Binne Zwanenburg

Structure and function of natural and synthetic signalling molecules in parasitic weed germination

The structures of naturally occurring germination stimulants for seeds of the parasitic weeds Striga spp. and Orobanche spp. are described. The bioactiphore in this strigolactone family of stimulants is deduced from a structure-activity relationship and shown to reside in the CD part of the stimulant molecule. A molecular mechanism for the initial stages of seed germination is proposed. The influence of stereochemistry on the stimulant activity is significant. Combining this molecular information leads to a model for the design of synthetic strigolactones. Nijmegen-1 is a typical example of a highly active, newly designed synthetic stimulant. The occurrence of natural stimulants not belonging to the strigolactone family, such as cotylenin and parthenolide, is briefly described. The biosynthesis of natural strigolactones from beta-carotene is analysed in terms of isolated and predicted stimulants. This scheme will be helpful in the search for new strigolactones from root exudates. Protein fishing experiments to isolate and characterise the receptor protein using biotin-labelled GR 24 are described. A receptor protein of 60 kD was identified by this method. Nijmegen-1 has been tested as a suicidal germination agent in field trials on tobacco infested by Orobanche ramosa L. The preliminary results are highly rewarding. Finally, some future challenges in synthesis are described. These include synthesising new natural and synthetic stimulants and establishing the molecular connection between strigolactones as germination stimulants, as the branching factor for arbuscular mycorrhizal fungi and as an inhibitor of shoot branching.

The Synthetic Potential of Three-Membered Ring Aza-Heterocycles

Functionalized aziridines, especially aziridine-2-carboxylic esters, are highly valuable small-ring heterocycles for the synthesis of a large variety of anomalous amino acids, new types of ligands for catalytic purposes, new synthons, and four- and five-membered ring heterocycles through ring expansion reactions. This review highlights the aziridine chemistry with the focus on the utility of aziridine esters. The synthesis and chemistry of the highly strained azirine carboxylic esters is also briefly reviewed.

A two-step chirality transfer from (−)-endo- to (−)-exo-tricyclo[5.2.1.026]deca-4,8-dien-3-one

Abstract An effective synthesis of enantiopure (−)-exo-tricyclo [5.2.1.02,6]deca-4,8-dien-3-one exo- 1 is realized starting from enantiopure (−)-endo- 1 applying Diels-Alder/retro-Diels-Alder methodology. The unusually high exo-stereoselectivity observed in the [4+2]-cycloaddition of (−)-endo- 1 with cyclopentadiene has been evaluated by semi-empirical AM1 transition state calculations.

A convenient synthesis of optically active 1H-aziridine-2-carboxylic acids (esters)

Optically active glycidic esters, prepared from allylic alcohols employing the Sharpless epoxidation, were treated with sodium azide. In a subsequent reaction with PPh3 the azido alcohols thus obtained were converted into aziridine-2-carboxylic esters of high optical purity in good yields.

Resolution of secondary alcohols by enzyme-catalyzed transesterification in alkyl carboxylates as the solvent.

Abstract The Porcine Pancreatic Lipase (PPL)- and Mucor Esterase-catalyzed resolution of 1-phenylethanol 5 in four different alkyl carboxylate solvents, viz. methyl acetate, propionate and butyrate and ethyl acetate, was evaluated. The beneficial influence of the addition of molecular sieves 4A to the reaction mixture is demonstrated. A Mucor Esterase-catalyzed resolution of 5 on a 0.5 mol scale is described. The kinetic resolution of a large variety of secondary alcohols ( 5 - 22b ) was investigated using both biocatalysts under the established optimal reaction conditions for substrate 5 .

Boroxazolidones as simultaneous protection of the amino and carboxyl group in α-amino acids

Abstract The synthesis of boroxazolidones 1 form a variety of α-amino acids is given. These compounds 1 show a strong intramolecular coordination between B and the amino group. These heterocycles can serve as protected α-amino acids in which the α-amino and the carboxyl function are simultaneously blocked, while the side chain remains free for further reactions. The boroxazolidines were reconverted into α-amino acids under mild acid conditions. Using this methodology aspartic acid was converted into β-benzyl aspartate and glutamic acid into γ-benzyl glutamate.

Stereocontrolled total synthesis of the macrocyclic lactone (−)-aspicilin

Abstract The essential features of the enantiocontrolled total synthesis of (−)-aspicilin are the strategic use of the photochemical rearrangement of α,β-epoxy diazomethyl ketones to 4-hydroxyalkenoates (Scheme 1) and the stereochemical control of the Sharpless epoxidation, α,ω functionalization of an alkynol using potassium 3-aminopropylamine (KAPA) as acetylene zipper, coupling between C 7 and C 8 by means of a Wittig reaction and lactonization by using 2,6-dichlorobenzoyl chloride.

PPL-catalyzed resolution of 1,2-and 1,3-diols in methyl propionate as solvent : an application of the tandem use of enzymes

Abstract The Porcine Pancreatic Lipase (PPL)-catalyzed transesterification of 1-phenyl-1,2-ethanediol 1, 2-phenyl-1,2-propanediol 2 1,2-decanediol 3 1,2-pentanediol 4 1,2-butanediol 5 1,2-propanediol 6 and 1,3-butanediol 7 in methyl propionate as solvent was evaluated. In alt substrates the primary hydroxy group is esterified exclusively. The enantioselectivity displayed in this PPL-catalyzed reaction is moderate. The enantiomeric excess of diol (-)-1 is enhanced by subjecting propionate (-)-8 with a moderate ee (obtained by a PPL-catalyzed esterification of racemic 1 in methylpropionate) to an enzyme-catalyzed hydrolysis (tandem principle).

Synthesis of isothiocyanate-derived mercapturic acids

Twelve mercapturic acids derived from saturated and unsaturated aliphatic and aromatic isothiocyanates were synthesised, by adding isothiocyanate to a solution of N-acetyl-L-cysteine and sodium bicarbonate, in a typical yield of 77%. Isothiocyanates were synthesised first by adding the corresponding alkyl bromide to phthalimide potassium salt. The obtained N-alkyl-phthalimide was hydrazinolysed yielding the alkyl amine, which subsequently was reacted with thiophosgene yielding the isothiocyanate with an overall yield of 16%. Mercapturic acids in urine can serve as a biomarker of intake to determine the health promoting potential of isothiocyanates present in cruciferous vegetables.

Enzymic optical resolution and flash vacuum thermolysis in concert for the synthesis of optically active cyclopentenones

Abstract A practical synthesis of enantiomerically pure cyclopentenones with a predeterminated absolute configuration has been realized, starting from optically active tricyclo[5.2.1.02,6]decadienones.