![]() 54 In the presence of iodine, TBCO generates I + and the non-nucleophilic 1,4,6-tribromophenolate, which does not interfere. To circumvent this drawback, a modification of the NIS procedure makes use of 2,4,4,6-tetrabromo-2,5-hexadienone (TBCO). Thus, glycosyl succinimides, a frequently observed side product, 53 owing to the marked inverse anomeric effect of that group, adopt predominantly an inverted 1 C 4( D) chair conformation. The counterion, as represented by succinimide anion obtained by heterolysis of NIS, is competing with the employed nucleophiles. This approach was developed and proven valid in several cases of α- l- and α- d-oligosaccharide formation using suitable iodonium donors, for example, NIS or IColl 2ClO 4. Nucleophilic attack from the opposite side affords the α-glycosidic linkage in 2-halo glycosides. Treatment of a glycal with an electrophile E + leads to an intermediate ‘onium’ ion, the configuration of which will be determined by the ‘inverse’ anomeric effect. However, stereoelectronic differences still exist in these sets of molecules, which make the α- and β-faces distinguishable for an approaching electrophile. In the case of glycals, C3, C2, C1, and O are in one plane. Steric guidance in 2- deoxy sugars for an approaching nucleophile is limited to ‘remote’ positions (see below). Klaffke, in Comprehensive Glycoscience, 2007 1.08.2.1 Addition of Halonium Ions Neighboring group participation of the 2-substituent, Y, favors α-glycoside formation from the manno isomers and β-glycoside formation from the gluco isomers. Y +X − addition can take place from the β-site (Y +) giving α,β-D- manno (M) configuration or from the α-site (Y +) giving α,β-D- gluco (G) configuration. This is demonstrated in Scheme 25 for a D-glucal derivative. (A detailed discussion of these two sugars met in glycoconjugates is beyond the scope of this chapter for anomeric O-alkylation of KDO see Section 1.2.5.) Therefore a temporarily anchimerically assisting group was required in the 2-position, which is most readily introduced starting from glycals. With modifications these statements are also true for KDO and neuraminic acid, which possess an electron-withdrawing caboxylate group at the anomeric position. Misfortune results from isolation.With 2- deoxy sugars, which lack an electron-withdrawing 2-substituent and also lack neighboring group participation, generally low stability of the glycosyl halides and low α/β-selectivity in glycoside and saccharide synthesis has been encountered. Holding power is like bringing a large torch into a sea of candles: the more the flame is passed to others, the greater the chance that the torch will be reignited if its flame dies out. If you strive for abundance so single-mindedly that you alienate those who are close to you, you will be left holding an empty bag. Line 6 (top line)Īrrogance in the handling of power tends to bring the opposite of the desired effect. The situation is like that of a wealthy man who hires the best investment advisors, and then listens to them! The rich get richer, in wisdom and wealth alike. Accepting good advice brings blessings and supreme good fortune. Those who bring modesty to high positions attract those who are capable of offering brilliant counsel. A search for wise counsel is indicated here. In order to prevent a temporary setback from becoming a permanent condition, effort must be made to find complementary strengths that support your position. When a shadow passes over a time of prosperity or success, an unsettled feeling can remain. Though you are not to blame, a time of loss and difficulty may follow-but only for a while. In the confusion, small-minded or self-serving people may be gaining power, which makes it impossible to accomplish much at present. Line 3Ī recent success is eclipsed by a fast-moving element that could snatch the reins from your hands. Hold inwardly to what is true, and the sunshine will re-emerge soon enough. To try to do battle with negative forces at this time would only cause you to stumble. It is often best to handle periods of temporary darkness by doing nothing, letting them pass and sink of their own weight. In times of abundance, plots and intrigues are like eclipses of the sun, which only happen occasionally. ![]() This line indicates that the timing is good for a partnership that can help you advance to the top. Indications are that they’re well matched and their partnership is well founded. This combination often comes from two people who are joining together in an effort, project, or relationship. To facilitate an experience of abundance, bring a mixture of energy and vision. Changing Line Interpretations Line 1 (bottom line) ![]()
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