2. Reactions at Non-Anomeric Hydroxyl Groups
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1 2. Reactions at Non-Anomeric Hydroxyl Groups 2.1 Ether-Type Protecting Groups u Alkyl and aryl ethers are relatively stable to acids and bases due to the high C bond energy (358 KJ/mol) u most useful ether-type protections utilize resonance stabilization (by delocalization) of the benzylic-type cation or radical to facilitate the cleavage u The most common ethers used as protecting groups are: Methyl Ethers Benzyl (Bn) Ethers Substituted Benzyl Ethers Allyl and Related Ethers Trityl (Tr) Ethers 2-Naphthylmethyl (NAP) Ethers Propargyl Ethers o-xylylene Ethers Bn Bn Bn PMB PMB PMB R Bn R PMB Me Me Me Trt Nap MM R Me R Bn Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
2 Methyl Ethers Alkylation Reactions: Ether-Type Protecting Groups Methyl ethers are not normally regarded as protecting groups The removal is difficult requiring conditions not compatible with other functional groups The selective removal of an ether adjacent to a hydroxyl group in carbohydrate substrates Selective removal of methoxy protecting groups Boto A, et al. (2004) rg Lett 6:3785 (Diacetoxyiodo)benzene (DIB) Benzyl (Bn) Ethers The classical permanent protecting group for carbohydrates It is very stable and can be readily removed under essentially neutral conditions Benzyl ether formation is usually achieved by the reaction of alcohols and benzyl halides Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
3 Benzyl (Bn) Ethers Benzylation of methyl α-d-glucopyranoside Selective benzylation of carbohydrate hydroxyl functions by direct one-step protection is difficult to achieve Several techniques for the selective protection have been developed Generally, one of the two C bonds in benzylidene acetals can be selectively cleaved the direction of the cleavage is dependent on steric and electronic factors as well as, on the nature of the cleavage reagent Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
4 Reductive pening of Benzylidene Acetals Ph R R Me Bn H R + R Me H Bn R R Me Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
5 Selective pening of Benzylidene Acetals General mechanism Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
6 Selective pening of Benzylidene Acetals Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
7 Regioselective benzylation by opening of benzylidene acetals Benzylidene acetals can also be opened under oxidative conditions, typically using NBS in CCl4 Substrate can be treated with the tin reagent forming one or two Sn bonds, enhancing the nucleophilicity of the oxygen atoms in the stannyl ether or stannylene acetal. The effect is different for the two oxygen atoms forming an Sn-acetal and can be used for higher regioselectivity. Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
8 Examples of stannyl-mediated regioselective benzylation Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
9 Examples of regioselective de--benzylation Selective de--benzylation of easily available polybenzylated precursors. This has been achieved in limited cases by: catalytic hydrogenolysis Catalytic hydrogen-transfer cleavage Acetolysis hypoiodite fragmentation iodine-mediated addition-elimination sequences Lewis acids By isobutylalanes or the combination CrCl2/LiI Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
10 Substituted Benzyl Ethers Substituted benzyl ethers can be selectively removed in the presence of unsubstituted benzyl ethers have been developed. Me Bn Bn R Bn v p-methoxy benzyl (PMB) is an important in carbohydrate chemistry due to its easy introduction and removal. v PMB group removal can be mediated by oxidizing agents or by Lewis acids By oxidation with DDQ DDQ-FeCl 3 DDQ-Mn(Ac) 3 CAN (Cerium Ammonium Nitrate) PMB ethers can be cleaved with ZrCl 4, SnCl 2 /TMS-Cl/anisole, CF 3 C 2 H in CH 2 Cl 2, CeCl 3.7H 2 /NaI, or I 2 /MeH Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
11 Removal of p-methoxybenzyl Ethers with DDQ Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
12 Removal of PMB with Acids Examples of deprotection of p-methoxybenzyl ethers with a Lewis acid (SnCl 4 ) Two-step deprotection of p-acetoxybenzyl protecting group Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
13 Examples of p-substituted benzyl-type protecting groups Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
14 Examples of p-substituted benzyl-type protecting groups Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
15 Allyl and Related Ethers Allyl-ethers The protection with allyl and related (prenyl, methylallyl, cinnamyl, homoallyl) groups is of great importance due to their stability under the conditions required for glycoside formation. They are moderately stable to acids and bases, and offer the potential for selective dealkylation Example of regioselective allylation from copper complexes Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
16 Removal of Allyl Ethers Common allyl deprotection methods are two-stage procedures that include isomerization to 1-propenyl group with a variety of agents The most frequently employed conditions are: treatment with t-buk,wilkinson catalyst, Pd/C, PdCl2, ruthenium(ii), and iridium(i) complexes followed by acid hydrolysis or oxidation. Selective cleavage of branched allyl ethers Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
17 Trityl-Ethers Trityl-ethers Its utility is attributed to the easy preparation and removal as well as to the high selectivity for primary positions observed in polyols Tritylation of methyl α-d-glucopyranoside Substituted trityl protecting groups Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
18 2- Naphtyl-methyl Ether (NAP) The NAP protecting group was introduced by Esko and Spencer and it has higher stability to acidic glycosylation conditions than PMB NAP can be removed by: hydrogenolysis, acids oxidation conditions (similar to PMB) Sarkar AK, Rostand KS, Jain RK, Matta KL, Esko JD (1997) J Biol Chem 272:25608 Gaunt MJ, Yu J, Spencer JB (1998) J rg Chem, 63:4172 Max Planck Institute of Colloids and Interfaces Daniel Varón Silva Carbohydrate Chemistry and Biology
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