Branching chain, scheme

A simplified reaction scheme is shown in Fig. 26.5 Again, the ability of rhodium to change its coordination number and oxidation state is crucial, and this catalyst has the great advantage over the conventional cobalt carbonyl catalyst that it operates efficiently at much lower temperatures and pressures and produces straight-chain as opposed to branched-chain products. 

The biodegradation of branched-chain alkanol ethoxyethylates was carried out by the standard OECD confirmatory tests and the metabolites fractionated after solid-phase extraction. The structures of the metabolites were determined by electrospray mass spectrometry and this made it possible to derive a scheme for the partial degradation of the compounds (Di Corcia et al. 1998). 

Clearly, branching chains containing other borazine units on the free aminoboryl site cannot be excluded, even though such chains are difficult to obtain because of steric hindrance. Nevertheless, a structure that is consistent with the characterization data is proposed in scheme 8. 

Henry reaction of nitro sugar 11 with formaldehyde allowed the introduction of two hydroxymethyl groups at the carbon bearing the nitro group, and hence opened a specific route for the preparation of branched-chain imino sugar 57 and analogues (Scheme 20).44... 

With this solid-phase methodology in hand, the synthesis of branched oligosaccharides could be investigated. This led to the preparation of pentasaccharide 15 related to /V-glycan chains (Scheme 4.5).3... 

A sensitive method for determination of branched-chain L-amino acids is based on labelling individual species with 13 C or 2H, isolating the amino acid by IEC and applying the derivatization scheme depicted in reaction 1. The L-amino acid is converted... 

The mechanism of the reaction depicted in Scheme 4.6 differs from the Sf.,1 or Sf.,2 mechanism in that it involves the stage of one-electron oxidation-reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of fluorenyl radical. The latter is unsaturated at position 9 near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthiolate ion. This leads to the anion-radical of the substitution product. Further electron exchange produces the substrate anion-radical and final product in its neutral state. The reaction consists of radical (R)-nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol Sj j l. Reactions of Sj j l type can have both branch-chain and nonchain characters. 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

In this reduced scheme, step I is the fuel-consumption step, which is seen also to consume radicals. Step II is the step for production of C2 species not in steady state, important for obtaining correct CH profiles. Step III is the water-gas shift that burns CO. The oxygen is consumed by step IV, which is the source of radical production through the hydrogen-oxygen branched chain. 

A useful application of the chemistry shown in Scheme 83 is the synthesis of branched-chain functionalized carbohydrates. For this purpose two epoxides 261 and 262 derived from D-glucose and 263 derived from D-fructose were prepared following reported methodologies, and were submitted to a DTBB-catalyzed lithiation as described above in Scheme 83. The expected intermediates 264-266 and final products 267-269 were prepared in a regio- and stereoselective manner in 15- 95% yield" Also here, the use of a prochiral electrophile gave equimolecular amounts of both diastereomers. 

A pentopyranoside-fused butenolide is the key intermediate for the synthesis of the natural micotoxin patulin [226, 227]. Its synthesis involves Wittig olefination of a 3,4-di-O-protected arabinopyran-2-uloside, followed by protecting group removal and dehydration (Scheme 47). In other research, the glucopyranosid-2-uloside 190 was converted into the butenolide derivative 191 by aldol condensation with diethyl malonate and transesterification [228]. The latter was shown to be prone to autoxi-dation, leading to 192. Subsequent Michael addition with hydroxide ion, followed by decarboxylation, furnishes C-branched-chain sugar 193. 

Scheme 47 Synthesis of a 2-C-branched-chain sugar via a pyranose-fused butenolide...

From prior attempts at FruA-catalyzed DHAP additions to glyoxal or glutaric dialdehyde no product had been isolated, probably because the dialdehydes can cause cross-linking of the protein and thereby irreversibly destroy its enzymatic activity. On the other hand, hydroxylated aldehydes were assumed to form stable intramolecular hemiacetals in aqueous solution, which may mask the reactivity of free dialdehydes. Using the branched-chain glutaric dialdehyde 38 as a potential precursor to carbon-linked disaccharide mimetics (Scheme 2.2.5.14), we... 

Kinetic resolution of branched-chain fatty acids has been reported recently by Franssen et al. [24]. With the help of immobilised Candida antarctica lipase B, racemic 4-methyloctanoic acid (responsible for sheep-like and goat-like flavours in sheep and goat milk and cheese, respectively) was esterified with ethanol. Only the R ester could be obtained, whereas (S)-4-methyloctanoic acid was not converted (Scheme 22.1). 

Variations on this basic theme have been reported to reach more or less complex Type I branched-chain sugars (Scheme 2). A particular emphasis has been given on the use of... 

The synthesis of type III branched-chain sugars is based mainly on the use of ketosugars treated under Wittig-type conditions (see path a, Scheme 4) [13]. Several other methods, such as aldolization-crotonization or direct alkylidenation at the a-position of the carbonyl group of a keto sugar have been developed (path b). 

Ozonolysis and subsequent reduction open the way to formyl and hydroxymethyl branched-chain derivatives [18]. High stereoselectivities are generally observed on Grignard addition on conformationally biased ketosugars such as 3 (Scheme 6), equatorial attack being strongly favored, giving the axial alcohol 4 as the major product. 

Another interesting approach is the use of l,l-dimethoxy-2-lithio-2-propene, as demonstrated by Depezay s group, for the synthesis of C-methylene branched-chain sugars [24,25], intermediates in the synthesis of hamamelose G, a naturally occurring branched-chain pentose [26,27]. In this cash, the condensation of this Kthio derivative was effected on the aldehydo group of D-glyceraldehyde 10 (Scheme 8) to give a 3 7 mixture of 11a... 

Spiro epoxides are also valuable intermediates for the synthesis of type I branched-chain sugars. These spiro epoxides are formed from ketosugars using diazomethane addition or sulfonium chemistry. Further ring opening of the epoxide allows the introduction of various nucleophiles [1], Chloro spiroepoxide 15 (Scheme 9) has been prepared recently from ketosugar 1 dichloromethyllithium [28]. 

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