Allyl carbonates alcohol protection

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... 

Type III (no homodimerization) Acrylonitrile," protected 3° allylamines" Vinyl trialkoxysilanes, vinyl siloxanes 1,1-Disubstituted olefins, " non-bulky trisubstituted olefms, vinyl phosphonates, " vinyl phosphine oxides,phenyl vinyl sulfone, acrylonitrile, 4° allylic carbons (all alkyl substituents), protected 3° allylic alcohols, 7,Aolefm of 2-subst. 1.3- butadienes, 7,Aolefin of electronically deactivated 1.3- butadienes ... 

Type IV (spectators to CM) 1,1-Disubstituted olefms " 1,1-Disubstituted olefms, disub. o ,/ -unsaturated carbonyls, 4° allylic carbon-containing olefins, perfluorinated alkane olefins, 3° allylamines (protected)" Vinyl nitro olefins, protected trisubstituted allyl alcohols, a,/ -olefin of 2-subst. 1.3- butadienes, a,/ -olefm of electronically deactivated 1.3- butadienes ... 

Allyl carbonates can function as protecting groups for alcohols, with removal of the protecting group being effected by R3SnH and Pd° (equation 101).299... 

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]. 

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. 

Alkyl allyl ethers are difficult to cleave. Therefore alcohols are protected as allyl carbonate. Deprotection of tlie cyclic carbonate 471 with HC02H/Et3N proceeded regioselectively at room temperature to afford the <5-hydroxy ester 472. The electronic effect is crucial in this case and hydride attacked an electron deficient carbon [176]. 

Protection of alcohols and amines as allylic carbonates and carbamates is at the origin of the topic of this section (see Sect. A and Scheme The mixed allylic carbonate is... 

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. 

A diorganonickel complex generally undergoes reductive elimination more rapidly than the corresponding palladium complex. Nickel can mediate retro-allylation, which is involved in allylation of allylic carbonate with homoallylic alcohols for efficient synthesis of 1,5-hexadienes [25]. A combination of Ni(cod)2 (cod = 1,5-cyclooctadiene) and triethyl phosphite catalyzes the allylation reaction of Boc-protected cinnamyl alcohol (Boc = f-butoxycarbonyl) with homoallylic alcohols (Scheme 5.34). The reactions with alkyl-substituted homoallylic alcohols (R = alkyl) are not regiospecific but are sterically controlled. The highest linear... 

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. 

Finally, the necessity arose for the synthesis of pentulose 21, labeled with, 3C on the central carbons, C-2 and C-3, for an independent biosynthetic study, which is reported in Section III.5.27 The doubly labeled ester 34 (Scheme 14) is readily available by a Wittig- Homer condensation of benzyloxyacetaldehyde with commercially available triethylphosphono-(l,2-l3C2)acetate. Chirality was introduced by the reduction of ester 34 to the allylic alcohol, which produced the chiral epoxide 35 by the Sharpless epoxidation procedure. This was converted into the tetrose 36, and thence, into the protected pentulose 37 by the usual sequence of Grignard reaction and oxidation. 

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. 

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