Donald A. Whiting

Experiments in the biosynthesis of curcumin

The biogenesis of natural diarylheptanoids is discussed, with particular reference to curcumin [1,7-bis-(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-dione], the pigment of Curcuma Ionga rhizome. Methods for the isolation, characterisation, and degradation of curcumin, suitable for biosynthetic work, are reported. In administration of labelled precursors to C. Ionga,[1- and 3-14C]phenylalanine were incorporated into curcumin without scrambling of the label. [1- and 2-14C]-Acetate and -malonate were also incorporated, and the fractional distribution of label along the heptane chain was determined; the results do not provide satisfactory support for the expected biosynthetic scheme, in which two cinnamate units condense with one malonate unit. Other interpretations are discussed. [3H]-4-Hydroxy-3-methoxy-, -4-hydroxy-, and -3,4-dihydroxy-cinnamic acids were prepared, and supplied to C. Ionga with [14C]phenylalanine. The first two cinnamic acids are incorporated into curcumin significantly better than the last, although none was utilised quite as efficiently as phenylalanine.

Cucurbitacin glycosides from Citrullus colocynthis

Abstract The chloroform extract of Citrullus colocynthis yielded four cucurbitacin glycosides which were identified spectroscopically as 2- O -β- d -glucopyranosyl-cucurbitacin I, 2- O -β- d -glucopyranosyl-cucurbitacin E, 2- O -β- d -glucopyranosyl-cucurbitacin L and the novel glycoside, 2- O -β- d -glucopyranosyl-(22–27)-hexanorcucurbitacin I. Detailed 1 H and 13 C NMR data are provided.

Synthesis of the thiazoline-based siderophore (S)-desferrithiocin

Abstract A total synthesis of the new thiazoline-based siderophore desferrithiocin 1, isolated from Streptomyces antibioticus, is described. Thus, a concise synthesis of (S)-2-methylcysteine hydrochloride 11 is first developed based on a modification of Seebachs “self-reproduction of chirality” protocol using the thiazolidine intermediate 8 derived from (S)-cysteine and pivaldehyde as a key intermediate. When a solution of (S)-2-methylcysteine hydrochloride is heated with 2-cyano-3-hydroxypyridine in the presence of triethylamine, (S)-desferrithiocin is produced in 97% yield. In a similar manner, use of (R)-2-methylcysteine in a cyclocondensation with 2-cyano-3-hydroxypyridine led to (R)-desferrithiocin, in a similar yield.

Syntheses of the (±)-[n]-gingerols (pungent principles of ginger) and related compounds through regioselective aldol condensations: relative pungency assays

The deprotonation of trimethylsilylzingerone (13) by lithium di-isopropylamide at –78 °C has been found to be regioselective (92 : 8 in favour of less-substituted enolate): the anion was condensed with alkanals and acyl imidazoles to give convenient syntheses of the (±)-[2]–[10]- and -[12]-gingerols (1) and [4]-, [6]-, and [8]-gingerdiones (9). Similarly, 3-methoxy-4-trimethylsilyloxybenzylideneacetone (17) gave the (±)-[2]–[10]-dehydrogingerols (8) and [4]-, [6]-, and [8]-dehydrogingerdiones (10).The aldol reaction to [6]-gingerol and methyl [6]-gingerol was also conducted through a vinyloxyborane or through the enol silyl ether (TiCl4 catalysis). Results of organoleptic assays on these compounds are discussed, and the relation between pungency in the gingerols and in capsaicin is commented on. The aldol method was also used to synthesise the natural β-ketols(±)-daphneolone (25) and (±)-hexahydrocurcumin (4).

Stereoselective total syntheses of the (±)-di-O-methyl ethers of agatharesinol, sesquirin-A, and hinokiresinol, and of (±)-tri-O-methylsequirin-E, characteristic norlignans of coniferae

Total syntheses of the (±)-di-O-methyl ethers of the norlignans sequirin-A, agatharesinol, and hinokiresinol, and of (±)-Tri-O-methyl sequirin-E are described. p-Methoxyacetophenone was converted into p-methoxybenzoylethylene and thence into 4-(p-methoxybenzoyl)-2,2-dimethyl-1,3-dioxolan (13). The glycidic acid (15) was obtained from (13) by Darzens condensation with benzyl chloroacetate and hydrogenolysis of the resulting ester. Decarboxylation–rearrangement in hot acetone of the glycidic acid was stereoselective providing the desired diastereoisomer (17)(>80%) of 2,2-dimethyl-1,3-dioxolan-4-yl-(p-methoxyphenyl)acetaldehyde. Reaction of (17) with p-methoxybenzylidenetriphenylphosphorane gave trans- and cis-(±)-dimethylagatharesinol acetonides The trans-acetonide (19) was hydrolysed to (±)-dimethylagatharesinol (21); pyrolysis of the mixed orthoformate of the latter provided (±)-dimethylhinokiresinol. The cis-isomer (18) was hydrolysed and cyclised to yield (±)-dimethylsequirin-A. Reaction of the aldehyde (17) with 3,4-dimethoxybenzylidenetriphenylphosphorane, followed by acid-catalysed cyclisation, afforded (±)-trimethylsequirin-E, via the acetonide (20). An alternative approach to dimethylagatharesinol is discussed.

O-hydroxyphenylacetaldehyde: a major novel metabolite of coumarin formed by rat, gerbil and human liver microsomes.

