Polyunsaturated natural products

The interesting stereospecific Pd-catalyzed cross-coupling reactions of 1-alkynylzinc chlorides 25 with a diastereoisomeric mixture of alkenyl halides E) or (Z)- 24, have been described. The ( )-bromoalkene reacts preferentially in these reactions to afford good yields of ( )-enynes 26 of very high stereoisomeric purity [Eq. (10)] [18], The enyne can easily be selectively transformed into the corresponding dienes, which may be used to prepare stereodefined polyunsaturated natural products. 

Molecular Linchpins in Sequential Cross-Coupling Approaches to Polyunsaturated Natural Products 40... 

HEXATRIENE-1,6-DISTANNANES AS MOLECULAR LINCHPINS IN SEQUENTIAL CROSS-COUPLING APPROACHES TO POLYUNSATURATED NATURAL PRODUCTS... 

At that time, we strived for the first total synthesis of a polyunsaturated natural product named xerulinic acid (22 Scheme 4). We envisaged using the hexatriene-l,6-distannane trans,trans,trans-lS (Fig. 5) as a Cg linchpin for joining the Z-configured y-(hromomethylidene)butenolide 20 and the polyunsaturated bromoalkyne 23 by sequential Stille couplings. 

Propiolaldehyde diethyl acetal has found numerous synthetic applications in the literature which may be briefly summarized. The compound has been utilized in the synthesis of unsaturated and polyunsaturated acetals and aldehydes by alkylation of metal-lated derivatives, " by Cadiot-Chodkiewicz coupling with halo acetylenes, " and by reaction with organocuprates. Syntheses of heterocyclic compounds including pyrazoles, isoxazoles, triazoles, and pyrimidines have employed this three-carbon building block. Propiolaldehyde diethyl acetal has also been put to use in the synthesis of such natural products as polyacetylenes " and steroids. ... 

The introduction of epoxidation techniques for polyunsaturated natural oils by Swern and colleagues [7,8] led to industrial interest in the preparation of epoxy compounds useful for resin production [9,10],... 

But chrysanthemic acid derivatives are by far not the only examples of cyclopropane-containing structures in nature. In fact, the highly strained three-membered carbocycle is virtually ubiquitous. It occurs, for example, in every green plant in the form of 1-aminocyclopropanecarboxylic acid (ACC) 2, a direct precursor to the plant hormone ethylene [3]. In addition, the cyclopropane unit is found in a variety of other natural products, inter alia in terpenes and in various cyclopropanated fatty acids [4]. The biochemical precursors of the latter are unsaturated fatty acids, and in view of the existence of polyunsaturated fatty... 

A formal total synthesis of oximidine II was achieved by G.A. Molander et al., using an intramoiecuiar Suzuki-type cross-coupiing between an alkenyl potassium trifluoroborate and an alkenyl bromide to construct the highly strained, polyunsaturated 12-membered macrolactone core of the natural product. " The stability of potassium trifluoroborates was exploited in order to establish the best conditions for the macrocyclization. 

Synthesis has been useful in the chemistry of phenolic lipids for structural confirmation and in enabling structure/property correlations to be made by the general applicability of syntheses to a range of non-natural isomers. With the polyunsaturated constituents synthesis is valuable since either argentation chromatography or preparative HPLC have usually proved laborious on the natural products and on transformed isomeric compounds required for evaluatory purposes in structure/activity studies. 

Linseed oil presented another interesting opportunity for the production of bioplastics. The highly polyunsaturated nature of the oil allowed for a large number of reactive sites to be introduced onto the material for subsequent polymerization. As has previously been mentioned, this increased reactivity has been shown to limit the physical properties of the materials and as such there has been little commercial interest in taking these materials further. Epoxida-tion, acrylation and maleinization of linseed oil before polymerization produced a heavily crosslinked polymer [59]. A formulation for potential in infusion resins was developed with a mixture of epoxidized linseed oil and phthalic anhydride with the curing catalysed by 2-methylimidazole [60]. 

Hydrophobic Natural Products Polyunsaturated fatty acids, sterols, vitamins, and carotenoids, for example, that are present in plant tissues. These... 

The first application of our hexatriene-l,6-distannane trans,trans,trans-lH (Fig. 5) in natural product synthesis was in our total synthesis of the polyunsaturated y-alkyUdenebutenolide dUiydroxerulin. ... 

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