Protective groups trityl ether

Acetates. Because of the significant interest in selective acetylation reactions of sucrose, the need for a convenient and unambiguous method of identification has been recognized (34,35). The position of an acetyl group in a partially acetylated sucrose derivative can be ascertained by comparison of its H-nmr acetyl methyl proton resonances after per-deuterioacetylation with those of the assigned octaacetate spectmm. The synthesis of partially acetylated sucroses has generally been achieved either by way of selectively protected derivatives such as trityl ethers and cychc acetals or by direct selective acetylation and deacetylation reactions. 

Ethers are among the most used protective groups in organic synthesis. They vary from the simplest, most robust, methyl ether to the more elaborate, substituted, trityl ethers developed for use in nucleotide synthesis. They are formed and removed under a wide variety of conditions. Some of the ethers that have been used to protect alcohols are included in Reactivity Chart 1. ... 

As previously discussed, ethyl chlorocarbonate reacts rapidly and selectively with an equatorial 3-hydroxyl group to give the corresponding cathylate. Trityl ethers, usually employed as a selective protecting group for primary hydroxyls, can be prepared from A -3j3-ols by heating with triphenylmethyl chloride in pyridine, and from 5a-3 -alcohols by more prolonged heat-... 

Clean removal of trityl ether groups from O-isopropylidenated furanoses by using NaHS04-Si02 was previously described by Das et al.116 and afforded the corresponding alcohols at room temperature. The chemoselective deprotection was accomplished in yields above 91% within 2-2.5 h, leaving other protective-groups intact (Scheme 27). 

According to the procedure described in the preceding section, D-xylose (77) was converted in four steps and 30% overall yield to the trityl ether 76. Aldol addition to piperonal (60) and protection of the hydroxy groups as... 

Boc,245 /-butyl ester,246 trityl ether247 and even tris(p-methoxyphenyl)methyl ether.248 The oxidation-sensitive PMB normally resists the action of PCC,249 as well as the sulfur-containing protecting groups dithioacetals250 and mono-thioacetals.251... 

The employment of trityl trifluoroborate is particularly interesting. This reagent is able to introduce trityl groups on both primary and secondary alcohols54 and to selectively oxidize secondary trityl ethers to ketones in the presence of primary trityl ethers.55 Thus, treatment of diols with trityl trifluoroborate leads to tritylation of both alcohols followed by oxidation of the secondary trityl ether, resulting in the formation of a ketone possessing a trityl-protected primary alcohol. A work-up by mild acidic hydrolysis provides the deprotection of the primary trityl ether and formation of a hydroxy ketone.54... 

R = Bz), [a]D -8.2° (chloroform), into ), [oi]d +22° (chloroform). The yield was quantitative and the single product was easily isolated just by conventional extractive processing. Therefore, if we can protect the hydroxyl group at C-6 with another protecting group, such as trityl ether, it will be possible to examine the coupling with KDO derivatives on both C-3 and 6 positions. 

When benzyl 6-0-trityl-a-D-mannopyranoside was allowed to react with allyl bromide under phase-transfer catalysis, the 3-allyl and 2-allyl ethers and the starting material were obtained in yields of 36%, 23%, and 24%, respectively [152]. The absence of the protective group at C-4 might be responsible for this difference in selectivity. 

The triphenylmethyl (trityl) group remains the most frequently used for the temporary protection of primary hydroxyl groups (for review see Refs. [2, 3, 254]) which can be conveniently regenerated from the corresponding ethers by mild acid treatment or by hydrogenolysis. Moreover, good crystallizing properties of trityl ethers, easy... 

The electron-donating effect of pyridine substituents becomes especially evident with 4-dimethylaminopyridine. Methyl a-D-glucopyranoside reacts with 1.1 molar equiv. of trityl chloride, 1.5 molar equiv. of triethylamine, and a catalytic amount of 4-dimethylaminopyridine in DMF to give 88% of the 6-trityl ether after only several hours at room temperature [326], With the classical method, the yield was 61 % and some difficulties with the formation of addition complexes of the product with pyridine were encountered [326]. Surprisingly, pyridine still remains frequently used as a solvent in combination with this effective catalyst [327, 328]. Of the two secondary hydroxyl groups of methyl 4,6-O-benzyl-a-D-mannopyranoside [119] or methyl 4-0-benzyl-a-L-rhamnopyranoside [122], the equatorial OH-3 was protected first. Tritylation of methyl 2-0-benzyl-a-L-fucopyranoside (46) proceeded readily to give (47) in 70% yield [109]. 

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