Radical chain elongation

An approach to the preparation of chiral 208 utilizes a free-radical chain-elongation reaction for the preparation of 206. Alcohol 141 is converted in two steps and 92% yield to (4iS, 5R)-4-benzyloxymethyl-5-iodomethyl-2,2-dimethyl-l,3-dioxolane (209). Treatment of... 

B. Giese, T. Linker, and R. Muhn, Biomimetic chain elongation of carbohydrates via radical carbon-carbon bond formation, Tetrahedron 45 935 (1989). 

Many important natural products are (formerly) derived by chain elongation at position 5 of pentoses, or at position 6 of hexoses. Uronic acids, which are easily prepared, can be converted into the 4 radical 90 by chemistry based on the thiohydroxamate 6.77 We postulated that, if the hindrance on the a-side of the molecule was great enough, the carbon-carbon bond formed by reaction of 90 with a suitable radicophilic olefin would be the natural / -bond. In fact, even a dimethyl-ketal as in 90 (B = natural base or protected derivative thereof) was sufficient to direct the bond formation very largely to the desired face.77 The diacetone ketal of glucuronic acid 91 upon conversion to its iV-hydroxy-2-thiopyridone derivative 92 and then photolysis in the usual way in the presence of methyl acrylate gave the expected derivative 93 as a mixture of... 

Recently, most radical cyclizations of halides have been performed using iodides with intramolecular alkene or alkyne functionality. As with chain elongation reactions, both chain and nonchain processes are possible, with the former being the most common. The nonchain processes are not covered here and the reader is referred to the excellent reviews in the literature for details. 

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet li t and solar radiation. Free radicals formed by the homol3rtic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involving the production of HCl and conjugated double bonds. 

The carbon-centered radical at the 5-position can be reacted by various trapping agents other than the nitrosyl radical. Z. ekovi and co-workers used electron-deficient olefins (Michael acceptors) such as acrylonitrile to trap the 5-carbon radial and obtain functionalized alkyl chains.In order to maximize the yield of the desired chain-elongated product, a high concentration of the acrylonitrile had to be used. The final radical was trapped by the nitrosyl radical. 

One molecule of oxygen accepts two pairs of electrons, one from palmitoyl-CoA and the other from NADPH or NADH. The electrons NAD(P)H are transported via cytochrome-bs reductase to cytochrome bs (microsomal electron transport Chapter 14). An enzyme-bound superoxide radical is responsible for the oxidation of acyl-CoA. Four desaturases specific for introducing cis double bonds at C9, Ca, C5, and C4, respectively, are known. If the substrate is saturated, the first double bond introduced is C9. With an unsaturated substrate, other double bonds are introduced between the carboxyl group and the double bond nearest the carboxyl group. Desaturation yields a divinylmethane arrangement of double bonds (—CH=CH—CH2—CH=CH—). Usually desaturation alternates with chain elongation. Desaturation is inhibited by fasting and diabetes. The oxidation of unsaturated fatty acids occurs in mitochondria. 

By an analogy to the radical syntheses above, the carbon chain elongation reaction was achieved by Barton et al. [109] in the reaction with ethyl a-trifluoroacetoxy acrylate 165. Under radical conditions 165 was coupled with the ester 164 prepared from pentaacetyl gluconic acid and /V-hydroxy-2-thiopiridone (Scheme 35) to afford an unstable intermediate 166, readily convertible to the unsaturated a,p-keto acid 167. The methoxymercuration-demercuration of 167 gave a low selectivity, providing the mixture of 3-deoxy-4-methoxy-D-man/jo and D-g/wco-octulosonic acids (4 3) in a moderate yield [110]. 

Unsaturated fatty compounds such as oleic acid [la], 10-undecenoic acid [2a], pet-roselinic acid [3a], erucic acid [4a], and the respective esters, alcohols, and native oils (Fig. 1) are alkenes and contain an electron-rich double bond that can be functionalized in many different ways by reactions with electrophilic reagents. It is therefore remarkable that >90% of oleochemical reactions have been focused on the carboxylic acid functionality and < 10% have been reactions of the alkyl chain and the C,C-dou-ble bond (1). A review on radical additions to unsaturated fatty compounds that appeared in 1989 (2) quoted only very few C,C-bond-forming reactions giving branched and chain-elongated fatty compounds. Since then, modem preparative radical chemistry has been developing and has been applied also to fat chemistry (3-5). We report here on radical additions of activated haloalkanes such as alkyl 2-haloalka-noates and 2-haloalkanenitriles to unsaturated fatty compounds [l]-[4] initiated by electron transfer from copper in solvent-free systems. These additions were also car-... 

A review of the four reagent systems developed for direct C-H oxidation and establishing three carbon-chain elongations via straightforward installation of carbon unit to C-H bonds is published. Highly reactive chemical species acting like oxygen radical are utilized as a key C-H activator. ... 

Addition of radical trapping agents can extend the functionality of the Barton nitrite ester reaction. The Cekovic group added Michael acceptors to the reaction, and the carbon-centered radical adds to the Michael acceptor to form a chain elongated product 10. A large excess of Michael acceptor is necessary for yields to be practical. 

Chain elongation has been accomplished intramolecularly, by addition of the radical derived from the bromoacetal group at 0-3 of the 5,6-dideoxy-hex-5-enofuranose... 

Bold numbers identify the species in the global reaction mechanism depicted in Fig. 1. All of them occur without identifiable transition states, and the comparison among theoretical and experimental results is shown in Table 8. In this initiation reactions appear the H and CHs radicals, as in reactions (l)-(3), but also other small radicals that can participate in chain elongation reactions, namely C2H3-, CiHs, and different isomers of CsHs-. 

Metathesis chain elongation with a vinyl borate followed by an oxidation step gave rise to aldehyde 451, which on radical cydization led to the tricydic framework (452) of cyanthiwigin [168],... 

Another example for block copolymer synthesis is the preparation of poly (methylene-fc-styrene), which was achieved by using a hydroxyl-terminated living polystyrene obtained by TEMPO-mediated living radical polymerization [75] (Scheme 55). The chain end hydroxyl group was transformed into an allyl ether moiety, which was subjected to hydroboration with BH3 to afford polystyrene macroinitiator for the polymerization of 11. After chain elongation with 11 oxi-dation of the C-B chain ends furnished poly(methylene- -styrene) bearing TEMPO and hydroxyl group at chain ends. 

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