Isopropenyl Side Chain Research Articles

Synthesis of (±)-4,5-dia-Parthenolide, an Unnatural Parthenolide Stereoisomer.

A short total synthesis of the novel unnatural parthenolide diastereomer (±)-4,5-dia-parthenolide was accomplished in 13 steps and an overall yield of 1.75% starting from commercially available (E,E)-farnesol. The challenging isopropenyl side chain oxidation was regioselectively achieved via a newly developed stepwise dihydroxylation procedure, employing a Bartlett-Smith iodocarbonate cyclization followed by iodide substitution and catalytic transesterification.

X-ray Crystallography Reveals How Subtle Changes Control the Orientation of Substrate Binding in an Alkene Reductase

Reductions of (S)- and (R)-carvone by wild-type Saccharomyces pastorianus Old Yellow Enzyme (OYE 1) and a systematic collection of Trp 116 variants revealed that, for (S)-carvone, six Trp 116 mutants displayed inverted diastereoselectivity compared to the wild-type. For example, Ile and Val showed inverted stereoselectivity, but Leu and Phe maintained the wild-type stereopreference. For (R)-carvone, only two Trp 116 mutants (Ala and Val) reduced this alkene with reversed selectivity; all other catalytically active variants including Leu and Ile retained the wild-type diastereoselectivity. The same set of mutant enzymes was also used to catalyze the dehydrogenation of (S)- and (R)-carvone under aerobic conditions. To understand how small changes to the active site structure of OYE 1 could significantly influence its catalytic properties, we solved X-ray crystal structures of the wild-type as well as six key Trp 116 variants after individually soaking with both (S)- and (R)-carvone. In many cases, pseudo-Michaelis complexes formed in crystallo, and these revealed the details of protein–substrate interactions. Taken together, our results showed that the wild-type OYE 1 reduces carvone from a less preferred substrate binding orientation. The indole ring of Trp 116 physically blocks access to a hydrophobic active site pocket. Relieving the steric congestion by mutating Trp 116 allows entry of the isopropenyl side-chain of carvone into this hydrophobic pocket and also makes the opposite face of the π system accessible to hydride addition, thereby yielding the opposite diastereomer after net trans-addition of H2.

A novel photoaffinity ligand for kainate sites labels two polypeptides with molecular mass 45 and 33.5 kDa in chick cerebellum.

The synthesis of (2S,3S,4S)-4-[1-(4-azidobenzamidomethyl)ethenyl]-2-carboxy-3- pyrrolidineacetic acid (ABCPA) is described. This novel kainic acid analogue, bearing a photolabile functionality on the isopropenyl side chain, was proven to be a good inhibitor of [3H]CNQX and [3H]kainic acid binding on chick cerebellar membranes. [3H]ABCPA was photoaffinity cross-linked on the membrane fraction of chick cerebellum. Electrophoretic analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two major radioactive bands with apparent molecular masses of 45 and 33.5 kDa. [3H]ABCPA incorporation in both bands was completely blocked by 2 mM CNQX. When photoaffinity labeling was performed in the presence of 2 mM kainic acid, incorporation of [3H]ABCPA was blocked by approximately 70% in the 45-kDa band and by 18% in the 33.5-kDa band. Incorporation of radioactivity in both bands was blocked by approximately 30% with 10 mM glutamate.

Biosynthesis of germacrene A carboxylic acid in chicory roots. Demonstration of a cytochrome P450 (+)-germacrene a hydroxylase and NADP+-dependent sesquiterpenoid dehydrogenase(s) involved in sesquiterpene lactone biosynthesis.

Sprouts of chicory (Cichorium intybus), a vegetable grown in the dark, have a slightly bitter taste associated with the presence of guaianolides, eudesmanolides, and germacranolides. The committed step in the biosynthesis of these compounds is catalyzed by a (+)-germacrene A synthase. Formation of the lactone ring is the postulated next step in biosynthesis of the germacrene-derived sesquiterpene lactones. The present study confirms this hypothesis by isolation of enzyme activities from chicory roots that introduce a carboxylic acid function in the germacrene A isopropenyl side chain, which is necessary for lactone ring formation. (+)-germacrene A is hydroxylated to germacra-1(10),4,11(13)-trien-12-ol by a cytochrome P450 enzyme, and is subsequently oxidized to germacra-1(10),4,11(13)-trien-12-oic acid by NADP+-dependent dehydrogenase(s). Both oxidized germacrenes were detected as their Cope-rearrangement products elema-1,3,11(13)-trien-12-ol and elema-1,3,11(13)-trien-12-oic acid, respectively. The cyclization products of germacra-1(10),4,11(13)-trien-12-ol, i.e. costol, were also observed. The (+)-germacrene A hydroxylase is inhibited by carbon monoxide (blue-light reversible), has an optimum pH at 8.0, and hydroxylates beta-elemene with a modest degree of enantioselectivity.

