Common hydroxy protecting groups

Hydroxyl group must be protected during oxidation, acylation, halogenation, dehydration 

Tetrahydropyranyl (THP) ethers 1.79 can be prepared from 3,4-dihydro-2H-pyran (DHP) 1.78 by an acid-catalyzed reaction.  

However, formation of THP ether introduces a new stereogenic centre. The introduction of the THP ether onto a chiral molecule therefore results in the formation of diastereoisomers. 

Phenols are protected as their methyl ethers ° tert-butyl ethers, allyl ethers and benzyl ethers. 

Miura and co-workers reported the protection of phenols by allyl alcohols in the presence of catalytic amounts of palladium(II) acetate and titanium(IV) isopropoxide. The reaction is remarkably general however, it fails in the case of 3,5-dimethoxyphenol because of the exclusive formation of a C-allylated product. 

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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... 

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

Table 1 lists the commonly used hydroxy protecting groups, together with their stability, selectivity to amino and carboxy protecting groups, and cleavage conditions. 

Table 1 Cleavage Conditions and Stability of Commonly Used Hydroxy Protecting Groups ...

Keto aldehyde 191 (Scheme 2.87) was prepared as a common precursor in the synthesis of a series of isoprenoid pheromones. One of the synthetic options required the selective reduction of a ketone carbonyl in this compound. Under mild conditions of acetalization (weak acid, methanol), only the aldehydic function of 191 was affected to form a mono-protected derivative, 192. Reduction of the keto group in the derivative with sodium borohydride and subsequent removal of the acetal protecting group gave the desired hydroxy aldehyde 193. ... 

A commonly used, protected carbohydrate containing a secondary hydroxy group is diiso-propylideneglucofuranose 23. Oxidation to the corresponding ketone 24 illustrates some of the most widely applied methods for oxidation of secondary alcohols (O Table 4). Again, the reactions can be divided into three main categories oxidations mediated by activated DMSO, oxidations with chromium(VI) oxides, and oxidations catalyzed by mthenium oxides. For oxidations with activated DMSO the Swern procedure is the most widely used [27]. 

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