Allylic alcohol groups protection

Alkyl-4-oxo-l,3-dioxolanes, to protect a-hydroxy carboxy groups, 267 5-Alkyl-4-oxo-l,3-dioxolanes, to protect a-hydroxy carboxy groups, 267 4-Alkyl-5-oxo-l,3-oxazolidines, to protect carboxy groups, 266-267 Allyl alcohols, to protect, 18 iV-Allylamides, to protect amides, 154, 397 /V-Allylamines, to protect amines, 362 Allyl carbamates, to protect amines, 248, 252, 331-332... 

Application Matsutake alcohol (a fragrance compound in a Japanese mushroom) is synthesized by reduction of one terminal double bond by LiAlH4—Cp2TiCl2 after protection of an allylic alcohol group as a diisobutylaluminium alkoxide ... 

The Hsung group (Wisconsin) reported an enantioselective synthesis of Kishi-like intermediate 186 that began with the preparation of 272. Formation of the enamine with cyclohexan-l,3-dione, followed by oxidation of the allylic alcohol and protection of the remaining primary alcohol, gave 273 (Gephyrotoxin-10). 

The stereoselective branching of L-rhamnal at C-1 and C-2 via a silicon tethered radical cyclization is illustrated in Scheme 7, and allyl alcohol groups which are protected as their (bromomethyl)dimethylsilyl ethers undergo a radical-induced cyclization to give branched-chain sugars as shown in Scheme ZP... 

In all cases examined the ( )-isomers of the allylic alcohols reacted satisfactorily in the asymmetric epoxidation step, whereas the epoxidations of the (Z)-isomers were intolerably slow or nonstereoselective. The eryfhro-isomers obtained from the ( )-allylic alcohols may, however, be epimerized in 95% yield to the more stable tlireo-isomers by treatment of the acetonides with potassium carbonate (6a). The competitive -elimination is suppressed by the acetonide protecting group because it maintains orthogonality between the enolate 7i-system and the 8-alkoxy group (cf the Baldwin rules, p. 316). 

Allylamines are difficult to cleave with Pd catalysts. Therefore, amines are protected as carbamates, but not as allylamines. Also, allyl ethers used for the protection of alcohols cannot be cleaved smoothly, hence alcohols are protected as carbonates. In other words, amines and alcohols are protected by an allyloxycarbonyl (AOC or Alloc) group. 

Hydroxyl groups are stable to peracids, but oxidation of an allylic alcohol during an attempted epoxidation reaction has been reported." The di-hydroxyacetone side chain is usually protected during the peracid reaction, either by acetylation or by formation of a bismethylenedioxy derivative. To obtain high yields of epoxides it is essential to avoid high reaction temperatures and a strongly acidic medium. The products of epoxidation of enol acetates are especially sensitive to heat or acid and can easily rearrange to keto acetates. 

Butyl ethers can be prepared from a variety of alcohols, including allylic alcohols. The ethers are stable to most reagents except strong acids. The /-butyl ether is probably one of the more underused alcohol protective groups, considering its stability, ease and efficiency of introduction, and ease of cleavage. 

BzCl or BZ2O, Pyr, 0°. Benzoyl chloride is the most common reagent for the introduction of the benzoate group. Reaction conditions vary, depending on the nature of the alcohol to be protected. Cosolvents such as CH2CI2 are often used with pyridine. Benzoylation in a polyhydroxylated system is much more selective than acetylation. A primary alcohol is selectively protected over a secondary allylic alcohol, and an equatorial alcohol can... 

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