Ether and acetal substituents

In this Section, ether and acetal substituents will be discussed. In some polysaccharides, the terminal reducing sugar is glycosidically linked to a non-sugar aglycon, and this will be discussed in a special part. 

Several methylated sugars have been identified in hydrolyzates of LPS, cell-wall polysaccharides, and extracellular polysaccharides. A considerable number of these have been found in the LPS from photosynthetic prokaryotes. Two polysaccharides from Mycobacterium species, a glucan and a mannan are remarkable in that they contain high percentages of methylated sugars. Glycolipids from Mycobacterium species are also rich in methylated sugars, some of which have not been found elsewhere, but this is beyond the scope of the present article. 

The different methylated sugars known as components of bacterial polysaccharides are summarized in Table 1. When possible, references to publications in which the methylated sugar is part of a known structure are preferred to references in which the component has merely been identified. References to sugars of undetermined configuration or absolute configuration have been omitted when there is reason to assume that they are identical to better characterized compounds from other sources. 

Some sugar residues in bacterial polysaccharides are etherified with lactic acid. The biosynthesis of these involves C)-alkylation, by reaction with enol-pyruvate phosphate, to an enol ether (34) of pyruvic acid, followed by reduction to the (R) or (5) form of the lactic acid ether (35). The enol ether may also react in a different manner, giving a cyclic acetal (36) of pyruvic acid. 

The first known 1-carboxyethyl ether of a sugar was 2-amino-3-0-[(/ )-l-carboxyethyl]-2-deoxy-D-glucose or muramic acid (37). It is a component of the polysaccharide moiety of the peptidoglycan in the bacterial cell-wall. It is partially replaced by the mamo isomer, 2-amino-3-6 -[(/ )-l-carboxy-ethyl]-2-deoxy-D-mannose, in the peptidoglycan from Micrococcus lyso-deikticus. 

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The hydrogenolysis of allylic ether and acetate (235 236, R=alkyl or COCHj) should also take place more easily when the compound can adopt a conformation in which the OR group can become parallel to the u orbital of the double bond (69). The same stereochemical requirement must also be necessary in the hydrogenolysis of a substituent in a benzylic position (70). 

With the exception of norbomene, internal olefins do not undergo hydrosilylation. Hydrosilylation of 3-phenylpropene with PhSiDj forms a unique product and the process tolerates a variety of fianctional groups, halides, ethers and acetals, despite the well known strong Lewis acidity of the catalysts. Cyclisation/silylation of 1,5-dienes or 1,6-enynes has been reported to give a single product (Scheme 14) [26]. In the case of metallocene complexes bearing a menthyl substituent, ee values near 70% were obtained for the asymmetric hydrosilylation of 2-phenyl-but-l-ene [31]. 

Stereoselective reactions with acetals. Noyori et al. (10,438) have used this Lewis acid to promote an aldol-type reaction between enol silyl ethers and acetals and have noted high. syn-selectivity in this process. Molander and Haar report that reaction of acetals with cyanotrimethylsilane promoted by TMSOTf results in a-alkoxy cyanides and that this reaction can be diastereoselective when the acetal is substituted at the 4-position by an alkoxy group. The diastereoselectivity depends on the nature of the acetal and the 4-alkoxy group. Dimethoxy acetals show slight diastereoselectivity, but diisopropoxy and dibenzyl acetals can show diastereoselectivity of 5-10 1. The diastereoselectivity also depends on the type of 4-substituent. Acetoxy and t-butyldimethylsilyloxy groups have no effect on the diastereoselectivity, but methoxy, benzyloxy, and allyloxy groups promote anri-selectivity. Since a metal template is not involved, the diastereoselectivity... 

Deoxygenation of Epoxides. In the presence of catalytic rhodium(II) acetate, dimethyl diazomalonate deoxygenates epoxides under neutral conditions (eq 11). This reaction tolerates functionality such as ketones, esters, alkyl and silyl ethers and halogen substituents, although alcohols and aldehydes undergo competing insertion reactions. 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

Treatment of commercially available and symmetrical 3,4,5-tri-methoxytoluene (37) with iodine, periodic acid, and acetic acid under the conditions of Suzuki19 results in the formation of symmetrical diiodide 38 in 93 % yield. Although only one of these newly introduced iodine atoms is present in intermediate 13, both play an important role in this synthesis. Selective monodemethylation of 38 with boron trichloride furnishes phenol 39 in 53% yield together with 13 % of a regioisomer. Evidently, one of the Lewis-basic iodine substituents coordinates with the Lewis-acidic boron trichloride and directs the cleavage of the adjacent methyl ether... 

The Lewis acid induced reaction of silyl enol ethers and silyl ketene (thio)acetals with 4-acetoxyazetidinones is often used for introduction of a carbon substituent in the 4-position of the jS-lactam ring. Numerous examples are known, both with and without substituents at nitrogen, some of which are shown. 

When a substituent is able to resonantly stabilize the positive charge of the ionic intermediate, there is no bromine bridging and the intermediate is an open P-bromocarbocation. These carbocations have been shown to occur in the bromination of a-methylstilbenes (ref. 9), 1 and 2, and of a variety of enol ethers (ref. 10) and acetates (ref. 11). 

Recent investigations have demonstrated that electron withdrawing substituents at the a-position increase the rate of this reaction strongly (12.). This reaction would have great potential for natural product syntheses provided that additional electron donating functional substituents could be introduced in the p- and a-positions, that enol ethers, enediol ethers, ketene acetals could react as dienophiles (see route E in Scheme 1). In addition. 

The kinetic and activation parameters for the decomposition of dimethylphenylsilyl hydrotrioxide involve large negative activation entropies, a significant substituent effect on the decomposition in ethyl acetate, dependence of the decomposition rate on the solvent polarity (acetone-rfe > methyl acetate > dimethyl ether) and no measurable effect of the radical inhibitor on the rate of decomposition. These features indicate the importance of polar decomposition pathways. Some of the mechanistic possibilities involving solvated dimeric 71 and/or polymeric hydrogen-bonded forms of the hydrotrioxide are shown in Scheme 18. 

As a consequence of the complimentary electron demand of the nitroalkene and the product nitronate, there exists the possibility of a one-pot, tandem reaction. In this case, the nitroalkene will react preferentially with the electron-rich alkene to produce an intermediate nitronate. This nitronate can then react with a second alkene bearing an electron-withdrawing substituent. Therefore subjection of the nitroalkene 210 to both ethyl vinyl ether and acrylonitrile provides only the nitroso acetal 211 in moderate yield (Eq. 20) (70). Moreover, this also allows the possibility of intramolecular variants of the process. 

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