Synthesis of Amino Acid Conjugates and Further Derivatives of 3α-Hydroxylup-20(29)ene-23,28-dioic acid

Triterpenes of betulinic acid type exhibit many interesting biological activities. Therefore a series of new 3α-hydroxy-lup-20(29)-ene-23,28-dioic acid derivatives 2a—22 with putative pharmacological activities were synthesized. As starting compounds 3α-hydroxy-lup-20(29)-ene-23,28-dioic acid (1a), isolated from Schefflera octophylla, or its 3-O-acetyl derivative 1b were used. Mono- and diesters (2a—b from 1a, and 4d from 4c) were prepared with CH2N2. Oxidation of the isopropenyl side chain with OsO4 yielded the 20,29-diols (4a—b from 1b, and 19 from 17), which were in the case of 4b further transformed to the 29-norketones 8a/mdash;b. Oxidation of the isopropenyl side chain with m-chloroperbenzoic acid afforded the 20,29-epoxide 12 (from 1b) and the 29-aldehydes and a-hydroxy aldehydes (13a—c from 2a, 14a—c from 2b, and 16a—c from 15a). Ring A was modified by a tosylation—elimination sequence using p-TsCl/NaOAc, which afforded diolefin 15a (from 2a) with Δ2,20(29) double bonds or 23-nor-Δ3,20(29)diolefin 17 (from 1a). Compounds 4b, 4c, and 8a were coupled with L-methionin, L-phenylalanin, L-alanin, L-serin, and L-glutaminic acid via amide bonds at positions 23 and 28 to afford the amino acid conjugates 5a—7b and 9a—11.

A Selective Photoaffinity Ligand for the Kainate Class of Excitatory Amino Acid Receptor

The isopropenyl side chain of kainic acid (KA) has been replaced with a trifluoromethyldiazoketone group, resulting in a derivative ("diazoKA") that gives efficient photo-induced crosslinking to the active site of the KA class of glutamate receptor. The incorporation of this functional group is accomplished via a simple, one step coupling of a carboxylic acid and 2,2,2-trifluorodiazoethane in a procedure that should be generally applicable to the preparation of other photoaffinity labels. Major advantages include excellent stability towards acidic conditions, even Boc and t-butyl ester deprotection, as well as a reduced tendency to undergo (non-productive) Wolff rearrangement after binding and irradiation. The observed crosslinking paves the way for identifying the active site residues of the KA receptor.

Synthesis of a New Kainic Acid Based Selective Ligand as a Potential Photoaffinity Label of Non-NMDA Excitatory Amino Acid Receptors in Chicken Brain.

The synthesis of two analogues of kainic acid (KA) incorporating photo-activatible moieties attached either on the gamma-carboxy function (gamma-amide 1) or the isopropenyl side-chain (amide 2) is described. The synthesis of the former amide involves coupling of N-(tert-butoxycarbonyl)-protected alpha-diphenylmethyl kainate with 2-(4-azidobenzamido)ethylamine (5) followed by trifluoroacetic acid mediated complete deprotection. Amide 2 was synthesized by palladium-mediated allylic amination, with 4,4'-dimethoxybenzhydrylamine (DMBA), of N-(9-fluorenylmethoxycarbonyl)-protected dimethyl kainate, followed by splitting the DMB-group with formic acid, coupling with N-hydroxysuccinimidoyl 4-azidobenzoate and finally complete deprotection by saponification. Preliminary pharmacological studies in chicken brain membranes showed that amide 2 is a stronger inhibitor of [3H]KA binding on chicken cerebellar membranes than is amide 1 and that amide 2 has specificity only for the cerebellar, as opposed to the telencephalon, type of non-NMDA binding sites.

Alicyclic claisen rearrangement with a two-atom ring contraction. A new synthetic approach to cembranoids

Abstract Methylenation of certain olefinic lactones with dimethyl titanocene, followed by thermal or trilsobutylaluminum-mediated Claisen rearrangement leads to cembrane-like 11-15-membered ring ketones bearing an isopropenyl side chain.