A major novel coumarin metabolite was isolated from rat hepatic microsomal incubations by high-performance liquid chromatography. In the presence of a rat liver cytosolic fraction and NADH it was rapidly metabolized to O-hydroxyphenylethanol. The metabolite co-chromatographed with an authentic sample of O-hydroxyphenylacetaldehyde and its identity was confirmed by mass spectral analysis. The formation of O-hydroxyphenylacetaldehyde from coumarin was NADPH-dependent. It was the major metabolite formed by rat, gerbil and human liver microsomes at a coumarin concentration of 1mM.

Oxidative Coupling of Phenols and Phenol Ethers

The ready oxidation of phenols to dimeric products has been well known for more than a century. The invaluable survey of the area by Musso1 lists over 20 papers published before 1900 on, for example: the formation of the dilactone (1) from gallic acid2 and its derivatives (1871);3 the production of both parapara and ortho-ortho coupled dimers from 1-naphthol (1873);4 the synthesis of the diphenoquinone (2),5 in 97% yield, from 2,5-dimethoxyphenol (1878); and the preparation of 2,2′-dihydroxybiphenyl from phenol itself (1878).6 Reagents used in such early work include iron(III) salts, potassium ferricyanide, oxygen and the halogens; electrochemical methods were also known, and enzyme-catalyzed reactions were reported in, for example, the oxidative dimerization of eugenol (1896).7 Extensive chemical investigations into this major reaction continued this century and further momentum was gained from the recognition of the role of oxidative coupling in biogenesis, signalled by the important and influential papers of Barton and Cohen,8 and of Erdtman and Wachtmeister.9 Subsequent biosynthetic investigations have confirmed the significance of the title reaction in the in vivo formation of many aromatic natural products including alkaloids (Battersby1 estimated that such coupling was involved in ca. 10% of known alkaloids), lignans, lignin, tannins, and plant and insect pigments. Many biomimetic studies of oxidative phenolic coupling were made, either for synthetic ends or to demonstrate possibilities in biological processes. Low chemical yields were often reported in such work, particularly in certain well-known alkaloid cases, and attention was turned to new reagents, e.g. manganese(III), vanadium(V) and thallium(III), which generally proved more effective. Further, oxidants such as vanadium(V) were found to effect coupling of phenol ethers, and interest was revived in the anodic oxidation of such aromatic substrates. The oxidation of phenol ethers has proved a valuable synthetic process, often more predictable and higher yielding than the corresponding phenol oxidations, and this reaction has been employed in recent years in a number of notable natural product syntheses.

Structures of the oat root resistance factors to ‘take-all’ disease, avenancins A-1, A-2, B-1 and B-2 and their companion substances

It is shown that avenestergenins, having a 12-oxo group, are not true aglycones of the avenacin series: the latter are 12, 13β-epoxides. Acid hydrolysis would be expected to lead to a 13α, 12-ketone, not the 13β, 12-ketone of the avenestergenins, and the chemistry of the process is modelled using the 12α, 13α- and 12β, 13β-epoxides from 3β-benzoyloxyolean-12-ene and isolating the 13α, and 13β, 12-ketones. The former is readily converted into the latter under acid conditions similar to those employed for hydrolysis of the avenacins. Search of the oat extractives has resulted in isolation of the true free aglycone of the avenacin A-1 series, named epoxyavenagenin A-1.By combination of f.a.b. m.s., methylation, 13C and 1H n.m.r. techniques, the trisaccharide chain of all four avenacines is shown to be [β-D-glucopyranosyl(1 → 4)]-[β-D-glucopyranosyl(1 → 2)]-α-L-arabinopyranosyl attached at the triterpene 3-β-hydroxy group. This completes structural and stereochemical details for avenacins A-1, A-2, B-1, and B-2.As minor components of healthy oat root extract, two compounds formulated as glucoavenacin A-1 and deglucoavenacin A-1 have been isolated. The latter is of particular interest as, along with bis-deglucoavenacin A-1, they are detoxified products of avenacin A-1 formed by the highly virulent Gaeumannomyces graminis var. avena which can attack oat roots.

Deguelin cyclase, a prenyl to chromen transforming enzyme from Tephrosia vogellii

Abstract Seeds and plant parts of Tephrosia vogellii were investigated in order to provide systems for the study of prenyl to chromen transformation in rotenoids, as exemplified by the conversion of rot-2 -enonic acid into deguelin. No hydroxylated intermediate was found. A cell free preparation has been obtained from T. vogellii seedlings or seeds and shown to catalyse the reaction. The water soluble enzyme has been partially purified using ammonium sulphate precipitation, gel chromatography and ion exchange procedures. Data for the enzyme, named deguelin cyclase, are reported—optima for pH and temperature and Km (which indicates strong binding between enzyme and substrate). Results relevant to Mr determination are discussed. The enzyme has a requirement for oxygen, but not for cofactors. It is inhibited by chloride ion and chelating agents, particularly 1,10-phenanthroline. Deguelin cyclase can convert the 11-hydroxyrotenoid sumatrolic acid into α-toxicarol and lapachol into dehydro-α-lapachone, though the prenyl to chromen conversion is not general. It does not convert rot-2′-enonic acid into rotenone under the conditions studied. Deguelin cyclase seems not to belong to the P450 group and resembles more closely the non-heme iron protein isopenicillin N synthase.

A new synthetic route to furofuranoid lignans via the intramolecular Mukaiyama reaction

The sequence set out in Scheme 1 provides a short and expedient synthesis of a number of (±)-furofuranoid lignans, including styraxin 3(antitumour), aptosimon 5, asarinin 6, pluviatilol 7, ‘MEL’4(inhibitor of germination) and related compounds.