Isolation and biosynthesis of cyclopentenylglycine, a novel nonproteinogenic amino acid in Flacourtiaceae

Asymmetric Eschenmoser-Claisen rearrangement for anti-beta-substituted gamma,delta-unsaturated amino acids

Optically active anti-beta-substituted gamma,delta-unsaturated amino acids are important synthetic building blocks in organic synthesis and for peptidomimetics. A novel asymmetric Eschenmoser-Claisen rearrangement with use of a C2-symmetric chiral auxiliary was developed to generate this type of amino acid. Excellent diastereoselectivities and high enantioselectivities (87-93% ee) were obtained after the chiral auxiliary was removed via iodolactonization/zinc reduction.

Highly selective addition of chiral, sulfonimidoyl substituted bis(allyl)titanium complexes to N-sulfonyl alpha-imino esters: asymmetric synthesis of gamma,delta-unsaturated alpha-amino acids bearing a chiral, electron-withdrawing nucleofuge at the delta-position

Selective addition of the chiral, sulfonimidoyl substituted bis(allyl)titanium complexes 5a-d, which are configurationally labile in regard to the Calpha-atoms, to N-toluenesulfonyl (Ts)-, N-2-trimethylsilylethanesulfonyl (SES)-, and N-tert-butylsulfonyl (Bus) alpha-imino ester (9a-c) in the presence of Ti(OiPr)(4) and ClTi(OiPr)(3) afforded with high regio- and diastereoselectivities in good yields the (syn, E)-configured beta-alkyl-gamma,delta-unsaturated alpha-amino acid derivatives 2a-g, which carry a chiral, electron-withdrawing nucleofuge at the delta-position and a cyclohexyl, an isopropyl, a phenyl, and a methyl group at the beta-position. Addition of the cyclic bis(allyl)titanium complex 14 to N-Bus alpha-imino ester 9c afforded with similar high regio- and diastereoselectivities the (E)- and (Z)-configured amino acid derivatives (E)-8 and (Z)-8. Reaction of complexes 5a-d with alpha-imino esters 9a-c in the presence of Ti(OiPr)(4) occurs stepwise to give first the mono(allyl)titanium complexes containing 2a-g as ligands, which react in the presence of ClTi(OiPr)(3) with a second molecule of 9a-c with formation of two molecules of 2a-g. Formation of (S,R,E)-configured homoallylic amines 2a-g entails Si,Re,E processes of alpha-imino esters 9a-c with the (R,R)-configured bis(allyl)titanium complexes (R,R)-5a-d and (R)-configured mono(allyl)titanium complexes (R)-17a-d, both of which are most likely in rapid equilibrium with their (S,S)-diastereomers and (S)-diastereomers, respectively. Interestingly, in the reaction of 5a-d with aldehydes, the (S,S)-configured complexes (S,S)-5a-d are the ones which react faster. Reaction of the N-titanated amino acid derivatives Ti-2a and Ti-2b with N-Ts alpha-imino ester 9a led to the highly diastereoselective formation of imidazolidinones 15a and 15b, respectively. Cleavage of the sulfonamide group of the N-Bus amino acid derivative 2d with CF(3)SO(3)H gave quantitatively the sulfonimidoyl functionalized amino acid H-2d. A Ni-catalyzed cross-coupling reaction of the amino acid derivative 2e with ZnPh(2) led to a substitution of the sulfonimidoyl group by a phenyl group and furnished the enantiomerically pure protected alpha-amino acid Bus-1. Two new N-sulfonyl alpha-imino esters, the SES and the Bus alpha-imino esters 9b and 9c, respectively, have been synthesized from the corresponding sulfonamides by the Kresze method in medium to good yields. The N-SES alpha-imino ester 9b and the N-Bus alpha-imino ester 9c should find many synthetic applications, in particular, in cases where the N-Ts alpha-imino ester 9a had been used before.

