reaction sequence that requires three enzymes, tRNA(Glu), ATP, Mg2+, NADPH, and pyridoxal phosphate. We have examined the distribution of the two ALA biosynthetic pathways among various genera, using cell-free extracts obtained from representative organisms. Evidence for the operation of the five-carbon pathway was obtained by the measurement of RNase-sensitive label incorporation from glutamate into ALA, using 3,4-[3H]glutamate or 1-[14C]glutamate as substrate. ALA synthase activity was indicated by RNase-insensitive incorporation of label from 2-[14C]glycine into ALA. The distribution of the two pathways among the bacteria tested was in general agreement with their previously established phylogenetic relationships and clearly indicates that the five-carbon pathway is the more ancient process, whereas the pathway utilizing ALA synthase probably evolved much later. The five-carbon pathway is apparently the more widely utilized one among bacteria, while the ALA synthase pathway seems to be limited to the alpha subgroup of purple bacteria.

Aminolevulinic Acid↗
PubMed1983

Analysis of porphyrin precursors, 5-aminolevulinic acid derivatives by isotachophoresis.

Porphyrin precursors, 5-aminolevulinic acid (ALA), 4,5-dioxovaleric acid (DOVA), and porphobilinogen (PBG) can be simultaneously and conveniently analyzed by isotachophoresis. The quantitative analysis requires a minimum of 12 nmol for ALA, 4 nmol for DOVA, and 35 nmol for PBG. The reproducibility determined as coefficient of variation for qualitative and quantitative determination is within 4 and 5%, respectively. Isotachophoresis permits simple, efficient and quantitative detection, and identification of ALA derivatives in a mixture.

Aminolevulinic Acid↗