Tandem reduction-chloroallylboration of esters: asymmetric synthesis of lamoxirene, the spermatozoid releasing and attracting pheromone of the laminariales (Phaeophyceae)

The asymmetric synthesis of all four stereoisomers of lamoxirene (cis-2-cyclohepta-2,5-dienyl-3-vinyloxirane), the spermatozoid-releasing and -attracting pheromone of the Laminariales (Phaeophyceae), is reported. Chiral ethyl cyclohepta-2,5-diene carboxylates, prepared by a divinylcyclopropane Cope rearrangement, were effectively alkylated by means of a novel tandem DIBAL-H reduction/asymmetric alpha-chloroallylboration using (Z)-gamma-chloroallyldiisopinocampheylboranes. The ensuing syn-alpha-chlorohydrins were transformed into the corresponding vinyloxiranes with DBU, providing all four isomers of the pheromone in good chemical and excellent optical yield (90-97% ee). Spermatozoid-release assays were conducted with the sympatrically growing species L. digitata, L. hyperborea, and L. saccharina and established (1'S,2R,3S)-1c as the most active isomer in all cases.     

Biosynthesis of Dodecano-4-lactone in Ripening Fruits: Crucial Role of an Epoxide-Hydrolase in Enantioselective Generation of Aroma Components of the Nectarine (Prunus persica var. nucipersica) and the Strawberry (Fragaria ananassa)

Early steps in the biosynthesis of dedecano-4-lactone in ripening nectarines (Prunus persica var. nucipersica) and strawberries (Fragaria ananassa) were studied using cis-9,10-([¹⁸O]epoxy)heptadecanoic acid ( 5 ), instead of the natural C₁₈-precursor epoxystearic acid 1 . In nectarines, an epoxide hydrolase catalyzed, in a highly regio- and enantioselective reaction, the attack of H₂O at C(9) of epoxy acid 5 yielding preferentially (9R,10R)-dihydroxyheptadecanoic acid 6a as an early intermediate and, after further metabolization, undecano-4-lactone 7a . In strawberries, the same transformation exhibited only a moderate regio- and enantioselectivity. Besides the previously established function of phytogenic epoxide hydrolases involved in generating oxygenated fatty-acid derivatives, which are used in cutin monomer production and plant defence, a new rǒle for these enzymes now emerges in the enantioselective production of aroma components of ripening fruits. The cis-9,10-([¹⁸O]epoxy)heptadecanoic acid (≥ 98% ¹⁸O; 5 ) was obtained in good overall yield (68%) from isopropyl (9Z)-heptadec-9-enoate using ¹⁸O₂ gas in a free-radical chain reaction in the presence of 2-methylbutanal.     

Angular Alkylation of cis-Decalin Epoxides with C-nucleophiles: Mechanism,scope, and limitations of a novel bridgehead functionalisation

The angular alkylation of cis-decalin epoxides like 5 or 7 can be achieved at C(8a)

1 For convenience, the arbitrary numbering given for 5 (Scheme I) is used throughout the General Part; for systematic names, see Exper. Part.

in good yield by using CuI and a large excess of Grignard reagents without an sp³ centre at C(2). The reaction proceeds via a carbenium-ion intermediate which is stabilised by homoconjugative interaction with the adjacent double bond. Due to 1,3-diaxial strain in the alkoxides resulting from alkylation or reduction at C(4a) of the epoxides 5 or 7 , the nucleophile is delivered selectively to C(8a). Grignard reagents possessing H-atoms at C(β), transfer a hydride to the epoxide yielding the trans-decalol 11 (Grignard reduction). The angular alkylation of 5 with allylic and benzylic Grignard reagents proceeds with good yield.     

Stereoselective syntheses of deuterium labelled marmesins; valuable metabolic probes for mechanistic studies in furanocoumarin biosynthesis

Stereoselective syntheses of deuterium labelled marmesins 3a-3e as metabolic probes for biosynthetic studies are described. Starting from iodo-umbelliferone (5), the title compounds were readily available by copper catalysed alkylation-cyclisation and subsequent transfer-hydrogenation of the resulting furanocoumarins 6a-6e. The bioconversion of marmesin by a microsomal preparation of A. majus yielded psoralene (4) and acetone in a single step and proceeded as a syn-elimination of the alkoxy-substituent together with a single hydrogen atom from a vicinal carbon atom (β-cleavage).     

