Methoxymethyl ethers cleaved

The methoxymethyl ether protecting groups of 33 were then cleaved using triphenylphosphine and carbon tetrabromide. The resulting hydroquinone function was oxidized by palladium on carbon under an atmosphere of air to afford the quinone 52 (70 %). A two-step procedure was implemented to install the diazo function. First, the ketone function of 52 was condensed with N,N -bis( tert-butyldimethylsilyl)hydrazine in the presence of scandium triflate, which formed the Af-tert-butyldimethylsilyl hydrazone 53. The hydrazone (53) was then oxidized using difluoroiodobenzene to afford kinamycin C (3) in 35 % yield. 

Tetrahydropyranyl (THP) ethers, another species known to be unstable to acid, have similarly been reported to be cleaved by solutions of iodine in methanol.209 At room temperature, cleavage of the THP ethers was complete in 1.5 to 8 h. As with the previous example using iodine in methanol at lower than reflux temperature, TBDMS ethers were stable to these conditions. The ability to tune the reactivity of the iodine in methanol system by simply controlling the temperature is of value in selective deprotection. This is even more useful when fluorine, known to remove only silyl ethers,105 is exploited. Given that methoxymethyl ethers, essentially acetals, are known to be cleaved under acidic conditions, it seems likely they too should be subject to removal by solutions of iodine in methanol. Sundry examples of deprotections using iodine in methanol are presented in Table IV. 

Lithium naphthalenide (prepared from lithium and 1.33 equivalents of naphthalene) also reductively cleaves benzyl ethers [Scheme 4.143],262 Some functionalities survive the reaction conditions like carbon-carbon double bonds, benzene rings, THP ethers, stlyl ethers and methoxymethyl ethers. A ketone group can be present but its prior conversion to an enolate is necessary. A similar transformation, but with a catalytic amount of naphthalene, has been reported.263 Although allyl ethers are also cleaved by the procedure, the selective deprotec-... 

The boron reagent 1 is recommended also for cleavage of mono MEM or methoxymethyl ethers of 1,2- or 1,3-diols, which are cleaved by acids mainly to cyclic diol foimals. 

Cleavage of MOM ethers. Methoxymethyl ethers are cleaved by this reagent at 0° in 80-97% yield. Acetals and THP, trityl, and r-butyldimethylsilyl ethers also are cleaved, but less readily.-... 

Cleavage of methoxyethoxymethyl ethers. The reagent selectively cleaves MEM ethers in the presence of benzyl, silyl, allyl, and methyl ethers as well as aeetals, acetates, and benzoates. However, methoxymethyl ethers and aeetonides are cleaved at about the same rate as MEM ethers. 

Cleavage of THP-ethers. The boron trifluoride etherate-ethanethiol mixture cleaves THP ethers at low temperatures without affecting the methylenedioxy group, acetonides, mesitaldehyde acetals, methoxymethyl ethers, and r-butyldimethylsilyl ethers. 

Transetherification and transesterification. Tin(II) bromide is a catalyst for converting p-methoxybenzyl ethers into methoxymethyl ethers by CH2(OMe)2 and MeOCH2Br. Benzyl ethers and trimethylsilyl ethers are cleaved and acetylated in one operation by the action of SnBr2-AcBr in CH2CI2 at room temperature. 

P2I4, CH2CI2, 0° - It, 30 min, 62-86% yield. These conditions also cleave methoxymethyl and methoxyethoxymethyl ethers. 

Bromocatecholborane. Ethyl esters are not affected by this reagent, but it does cleave other groups see the section on methoxymethyl (MOM) ethers. 

Chloromethyl ethers.1 BC1, cleaves methoxymethyl (MOM) ethers to give chloromethyl ethers. 

When the ylid derived from the oxide 7 was treated with benzaldehyde, a 2-methoxy-l,3-diene was formed. This led us to attempt a synthesis of (i) ar-turmerone by a sequence (Scheme 3) in which 7 effectively acts as an acyl anion equivalent. The required aldehyde 8 was synthesized in good yield using initially diphenyl(methoxymethyl)phosphine oxide (3), and then chlorotri-methylsilane-sodium iodide (12) to cleave the enol ether. 

Acetal esters such as methoxymethyl (Mom),P9 (2-methoxyethoxy)methyl (Mem),t l benzyloxymethyl (Bom),P l tetrahydropyranyl,P l 2-(trimethylsilyl)ethoxymethyl (SEM)t l and (methylsulfanyl)methyl (hltm) are sensitive to noild aqueous acids in THE as well as to mild Lewis acids such as magnesium bromide in diethyl ether (2 equiv, rt, l-24h).t l In addition. Bom esters may be cleaved by catalytic hydrogenation.P l In the absence of acid, acetal esters display reactivities typical for unhindered alkyl esters. Surprisingly, these esters... 

Petrini and co-workers used the bis(methoxymethyl)-protected nitrone 150, also derived frern L-tartrate, as an electrophile rather tfaiui as a 1,3-dipole (Scheme 21, top line) (89). In their key step, reaction with 4-ben loxybutylmagnesium bromide gave the cyclic hydroxylamine 151 in 82% yield (de 90%). Transfer hydrogenation with ammonium formate and a palladium catalyst cleaved both the hydroxylamine and the benzyl ether, affording the aminoalcohol 152. Cyclization via the corresponding primary chloride created the protected indolizidine 153, acidic hydrolysis of t ch completed this short synthesis of ( + )-132 in 16%... 

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