Effect of diastereoisomeric epoxyrotenones on hepatic epoxide hydrase activity.

The effect on epoxide hydrase activity of the two diastereomeric (6'R) and (6'S)-6',7'-epoxyrotenones formed during the oxidative metabolism of rotenone on the isopropenyl side chain is reported. The activity of microsomal epoxide hydrase was determined using styrene oxide as substrate. The results indicate that the two diastereomeric 6',7'-epoxyrotenones are good substrates for epoxide hydrase and are noncompetitive inhibitors for the hydration of styrene oxide. The more polar of the two isomers is more active as inhibitor of this enzyme.

Elimination reaction of angular hydroxymethyl groups of 20(29)-lupene derivatives

Solvolysis of 28-p-toluenesulfonyloxy-20(29)-lupene derivatives IV-VII proceeds with isomerization of the isopropenyl side chain to the isopropylidene chain and expansion of the ring E to a six-membered ring, containing trisubstituted double bond; for „anhydrobetulin" and its derivatives formulae VIII-XI with homoconjugated double bonds are suggested. Formation of a conjugated diene system is hindered by steric interactions of the isopropylidene chain with the ring C (with C(12)). Only the trisubstituted double bond in the dienes VIII and X undergoes catalytic reduction, the hydrogen approaching from the α-side (XII-XVI) as demonstrated by the Cotton effect of trinorketone XXI and its 20,20-dibromo derivative XXIII.

Biosynthesis of carvone in Mentha spicata

Abstract Degradation of, and measurement of isotope ratios in, (−)-carvone that had been biosynthesized in Mentha spicata from 3H- and 14C-labelled geraniol and mevalonate indicate that (a) oxidation of limonene or its biogenetic equivalent to form carvone involves shift of the endocyclic double bond; (b) (+)-limonene and (−)-carvone are biogenetically related and are probably formed on divergent pathways from a common intermediate; and (c) the exocyclic double bond of carvone is not formed regiospecifically. These results enable the mechanisms for the introduction of the carbonyl group and for the formation of the isopropenyl side-chain to be delimited.

Eine Untersuchung der in vivo Oxidation des Capsidiols mit Hilfe der 13C-NMR Spektroskopie / A 13C-NMR Study of the Biological Oxidation of Capsidiol

Abstract The mechanism of the biological hydroxylation of the isopropenyl side-chain of the sesquiterpene capsidiol, to give 13-hydroxycapsidiol, has been investigated with 13C-NMR techniques, using capsidiol biogenetically enriched with 13C from [1,2-13C2] acetate. The results indicate that neither an allylic rearrangement nor the formation of an epoxide intervene in the process.

Stereochemical considerations in the formation and biological activity of the rotenone metabolites

The rotenone metabolites formed by enzymatic hydroxylation at one or more of the 12a-, 8′- or 6′, 7′-positions are now available from synthesis. Cochromatographic comparisons of these authentic compounds with the metabolites confirm that in addition to the hydroxylations in the 5′-isopropenyl side chain the enzymatic attack at the B C ring juncture introduces a hydroxyl group in the 12aβ- or 12aα-position with retention of the original stereochemical configuration at other positions. 5′-Hydroxyrotenone or its equivalent keto phenol and rotenolones derived from this material are not present as rotenone metabolites. The only metabolites with high potency as NADH oxidase inhibitors are those retaining the original 6aβ, 12aβ-configuration and with one hydroxyl group at the 12aβ- or 8′-position, but even these two metabolites are much less inhibitory than rotenone. The relatively high intraperitoneal toxicity to mice of 6aβ, 12aβ-rotenolone and 8′-hydroxyrotenone despite their reduced activity as NADH oxidase inhibitors suggests that these compounds are not metabolically detoxified as rapidly as rotenone.

Extractives of Australian timbers. V. Ceanothic acid

Further evidence supporting structure (II) for ceanothic acid is given. The isopropenyl side chain has been degraded to yield a trisnorketone; reactions involving the isopropenyl and carboxyl groups attached to ring E and the enlargement of ring E are discussed and a close parallel with the corresponding chemistry of betulic acid is presented. The action of alkali effects isomerizations involving the ring A substituents and this, together with spectroscopic evidence, has been shown to support the stereochemistry assigned to the carboxyl and hydroxyl groups attached to ring A as in (II).