Donald G. Smith

1H- and 13C-n.m.r. spectra of the methyl mono-, di-, and tri-O-acetyl-α- and -β-d-xylopyranosides

Abstract The 1 H- and 13 C-n.m.r. data for the mono- O -acetyl derivatives of methyl α- and β- d -xylopyranoside are compared with those for the parent compounds, to determine the effect of the acetyl groups on the chemical shifts of the methine protons and the skeletal carbons, and on the one-bond 1 H- 13 C, coupling constants. These substituent effects were used to confirm the assignments in the spectra of the di- and tri- O -acetyl derivatives of methyl α- and β- d -xylopyranoside.

High-molecular-weight phloroglucinol-based tannins from brown algae: Structural variants

Brown algae (Phaeophyceae) accumulate tannins which may help protect the alga against predation by herbivores (Hunger 1902; Geiselman & McConnell 1981; P. D. Steinberg, pers. commun.). These tannins are constructed of phloroglucinol (1,3,5-trihydroxybenzene) units linked together to form a series of polyphloroglucinols of low (102–103), intermediate (103–104) and high (≥104 amu) molecular weights. Low-MW polyphloroglucinols often represent only a small fraction of the total, but owing to the relative ease with which individual isomers may be purified, they have received considerable attention. To date, structures of about three dozen low-MW polyphloroglucinols have been published (Glombitza 1979; Glombitza et al. 1981; Koch et al 1981). Purification of individual polyphloroglucinols of intermediate MW presents a more formidable challenge; however, available data indicate a structure fundamentally similar to that of the lower-MW congeners (Ragan & Jamieson 1982).

Tenellin and bassianin, metabolites of Beauveria species. Structure elucidation with 15N- and doubly 13C-enriched compounds using 13C nuclear magnetic resonance spectroscopy

13 C n.m.r. spectroscopy and biosynthetic labelling with [15N]nitrate and [1,2-13C]acetate proved to be valuable adjuncts to established chemical and spectroscopic methods in elucidating the structures of tenellin and bassianin as 3-acyl derivatives of 1,4-dihydroxy-5-p-hydroxyphenyl-2(1H)-pyridone.

New techniques in biosynthetic studies using 13C nuclear magnetic resonance spectroscopy. The biosynthesis of tenellin enriched from singly and doubly labelled precursors

13 C n.m.r. spectroscopy was used for the dual purpose of assessing the relative merits of singly and doubly labelled precursors while establishing the biosynthesis of tenellin from 13C-labelled acetate, phenylalanine, and methionine; precursors in which two adjacent sites are labelled can provide superior information, and may prove useful for obtaining enrichment data at low incorporations.

Homonuclear 13C decoupling in 13C nuclear magnetic resonance studies of biosynthesis using doubly labelled precursors. Assembly pattern of the acetate units in bikaverin

In a 13C n.m.r. study of bikaverin biosynthesis by Fusarium oxysporum the use of (1,2-13C)-acetate and homonuclear 13C-decoupling overcame the difficulties of low 13C-enrichment, so that the polyacetate origin and the arrangement of precursor units in the metabolite could be established.

Biosynthesis of chlorflavonin in Aspergillus candidus. 13C- and 14C-labelling evidence for a new route to the flavonoid structure

Aspergillus candidus growing in a glucose–leucine–salts medium synthesized chlorflavonin (1) from isotopically labelled phenylalanine, cinnamate, benzoate, or acetate. Radioactivity from [β-14C]cinnamic acid and [α-14C]-benzoic acid was incorporated less efficiently than that from L-[β-14C]phenylalanine but decarboxylation of 3-chlorosalicylic acid (3) formed by alkaline degradation of the chlorflavonin samples located all of the radioactivity at C-2 in the flavonoid. 4,5-Dimethoxyresorcinol (2), obtained by alkaline degradation of chlorflavonin labelled from sodium [1-14C]acetate and [α-14C]cinnamic acid, accounted for only part of the radioactivity. The label from both precursors was distributed between ring A and one or more of the C-3, C-4, and C-3 methoxy-carbon atoms. 13C N.m.r. spectrometry of chlorflavonin labelled from sodium [1-13C]acetate showed 13C incorporation into C-4, C-5, C-7, and C-8a. Sodium [1-13C0;1;1,2-13C1;0;1]acetate was incorporated intact into (C-3,C-4) and all adjacent pairs of ring-A carbon atoms. The results indicate a pathway of flavonoid biosynthesis differing from that of higher plants in that a C6-C1 precursor unit is condensed with four C2 units. In the route proposed, the heterocyclic ring is formed before ring A is substituted at C-8 and while it is free to rotate at the enzyme surface.