Alcohols allylic carbonates, protection using

Alcohols are protected as allyl ethers, which are difficult to cleave with the Pd catalyst and deprotected by other methods [149]. Alcohols are conveniently converted to allyl carbonates 334 by treatment with allyl chloroformate (333). The allyl carbonates are deprotected using HCO2H [150], and HSnBu3 [151]. This method is called the AOC (allyloxycarbonyl) method. Phenols are protected as allyl phenyl ethers, which can be cleaved with HSnBu3 [152]. 

Diallyl dicarbonate was used for the allyloxycarbonyl protection of amino compounds including amino acids, amino sugars and nucleosides. Except for the reaction with amino acids, the reagent does not require an additional base, and the only by-products, allyl alcohol and carbon dioxide are both volatile. For example, N-allyloxycarbonyl glucosamine was obtained analytically pure by simple evaporation of the reaction mixture. 

An allyl carbonate linker 2k has been used [122] to synthesize pseu-doargipinine III on a 4-methylbenzhydrylamine base resin (to which alanine had been attached as a spacer and internal standard). The nitrophenylallyl carbonate was prepared from the allyl alcohol and used to anchor mono-protected diamines. During further synthetic steps the anchoring group was shown to be stable toward concentrated TEA solutions and piperidine but could be efficiently cleaved by Pd-catalyzed allyl transfer. 

However, we should mention that use of allyl carbonates to protect alcohols, including deprotection under Pd(0) catalysis, was reported in 1981 by Guibd and Saint M Leuxt (Scheme 5). The same strategy to protect and deprotect amines was reported by Jeffrey and McCombie in 1982 and patented by McCombie in 1980 with an application to the... 

Deprotection of allyl carbonates has been accomplished using a wide variety of nucleophiles (Scheme 9), including formate, sodium borohydride, tri-n-butyl hydride, dimedone, diethylamine, and sodium azide. A recent, noteworthy development is the use of sulfinic acids as nucleophiles in the deprotection of allyl carbonates. Not only were allyl carbonates quantitatively deprotected in 30 min, but the authors also reported the successful deprotection of an allyl ether using catalytic palladium(O). Should this result prove to be general, it would represent in a major development in allyl-based protection of alcohols. 

Monoprotection of Alcohols. 2-Methoxypropene is used as a protective group for aliphatic, " allylic, and propargylic alcohols, masking them as their mixed acetals (eq 1). Deprotection can be accomplished by stirring in MeOH over ion exchange resin, by reaction in methanol with catalytic Acetyf Chloride, by Potassium Carbonate in methanol, or by 20% Acetic Acid. 

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. 

A-Protected amines were assembled on solid-phase via sulfonamide-based handle 58 (Scheme 27) [67]. Tertiary sulfonamides were generated upon reaction with allylic, benzylic and primary alcohols under Mitsu-nobu conditions. Secondary amines were released from the support using mild nucleophilic conditions such as treatment with thiophenol and potassium carbonate. 

In general, the breaking of the carbon—oxygen bond appears difficult to achieve and needs activation by strongly polar groups. For these reasons, alcohol deprotection is carried out cathodically under the form of benzylic[91] and allylic [92] ethers as well as tosylates [91]. Thus, Torii [93] used the 4-nitrobenzyl group to protect alcohols. The deprotection was carried out in two steps (1) reduction of the nitrogroup of the amine and (2) oxidation of the amine at a platinum electrode (yield up to 93%). 

Hydrocarboxylation can also be used in the synthesis of heterocycles if the heterofunction is introduced at an appropriate position within the substrate. Thus, preparation of 2,4-disub-stituted 2-buten-4-olides can be achieved via stoichiometric transformation of a protected optically active propargyl alcohol without loss of optical activity30. Sequential treatment with zirconocene chloride hydride (Schwartz s reagent), carbon monoxide, and iodine gives a 55% yield of (S)-2-ethyl-4-isobutyl-2-buten-4-olide30. This reaction resembles the intramolecular hydrocarboxylation of allylic alcohols. 

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