O-protective groups

In the case of an oxygen substituent at C-3 varying selectivities are observed, depending on the O-protective group, the Lewis acid, and the nucleophile. 

Selective activation of anomeric groups (e.g., X, Y exhibit different reactivities as in orthogonal groups Y can be exchanged to X Y can be activated after O-protective group manipulation or change of promoter)... 

One cyclization procedure that depends on this approach has been described in an earlier section. A further method that affords double activation of a methylene group at C-6 of aldose derivatives, and simultaneous release of an aldehydic group, leads to C-6- to C-l-bonding under extremely mild conditions and is compatible with the presence of most O-protecting groups (Scheme 11). For this reason, and because the cyclization step is normally very efficient and stereoselective, this strategy has been used extensively, ft... 

V-Benzylamines derived from di-O-protonated glyceraldehydes react with phenyl-magnesium bromide to give protected aminodiols with total diastereoselectivity the nature of the O-protecting group determines the direction of the selectivity.40... 

Sendzik, M. Guamieri, W. Hoppe, D. Monocarbamates derived from (S)-2-(dibenzylami-no) butane-1,4-diol and the influence of the second O-protecting group on the regioselec-tivity of deprotonation. Application to the synthesis of the Boletus toxin (2S,4S)-y-hydroxy-norvaline. Synthesis 1998, 1287-1297. 

Small ring r/wt.v-epoxides are often available by peracid epoxidation of a derivative with a sterically demanding O-protecting group (see Houben-Weyl, Vol. E13/2, pp 1263, 1264 and reference 49). 

As has been demonstrated, 2-(trimethylsilyl)ethoxymethyl (SEM) esters are selectively removed from amino acids and peptide derivatives in the presence of the most common N- and O-protecting groups applied in peptide chemistry including the urethane-type Boc, Z, Fmoc, and Troc as well as Bzl, tBu, and TBDMS ethers.The SEM ester is removed by acidolysis or with a fluoride ion source, e.g. TBAF in THF or HMPA or with aqueous HF in MeCN (—10°C).f l Deprotection with magnesium bromide in EtjO represents an even milder alternative that allows increased selectivity toward fluoride-labile silyl ethers or Fmoc groups. The SEM esters are prepared in 60-80% yield by stirring a mixture of 0.25 M N-protected amino acids in DMF with 0.8 equivalents of SEM-Cl and 1.1 equivalents of lithium carbonate at room temperature for 16 hours. 

The foregoing ring-closure reaction requires that O-protecting groups be stable to basic conditions however, the sulfonate ester group survives both the cyclization and the subsequent further treatment with base. 

The reactivity of the glycosyl halides increases in reverse order compared with stability protective groups at different positions exert remote electronic effects . The influence of these groups does not depend only on the distance to the anomeric carbon atom thus electron-withdrawing O-protective groups at C-4 lower the C-1 reactivity quite strongly due to through-bond interactions. 

E. Eichler, F. Yan, J. Sealy, and D. M. Whitfield, 1-Methyl l -cyclopropylmethyl - An acid labile O-protecting group for polymer-supported oligosaccharide synthesis, Tetrahedron, 57 (2001) 6679-6693. 

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