Protection alcohol groups

Diels-Alder cyclization of IfiAO-undecatrienals.5 These unsaturated aldehydes undergo intramolecular Diels-Alder cyclization, particularly under Lewis acid catalysis. The reaction is highly endo-selective. Silyl-protected alcohol groups at C4 and Q can be present, and t-butyldimethylsilyl ethers show a strong axial preference. 

TIPS ether) A silyl ether of formula R -O-Sid-Prh commonly used to protect alcohol groups. Formed from an alcohol with TIPSC1 and a tertiary amine. Deprotected using aqueous fluoride salts, (p. 645)... 

Hydrolysis of the protected alcohol group in 19 is realized by reaction of aqueous sodium hydroxide in iV.iV-dimethylformamide at room temperature and under argon. Chromatographic purification affords the benzylic alcohol derivative 20 in 65% yield (Scheme 9.15). 

The glycals react with water, alcohols, phenols, carboxylic acids, and certain bases, in the presence of an acidic catalyst, in the same way as does 2,3-dihydro-4/f-pyran (which gives, for example, 2-hydroxy-, 2-alkoxy-, and 2-aryloxy-tetrahydropyrans in high yield ). (Tetrahydro-pyranyl ethers are useful for protecting alcoholic groupings during reactions in basic media, and are readily hydrolyzed with acid,) The first step of these additions would be expected to be protonation at C-2, followed by attack of the nucleophilic reagent on the resonance-stabilized, C-1 carbonium ion. 

Silyl ethers are used to protect alcohol groups. 

In the piepaiation of ioveisol (12) (41), the key intermediate (23) is prepared from the diacid (20) by the action of thionyl chloride followed by 3-amino-l,2-propanediol. The alcohol groups of (23) are protected as the acetates (25), which is then N-acylated with acetoxyacetyl chloride and deprotected in aqueous methanol with sodium hydroxide to yield (26). N-alkylation of (26) produces ioversol (12). 

The /-butyldimethylsilyl group introduced by TBDMIM has a number of advantages in protecting alcohols (6). The sdylated alcohol hydroly2es more slowly than an alcohol sdylated with TMS by a factor of 10 . The sdyl ether is also stable to powerful oxidi2ing and reducing agents, but it can easily be removed by aqueous acetic acid or tetrabutylammonium fluoride in tetrahydrofuran. 

Photolytic cleavage reactions (e.g., of o-nitrobenzyl, phenacyl, nitrophenylsul-fenyl derivatives) take place in high yield on irradiation of the protected compound for a few hours at 254-350 nm. For example, the o-nitrobenzyl group, used to protect alcohols, amines, and carboxylic acids,has been removed by irradiation. Protective groups that have been removed by photolysis are described at the appropriate places in this book in addition, the reader may wish to consult five review articles. 

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. ... 

The ability to convert a protective group to another functional group directly without first performing a deprotection is a potentially valuable transformation. Silyl-protected alcohols have been converted directly to aldehydes, ketones, bro-mides, acetates, and ethers without first liberating the alcohol in a prior deprotection step. 

In an attempt to synthesize the macrolide antibiotic chlorothricolide, an unhindered —COOH group was selectively protected, in the presence of a hindered —COOH group, as a MEM ester that was then reduced to an alcohol group. ... 

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