Hydroxy protection

The MOM group was also used for hydroxy protection to give fragment E as shown below ... 

The benzyl group can serve as a hydroxy-protecting group if acidic conditions for ether cleavage cannot be tolerated. The benzyl C—O bond is cleaved by catalytic hydrogenolysis,176 or by electron-transfer reduction using sodium in liquid ammonia or... 

These reaction conditions do not affect most of the other common hydroxy-protecting groups and the methoxybenzyl group is therefore useful in synthetic sequences that require selective deprotection of different hydroxy groups. 4-Methoxybenzyl ethers can also be selectively cleaved by dimethylboron bromide.182... 

Methoxyphenyl (PMP) ethers find occasional use as hydroxy protecting groups. Unlike benzylic groups, they cannot be made directly from the alcohol. Instead, the phenoxy group must be introduced by a nucleophilic substitution.185 Mitsunobu conditions are frequently used.186 The PMP group can be cleaved by oxidation with CAN. 

Allyl carbonate esters are also useful hydroxy-protecting groups and are introduced using allyl chloroformate. A number of Pd-based catalysts for allylic deprotection have been developed.209 They are based on a catalytic cycle in which Pd° reacts by oxidative addition and activates the allylic bond to nucleophilic substitution. Various nucleophiles are effective, including dimedone,210 pentane-2,4-dione,211 and amines.212... 

Table 3.1 gives the structure and common abbreviation of some of the most frequently used hydroxy-protecting groups. 

Figure 6.46 (A) Hydroxy-protected thiourea 141 and 142 lacking the hydroxy function and their catalytic efficiency in the Friedel-Crafts alkylation of indole with frans-P-nitrostyrene (139 78% yield 85% ee under identical conditions). (B) Proposal for the key hydrogen-bonding interactions between 139 and the model substrates.

Fluoro-6-iodopyridazine (6) required for palladium-catalyzed Heck coupling with hydroxy-protected prop-2-yn-l-ol has been prepared by simply heating the 3,6-diiodopyridazine (5) with potassium fluoride in dimethyl sulfoxide at 140°C for 18 hours.121... 

INFLUENCE OF HYDROXY PROTECTING GROUPS ON SIALYL DONOR REACTIVITY 147... 

Deoxysugars, nucleosides and related molecules have also been prepared from hydroxy-protected, halogenated precursors using tributyltin hydride 55,58,. Thus in one example, the key step of the preparation of a 3 -deoxy derivative of S-formycinyl-L-homocysteine was the reduction of a halide precursor (equation 7)62. 

The classical method for tert-butyl esterification involves nnineral add catalyzed addition of the amino acid to isobutene. Both, N-protected, e.g. Z-Xaa-OH, and unprotected amino acids form tert-butyl esters with isobutene in the presence of catalytic amounts of sulfuric acid or TosOH. " Another efficient method is the transesterification of an acetic acid tert-bvXy ester catalyzed by perchloric acid. " Amide tert-butylation was recognized as a side reaction in the presence of perchloric acid, but could be completely suppressed by using sulfuric acid. Both methods, acid-catalyzed addition to isobutene and the transesterification of acetic add terf-butyl esters, result in simultaneous terf-butyl-ation of hydroxy and sulfanyl groups. A synthetic route to Z-Thr-OtBu, Z-Ser-OtBu, and Z-Hyp-OtBu without conconnitant O-alkylation involves the hydroxy protection by the acetoacetyl group, which is readily cleaved by treatment with 2 equivalents of hydrazine in EtOH for 30 minutes. ... 

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