Carbohydrates alkenes

Unsaturated branched-chain sugars are mentioned in Chapters 13 and 16, and reactions of nitro-alkene carbohydrates with sulphur ylides are covered in Chapter 9. 

Heterogeneous reaction (Section 6 1) A reaction involving two or more substances present in different phases Hydro genation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst Heterolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that both electrons are retained by one of the initially bonded atoms Hexose (Section 25 4) A carbohydrate with six carbon atoms High density lipoprotein (HDL) (Section 26 11) A protein that carries cholesterol from the tissues to the liver where it is metabolized HDL is often called good cholesterol Histones (Section 28 9) Proteins that are associated with DNA in nucleosomes... 

Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

Asymmetric epoxidation is another important area of activity, initially pioneered by Sharpless, using catalysts based on titanium tetraisoprop-oxide and either (+) or (—) dialkyl tartrate. The enantiomer formed depends on the tartrate used. Whilst this process has been widely used for the synthesis of complex carbohydrates it is limited to allylic alcohols, the hydroxyl group bonding the substrate to the catalyst. Jacobson catalysts (Formula 4.3) based on manganese complexes with chiral Shiff bases have been shown to be efficient in epoxidation of a wide range of alkenes. 

The homo- and cross-addition of alkenes catalyzed by a transition-metal provided another economical way of forming C-C bonds.155 These reactions are carried out by using nickel, palladium, or ruthenium phosphine complexes to yield vinylarenes and some can occur in aqueous media. By using carbohydrate-derived ligands, asymmetric hydrovinylations can be carried out in aqueous conditions.156... 

Asymmetric tandem cycloaddition of a chiral carbohydrate nitroalkene with ethyl vinyl ether in the presence of electron-withdrawing alkenes produces a facile assembly of bicyclic systems, which can further be selectively cleaved to give homologated carbohydrates (Eq. 8.110).176... 

Fluorescent pseudomonads are capable of synthesizing poly(3HAMCL)s from a large number of substrates. Work on the biotechnological production of poly(3HAMCL) has focused mainly on two model systems - Pseudomonas oleo-vorans and P. putida. P. oleovorans is able to use alkanes and alkenes as substrate due to the presence of the OCT-plasmid while P. putida, which does not have this plasmid, cannot. In contrast to P. oleovorans, however, P. putida can use carbohydrates such as glucose and fructose for the production of poly(3HAMCL). 

Although reaction of carbohydrates with such activated alkenes as acrylonitrile and methyl vinyl sulfone has industrial importance, the... 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

The epoxidation of nonfunctionalized alkenes may also be effected by chiral dioxiranes. These species, formed in situ using the appropriate ketone and potassium caroate (Oxone), can be formed from C-2 symmetric chiral ketones (29)[93], functionalized carbohydrates (30)[94] or alkaloid derivatives (31)[95]. One example from the laboratories of Shi and co-workers is given in Scheme 19. 

Sulfated alkenes, 23 538 Sulfated carbohydrate products, 23 538 Sulfated cyclodextrin-based chiral stationary phase, 6 87 Sulfated fatty acids, 23 538 Sulfated fatty alcohol ethoxylates, 23 537 Sulfated fatty oils, 23 538 Sulfated products... 

Cycloaddition reactions between alkenes and noncarbohydrate, carbonyl compounds have been described in discussing the reactions of alkenes (see Table I and Scheme 1). The depiction of the excited carbonyl given in Scheme 6 is useful in understanding the regiochem-istry of the cycloaddition process, as it suggests that the electron-deficient oxygen atom in the excited carbonyl will react with the alkene to produce the (more-stable) 1,4-diradical. Table VIII lists cycloaddition reactions in which the excited carbonyl is part of a carbohydrate. 

Roberts and co-workers have employed a number of chiral carbohydrate-derived thiols as polarity reversal catalysts in the radical hydrosilylation of electron-rich prochiral alkenes [68-70]. In these thiols, the SH group is attached to the anomeric carbon atom. Scheme 21 demonstrates the non-catalyzed reaction and in step b, the hydrogen atom transfer from the silane... 

The asymmetric tandem cycloaddition of the chiral carbohydrate nitroalkene (35) with ethyl vinyl ether involves the initial formation of the nitronate (36) which reacts exclusively with electron-withdrawing alkenes by 3 -I- 2-cycloaddition to yield chiral bicycles (37) and (38) (Scheme 12). ... 

Carbohydrates remain an attractive source of chirality in preparation of ligands for asymmetric catalysis. Functionalized phospholanes, 192 [167], and chiral bisphosphinites 193 [168] with an attached crown ether unit were obtained recently from D-mannitol and from phenyl 2,3-di-0-allyl-4,6-0-benzylidene-p-D-glucopyranoside, respectively (Figure 18). Compounds 194 and 195 were obtained in the photochemical addition of H2P(CH2)3PPH2 onto the crresponding alkenes - Pd-complexes of these new bisphosphines were successfully applied as catalysts in the copolymerization of CO and... 

Reactions of alkenes such as 120 with a-chloronitrosoalkanes of type 119 proceed under very mild conditions and result in the formation of nitrones 121 that can be easily hydrolyzed into hydroxylamines 122 (equation 86) . Chiral carbohydrate-derived a-chloronitrosoalkenes 123 possess enhanced reactivity and produce good stereoselectivity in reaction with prochiral alkenes such as 124 (equation 87) . ... 

Intramolecular oxime-alkene cycloaddition has been proved to proceed with complete stereoselectivity in carbohydrate derived hydroxylamine 171 (equation 112). 

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