MEM group

The MEM group has been introduced on one of two sterically similar but electronically different alcohols in a 1,2-diol. ... 

Benzyl, allyl, methyl, THP, TBDMS, and TBDPS ethers are all stable to these conditions. A primary MEM group could be selectively removed in the presence of a hindered secondary MEM group. 

From a dialkyl acetal Bu3SnSPh, Bp3 Et20, toluene, —78° -> 0°, 64-100% yield. These conditions also convert MOM and MEM groups to the corresponding phenylthiomethyl groups in 64-77 % yield. 

In a study of the deprotection of the MEM ethers of hydroxyproline and serine derivatives, it was found that the MEM group was stable to conditions that normally cleave the t-butyl and BOC groups [CF3COOH, CH2CI2, 1 1 (v v)]. The MEM group was also stable to 0.2 N HCl, but not stable to 2.0 N HCl or HBr-AcOH. ... 

Methanolic HCl, 16 h, 80-94% yield. These conditions do not cleave the MEM group.1% Sulfuric acid in methanol has also been used. ... 

MeOH, reagent prepared by heating Bu2SnO and Bu3SnP04, heat 2 h, 90% yield." This method is effective for primary, secondary, tertiary, benzylic, and allylic THP derivatives. The MEM group and ketals are inert to this reagent, but TMS and TBDMS ethers are cleaved. 

Tonsil, a Mexican Bentonite, acetone, 30 min, it, 60-95% yield. MOM and MEM groups are stable and phenolic THP ethers were also cleaved." ... 

P2I4, CH2CI2, 0° It, 30 min, 70-90% yield.This method is also effective for removal of the SEM and MEM groups. 

From a dialkyl acetal MgBr2, Et20, rt, PhSH, 91 % yield." MOM groups are converted to phenylthiomethyl groups in 75% yield, but MEM groups do not react. 

Me2Sn(SMe)2, BF3 Et20, PhCH3, 0°, 3-24 h AcOH, 75-100% yield.An ethyl ester can be hydrolyzed in the presence of an MEC ester with 1 N aqueous NaOH-DMSO (1 1), and MEC esters can be cleaved in the presence of ethyl, benzyl, cinnamyl, and t-butyl esters, as well as the acetate, TBDMS, and MEM groups. 

From the intramolecular coupling product, just the two MEM-groups protecting the phenol functions (MEM = 2-methoxyethoxymethyl) have to be removed, in order to obtain the target molecule. 

The methoxymethyl (MOM) and (3-methoxyethoxymethyl (MEM) groups are used to protect alcohols and phenols as formaldehyde acetals. These groups are normally introduced by reaction of an alkali metal salt of the alcohol with methoxymethyl chloride or (3-methoxyethoxymethyl chloride.157... 

The intron group I ribozymes feature common secondary structure and reaction pathways. Active sites capable of catalyzing consecutive phosphodi-ester reactions produce properly spliced and circular RNAs. Ribozymes fold into a globular conformation and have solvent-inaccessible cores as quantified by Fe(II)-EDTA-induced free-radical cleavage experiments. The Tetrahy-mem group I intron ribozyme catalyzes phosphoryl transfer between guanosine and a substrate RNA strand—the exon. This ribozyme also has been proposed to use metal ions to assist in proper folding, to activate the nucleophile, and to stabilize the transition state. ... 

As demonstrated by various techniques, including cyclic voltammetry, ESR and NMR spectroscopy, instead of 1, which is an open shell system, the diamagnetic dimer 2 was isolated. The suggested mechanism for the formation of 2 involves, as first step, the acid-catalyzed cleavage of the MEM group followed by an intramolecular ring formation to an 1,3-oxazetidinium system. This intermediate then loses formaldehyde and CO to form the azafulleronium ion which is... 

To assess the effect of intramolecular chelation in this class of organolithium, Gawley also made 157 and treated it under similar conditions.57 In THF alone, the MEM-protected 158 has greater chemical stability than 155, and is configurationally stable up to about -60 °C. Like the lithiated Boc-pyrrolidine 138 (but unlike the lithiated /V-methyl pyrrolidines 155) TMEDA tends to decrease its configurational stability and a direct comparison between MEM protected 158 and 155 in the presence of TMEDA shows that the MEM group also... 

Methoxyethoxymethyl (MEM)-protected arylquinolizidines 25 and 27 were prepared from MEM-protected isovanillin (29) through the same sequence as shown in Scheme 2. Treatment of the alcohol 28, obtained by basic hydrolysis of 27, with the anhydride 30 gave 31 in 73% yield. Removal of the MEM groups with trifluoroacetic acid in methylene chloride afforded 10-epidemethoxyabreso-line (3) in 12% overall yield from 29. 

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