Anomeric olefin

Anomeric hydroperoxides are readily prepared by treatment of 2-deoxy sugars with H202 in the presence of acid (Fig. 59). They are used as reagents for enantioselective epoxidation of a,(Tunsaturated olefins (e.g. chalcone) in the presence of sodium hydroxide, the epoxidations showed exceptionally high asymmetric induction.76... 

The activation of NPGs during a glycosylation reaction (Scheme 5.7a) depends on electrophilic addition to the olefin (—>111), followed by intramolecular displacement by the anomeric oxygen to form the oxonium species IV. Trapping with a glycosyl... 

A. Kittaka, T. Asakura, T. Kuze, H. Tanaka, N. Yamada, K. T. Nakamura, and T. Miyasaka, Cyclization reactions of nucleoside anomeric radical with olefin tethered on base Factors that induce anomeric stereochemistry,/. Org. Chem., 64 (1999) 7081-7093. 

The anomeric radical 11 adds to the olefin 12 to give the intermediate 13. Interception of this adduct radical 13 by tin hydride 14 yields the saturated product 16 and the organometallic radical 15. Eventually, the latter reacts with the precursor 17 to produce the chain-carrying radical 11 and the tin compound 18. 

Because such alkylation proceeds by S l mechanism, even cobalt complexes derived from unreactive (in an SN2 sense) halides can be formed. The cobalt complexes are air-stable compounds, but are affected by direct daylight. The incorporated Co—C bond is weak and, therefore, photolysis of 33 sets free the anomeric radical 11. In the presence of olefins 12 this radical adds to the double bond, followed by subsequent combination to give the insertion product 35 (Scheme 9). 

Anomeric effect, 82, 310-311, 305 Antarafacial, 163 examples, 164 sigma bonds, 167 Anti-Bredt olefin, 102 Approximations of MO theory Born-Oppenheimer, 22 Hartree-Fock, 222 Huckel, 35, 86 independent electron, 35 LCAO, 229 nonrelativistic, 22 SHMO, 87... 

Azide addition to enolizable ketones is regiospecific and may be considered as a 1,3-dipolar cycloaddition occurring at the double bond of the enolate, similar to the addition of azides to electron-rich olefins. However, a stepwise reaction appears more probable because glycosyl azides exhibit anomerism when they react with activated methylene compounds, thus indicating the presence of a triazene intermediate.264 On the other hand, the formation of the triazene intermediate may be considered as a limited case of 1,3-dipolar cycloaddition where one of the bonds is formed completely before the other one starts,2 such a limited case being observed for the Diels-Alder reaction.265... 

Figure 5.1.4 shows examples of what kind of structural information can be obtained from such an LC-NMR-MS run. This figure shows the NMR chromatogram of a total asterosaponin fraction obtained from MSPD extraction. For clarity, the regions of the methyl and olefinic/anomeric resonances are shown. 

As indicated in Scheme VII/32, cyclononanone (VII/165) is transformed into hydroperoxide hemiacetal, VII/167, which is isolated as a mixture of stereoisomers. The addition of Fe(II)S04 to a solution of VII/167 in methanol saturated with Cu(OAc)2 gave ( )-recifeiolide (VII/171) in quantitative yield. No isomeric olefins were detected. In the first step of the proposed mechanism, an electron from Fe2+ is transferred to the peroxide to form the oxy radical VII/168. The central C,C-bond is weakened by antiperiplanar overlap with the lone pair on the ether oxygen. Cleavage of this bond leads to the secondary carbon radical VII/169, which yields, by an oxidative coupling with Cu(OAc)2, the alkyl copper intermediate VII/170. If we assume that the alkyl copper intermediate, VII/170, exists (a) as a (Z)-ester, stabilized by n (ether O) —> <7 (C=0) overlap (anomeric effect), and (b) is internally coordinated by the ester to form a pseudo-six-membered ring, then only one of the four -hydrogens is available for a syn-//-elimination. [111]. This reaction principle has been used in other macrolide syntheses, too [112] [113]. 

An early example of the reaction of protected furanoses with olefins is shown in Scheme 7.12. In this study, Cupps et al. [69] utilized tin tetrachloride to effect the formation of a 75% yield of the illustrated C-glycoside. This reaction showed little anomeric selectivity. Additionally, the chloride was identified as a byproduct arising from the intermediate carbocation. 

Two examples of the applicability of anomeric carbenes were reported by VaseUa et al. [167,168]. The first demonstrated the utility of olefinic substrates such as Al-phenylmaleimide, acrylonitrile and dimethylmaleate for the formation of glycosidic cyclopropanes. The carbene precursor in this example was the glucose-derived diazirine. As shown in Scheme 7.55, the use of dimethylmaleate produced a mixture of diastereomers with a combined yield of 72%. Although not presented as a schematic, the second and more dramatic example was used to functionalize fullerenes. 

The reaction takes place with cleavage of the anomeric C-0 bond by electrophilic addition to the olefin followed by intramolecular displacement by the ring oxygen and eventual expulsion of the pentenyl chain, in the form of a halomethyltetrahydrofuran, to form an oxonium species (O Scheme 74). Trapping with water then leads to the reducing sugar. This transformation has also been extended to the use of n-pentenoyl esters [406,407]. 

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