Plant analysis by butterflies: occurrence of cyclopentenylglycines in Passifloraceae, Flacourtiaceae, and Turneraceae and discovery of the novel nonproteinogenic amino acid 2-(3'-cyclopentenyl)glycine in Rinorea

Following records about feeding habits of nymphalid butterflies, a novel nonproteinogenic L-amino acid, (S)-2-(3'-cyclopentenyl)glycine (11), was discovered in Rinorea ilicifolia, a species where the presence of a cyclopentanoid natural product of this kind was neither known nor anticipated from the taxonomic point of view. Another novel amino acid, (2S,1'S,2'S)-2-(2'-hydroxy-3'-cyclopentenyl)glycine (12), the stereochemistry of which was determined by single-crystal X-ray diffraction, was shown to occur in species belonging to Flacourtiaceae, Passifloraceae, and Turneraceae. These species, many of which serve as hosts for nymphalid butterflies (Acraeinae, Heliconiinae, Argynninae), also produce 2-(2'-cyclopentenyl)glycine. Cyclopentenylglycines are proposed to be novel chemical recognition templates for plant-insect interactions. Ratios between the epimers of (2S)-2-(2'-cyclopentenyl)glycine, which co-occur in plants, were determined by (1)H NMR spectroscopy. Contrary to a previous report, the (2S,1'R) epimer always appears to predominate over the (2S,1'S) epimer. Stereochemical aspects of biosynthesis of natural cyclopentanoid cyanogenic glycosides are discussed in relation to these findings.

Preparation and reactions of stannylated amino acids

Hydrostannations of propargylic glycine esters with the new hydrostannation catalyst [Mo(CO)3(CNtBu)3] (MoBI3) gave rise to alpha-stannylated allylic esters in good yield and with high regioselectivity. The chelate Claisen rearrangements of these esters allow the syntheses of gamma,delta-unsaturated amino acids with a vinylstannane moiety in the side chain. The amino acids obtained can be further modified by cross-coupling with various types of electrophiles.

Synthesis of epimers of L-cyclopentenylglycine using enzymatic resolution

Both epimers of the naturally occurring nonproteinogenic amino acid L-cyclopentenylglycine, (2S,1'S)- and (2S, 1'R)-2-(cyclopent-2'-enyl)glycine, were obtained via a procedure involving condensation of 3-chlorocyclopentene with diethyl acetylaminomalonate, deethoxycarbonylation, chromatographic separation of the resulting two pairs of enantiomers, and enzymatic resolution of the racemates employing enantioselective hydrolysis of the ethyl ester group with alpha-chymotrypsin. The method was used for preparation of (13)C-labeled compounds of interest for biosynthetic tracer experiments. Enantiomeric purity of the products was determined by chiral HPLC on a Crownpak CR(+) column. The biologically active (2S,1'R) isomer was obtained as a pure compound and characterized for the first time. The (2R,1'R) and (2R,1'S) isomers were obtained as N-acetyl ethyl ester derivatives.

Biosynthesis of cyanohydrin glucosides from unnatural nitriles in intact tissue of Passiflora morifolia and Turnera angustifolia

Passiflora morifolia, which under natural conditions contains cyanohydrin glucosides linamarin, lotaustralin and epilotaustralin, converted cyclopentanecarbonitrile, 2-cyclopentenecarbonitrile and 3-methylbutanenitrile into the corresponding cyanohydrin glucosides. Turnera angustifolia, which normally produces glucosides of cyclopentenone cyanohydrin, converted cyclopentanecarbonitrile, 2-methylpropanenitrile and 2-methylbutanenitrile, but not 3-methylbutanenitrile, into the corresponding cyanohydrin glucosides. Mixtures of epimers were produced when these glucosides contained chiral cyanohydrin carbon atoms. Feeding with cyclopentanecarbonitrile resulted in formation of 1-(beta-D-glucopyranosyloxy)cyclopentanecarbonitrile, a saturated analogue of deidaclin and tetraphyllin A. Neither plant utilized cyclopropanecarbonitrile as substrate. The experiments demonstrate broad substrate specificity of nitrile hydroxylases present in these plants. A novel glycoside, 2-[6-O-(beta-D-xylopyranosyl)-beta-D-glucopyranosyloxy]propane (isopropyl primeveroside), was isolated from P. morifolia. The compound represents a rare example of natural isopropyl glycoside; its characterization included assignment of all 1H and 13C NMR signals of the primeverosyl group using two-dimensional NMR methods. Biosynthesis of the isopropyl moiety of the primeveroside is unclear, but the formation of alcohols corresponding to natural cyanohydrins may be a previously unrecognized extension of the cyanohydrin biosynthesis pathway in higher plants.