MOM ethers

The oxidation of higher alkenes in organic solvents proceeds under almost neutral conditions, and hence many functional groups such as ester or lac-tone[26,56-59], sulfonate[60], aldehyde[61-63], acetal[60], MOM ether[64], car-bobenzoxy[65], /-allylic alcohol[66], bromide[67,68], tertiary amine[69], and phenylselenide[70] can be tolerated. Partial hydrolysis of THP ether[71] and silyl ethers under certain conditions was reported. Alcohols are oxidized with Pd(II)[72-74] but the oxidation is slower than the oxidation of terminal alkenes and gives no problem when alcohols are used as solvents[75,76]. 

Selective formation of MOM ethers has been achieved in a diol system. ... 

HgCl2, CH3CN, H2O, 25°, 1-2 h, 88-95% yield. If 2-methoxyethanol is substituted for water, the MTM ether is converted to a MEM ether. Similarly, substitution with methanol affords a MOM ether. If the MTM ether has an adjacent hydroxyl, it is possible to form the formylidene acetal as a byproduct of cleavage. 

MezBBr, CH2CI2, -78° NaHC03, H2O, 87-95% yield. This method also cleaves MTM and MOM ethers and ketals. 

Pyridinium p-toluenesulfonate, r-BuOH or 2-butanone, heat, 80-99% yield. This method also cleaves the MOM ether and has the advantage that it cleanly cleaves allylic ethers that could not be cleaved by Corey s original procedure. 

For a further discussion of this reagent, refer to the section on MOM ethers."... 

MgBr2, Et20, rt, 66-95% yield.l-Butyldimethylsilyl and MEM ethers are not affected by these conditions, but the MOM ether is slowly cleaved. The THP derivatives of benzylic and tertiaiy alcohols give bromides. 

Dichlorodicyanoquinone (DDQ), CH2CI2, H2O, 40 min, it, 84-93% yield.This method does not cleave simple benzyl ethers. This method was found effective in the presence of a boronate. The following groups are stable to these conditions ketones, epoxides, alkenes, acetonides, to-sylates, MOM ethers, THP ethers, acetates, benzyloxymethyl (BOM) ethers, and TBDMS ethers. 

MeO)2CH2, 2,6-lutidine, TMSOTf, 0°, 15 min. Similar conditions have been used to introduce MOM ethers on alcohols. 

Me2BBr, CH2CI2, — 78°, 90-97% yield.This reagent also cleaves MTM, MEM, and MOM ethers (87-95% yield). 

The section on the formation of MOM ethers should be consulted since many of the methods described there should also be applicable to the formation of MOM esters., ... 

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

CH2(OMe)2, CH2CI2, TfOH, 4 h, 25°, 65% yield. This method is suitable for the formation of primary, secondary, allylic, and propargylic MOM ethers. Tertiary alcohols fail to give a complete reaction. 1,3-Diols give methylene acetals (89% yield). 

MTM ethers can be prepared from MEM and MOM ethers by treatment with Me2BBr to form the bromomethyl ether, which is trapped with MeSH and (t-Pr)2NEt. This method may have some advantage, since the direct preparation of MTM ethers is not always simple. ... 

A facile way of promoting the cyclization is to increase the nucleophilicity of the aryl system when possible. In the total synthesis of diazadiquinomycins A and B, for example, the authors were able to effect a double Knorr cyclization with concomitant in situ oxidation to the internal diquinone 23 by deprotecting the hydroquinone thereby lowering the activation barrier for the desired transformation. If the hydroquinone is left protected as the di-MOM ether, the reaction does not take place. ... 

The construction of the five contiguous stereocenters required for a synthesis of compound 3 is now complete you will note that all of the substituents in compound 5 are positioned correctly with respect to the carbon backbone. From intermediate 5, the completion of the synthesis of the left-wing sector 3 requires only a few functional group manipulations. Selective protection of the primary hydroxyl group in 5 as the corresponding methoxymethyl (MOM) ether, followed by benzylation of the remaining secondary hydroxyl, provides intermediate 30 in 68 % overall yield. It was anticipated all along that the furan nucleus could serve as a stable substi-... 

The completion of the synthesis of gilvocarcin V (2) only requires a few functional group manipulations. Hydrogenolysis of the four benzyl groups, followed by acetylation of the liberated hydroxyl groups, provides 30 in 68 % overall yield. After cleavage of the MOM ether in 30 with bromotrimethylsilane, application... 

Another elegant example of the thermal generation and subsequent intramolecular cycloaddition of an o-QM can be found in Snider s biomimetic synthesis of the tetracyclic core of bisabosquals.2 Treatment of the starting material with acid causes the MOM ethers to cleave from the phenol core (Fig. 4.3). Under thermal conditions, a proton transfer ensues from one of the phenols to its neighboring benzylic alcohol residue. Upon expulsion of water, an o-QM forms. The E or Z geometry of the o-QM intermediate and its propensity toward interception by formaldehyde, water, or itself, again prove inconsequential as the outcome is decided by the relative thermodynamic stabilities among accessible products. 

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