A cucurbit[6]uril analogue: host properties monitored by fluorescence spectroscopy

This paper describes the host properties of a new cucurbit[6]uril analogue, studied by fluorescence and 1H NMR spectroscopy. This host has an elongated cavity with oval-shaped portals. It is intrinsically fluorescent, and more importantly, this fluorescence is sensitive to guest encapsulation, allowing for the study of the inclusion of nonfluorescent guests by fluorescence spectroscopy. In the case of benzene as guest, significant enhancement of the cucurbit[6]uril analogue host fluorescence was observed upon addition of benzene; this allowed for the determination of the binding constant for 1:1 host-guest complexation, yielding a value of K = 6900 +/- 1100 M(-1). This complexation was also studied by 1H NMR, yielding a similar value of K = 8980 +/- 500 M(-1). The binding of a much larger guest, the dye Nile Red, was also studied, but in this case using guest fluorescence. Significant suppression of the Nile Red fluorescence was observed upon 1:1 complexation with the cucurbit[6]uril analogue, with an extremely large binding constant of 8.2 +/- 0.5 x 10(6) M(-1), indicating a very strong host-guest interaction and an excellent size and shape match. In both cases, binding was much stronger than in the case of the same guests with cucurbit[6]uril itself, and in the case of Nile Red, binding was also much stronger than with modified beta- or gamma-cyclodextrins. This is partly a result of the partial aromatic nature of the host walls, which allow for pi-pi interactions not possible in cucurbiturils or cyclodextrins. The ability to study its inclusion complexes using either host or guest fluorescence, and the very high binding constants observed, illustrates the versatility and potential usefulness of this new host compound.     

Novel C9 and C11 Hydrocarbons from the Brown Alga Cutleria multifida; Sigmatropic and Electrocyclic Reactions in Nature. Part VI

In addition to the known C₁₁H₁₆ hydrocarbons multifidene ( 4 ), aucantene ( 2 ), and ectocarpene ( 5 ), the marine brown alga Cutleria multifida produces trace amounts of the C₉H₁₂ hydrocarbon 7-melhylcycloocta-1,3,5-triene ( 8 ) and its valence tautomer 7-methylbicyclo[4.2.0]octa-2,4-diene, A second novel C₉H₁₂ hydrocarbon is 6-vinyicyclo-hepta-1,4-diene ( 9 ), a lower homologue of ectocarpene ( 5 ). Among the C₁₁H₁₆ hydrocarbons, 7-((1E/Z)-prop-l-enyl)cycloocta-1,4-diene ( 10 / 11 ) is found for the first time. The structure of all new products is confirmed by synthesis and spectroscopic data. The biosynthesis of the new hydrocarbons 8 – 11 is obviously linked to the pathways which lead to the major products giffordene ( 7 ), (6S)-ectocarpene ((6S)- 5 ), and (4R,5R)-aucantene ((4R,5R)- 2 ). Consecutive reactions of certain thermolabile primary products proceed via electrocyclic ring closure, 3,3-sigmatropic rearrangement, or a 1,7-sigmatropic H-shift.     

Efficient and flexible synthesis of chiral gamma- and delta-lactones

An efficient and highly flexible synthesis for chiral gamma- and delta-lactones with high enantiomeric purity is described (>99% ee and 57-87% overall yield). The protocol involves alkylation of chiral 1,2-oxiranes with terminally unsaturated Grignard reagents. Subsequent oxidative degradation (OsO(4)-Oxone) of the terminal double bond from chiral alk-1-en-5-ols and alk-1-en-6-ols affords 4- or 5-hydroxy acids and gamma- and delta-lactones after acidic workup. The flexibility and efficiency of the protocol is illustrated by the synthesis of several alkanolides and alkenolides, hydroxy fatty acids and dihydroisocoumarins.     

Designing stable redox-active surfaces: chemical attachment of an osmium complex to glassy carbon electrodes prefunctionalized by electrochemical reduction of an in situ-generated aryldiazonium cation

The production of stable redox-active layers on electrode surfaces can lead to improvements in electronic device design. Enhanced stability can be achieved by pretreatment of electrode surfaces to provide surface chemical functional groups for covalent tethering of redox complexes. Herein, we describe pretreatment of glassy carbon electrodes to provide surface carboxylic acid groups by electro-reduction of an in situ-generated aryl diazonium salt from 3-(4-aminophenyl)propionic acid. This surface layer is characterized by attenuated total reflection infrared spectroscopy, atomic force microscopy, and electrochemical blocking studies. The surface carboxylic acid generated is then used to tether an osmium complex, [Os(2,2'-bipyridyl)2(4-aminomethylpyridine)Cl]PF6, to provide a covalently bound redox-active monolayer, E(0) ' of 0.29 V (vs Ag/AgCl in phosphate buffer, pH 7.4), on the pretreated glassy carbon electrode. The layer proves stable to pH, temperature, and storage conditions, retaining electroactivity for at least 6 months.