Protection Cerium ammonium nitrate

Yamashiro et al. 1972), boron trifluoride etherate in acetic acid (Schnabel et al. 1971), trimethylsylil triflate (Schmidt et al. 1987), trimethylsilyl perchlorate (Vorbrueggen Krolikiewicz 1975), and, most frequently, trifluoroacetic acid (Farowicki Kocienski 1995 and references therein). Deprotection of the /-HOC group under neutral conditions was not described until recently, yet it is highly desirable. Now it has been found that the tert-butoxycarbonyl protecting group for amines, alcohols, or thiols is removed efficiently (90-99% yield) with use of 0.2 equivalent of cerium ammonium nitrate in acetonitrile at 80°C (Hwu et al. 1996) ... 

PMB ethers can be cleaved oxidatively with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)11 in dichloromethane/water tor with cerium ammonium nitrate (CAN) in acetonitrile/water.12 Many other protecting groups such as esters, isopropylidene acetals, benzyl ethers, allyl ethers and f-butyldiphenyl silyl (TBDMS) ethers are stable to these conditions (Scheme 2.4). The cleavage reaction, with DDQ is initiated with a single-... 

At the exotic end of the Lewis acid scale is tetrafluorosilane (mp -90 5C, bp -86 UC) first proposed by Corey and Yi as a mild and selective reagent for the cleavage of silyl-protected alcohols with the reactivity order being EtiSi > f-Bu-Me2Si f-BuPhiSi/ 1 The substrate in dichloromethane or acetonitrile, is stirred at room temperature under an atmosphere of excess tetrafluorosilane provided by a gas-filled balloon. The reaction is slow in dichloromethane but quite fast (ca. 15 min) in acetonitrile. In the final step of Yamamoto s synthesis of the Hemibrevetoxin B [Scheme 4.40]61 the secondary TIPS and TBS ethers were removed from 40.1 with tetrafluorosilane. Identical conditions were used by Nicolaou et al to remove two TBS ethers in the final step of their synthesis of Hemibrevetoxin B.62 In the example shown in Scheme 4,41, deprotection with fluoride (basic) or cerium(lV) ammonium nitrate (CAN) in methanol (neutral) isomerised the angelate to the more thermodynamically stable tiglate.63 However, with tetrafluorosilane, no isomerisation occurred during the deprotection step. 

Cerium(IV) ammonium nitrate in aqueous acetonitrile will also cleave a p-methoxyp/zeny/ ether (presumably to benzoquinone).335 The reaction was used in a synthesis of Stigmastellin A [Scheme 4.183] to release a primary alcohol in the presence of a benzyl ether.339 In an extension of the method, a bidirectional strategy for the synthesis of 2,3 5-trisubstitute d tetrahydrofuran components of annonaceous acetogenins benefited from the simultaneous protection of two primary hydroxyls as their Cj-symmetric hydroquinone ether [Scheme 4.184].340... 

The oxidative cleavage of Al-Boc groups with ammonium cerium(IV) nitrate (CAN) in acetonitrile has been reported,0 1 but the fact that, again, acidic or Lewis acidic rather than oxidative reaction conditions noight affect the reaction (CAN is, at least in aqueous solution, strongly acidic) should be kept in nnind. Boc-protected amino acids have been reported to be stable against other oxidative reagents. 

C-Lithiation of 1-substituted tetrazoles occurs readily and the resulting lithio derivatives can be trapped with electrophiles, but there is a strong tendency for decomposition of the lithio compounds, which depends on the 1-substituent. 5-Lithiated 2-substituted tetrazoles appear to be significantly more stable than the 1-isomers, as shown below, by a comparison of the two A -benzyloxymethyl (BOM)-protected isomers. p-Methoxybenzyl can be removed finally by hydrogenation or oxidation with cerium(lV) ammonium nitrate. 

Among the various cerium(IV) complexes, cerium(IV) ammonium nitrate (CAN) is the most important oxidant in organic synthesis, since it is sufficiently stable in different solvents and is commercially available. Besides its propensity of introducing and removing protecting groups via single-electron transfer or Lewis acid catalysis... 

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