MEM, protecting groups

Martin periodinane to deliver the desired key intermediate, ketone 208. The total synthesis of 181 was finalized in two more steps (49% overall yield) by removal of the acetal and the MEM protective group with simultaneous formation of the desired intramolecular ketal 216, followed by cleavage of the Cl 1 TBDMS ether and in situ oxidation with IBX. The highly complex secondary metabolite 181 was thus accessed in 16 linear steps starting from 210 with an impressive 14% overall yield (Fig. 36). 

The anion of 2-(methoxyethoxymethoxy)cyclopropyl phenyl sulfone (methoxyetho-xymethoxy = MEM) is a d -synthon for a,)8-unsaturated aldehydes ". The anion is readily formed by treating 2-(MEM)cyclopropyl phenyl sulfone (359) with n-butyllithium in THE at — 78°C. Treatment of the anion 360 with aliphatic primary bromides or allyl bromide produces the alkylated sulfone 361 in very good yields. Hydrolysis of the MEM-protecting group was readily performed by treatment with aqueous tetrafluoroboric acid to furnish the cyclopropanol sulfone (362). Treatment with aqueous sodium bicarbonate produced the corresponding aldehydes 363 in 70-90 % yields. 

Acyclic 0,0-acetals are used for the temporary protection of mono-alcohols. Most commonly used are the tetrahydropyranyl (THP), the methoxymethyl (MOM), the benzyloxymethyl (BOM), or the methoxyethoxymethyl (MEM) protecting groups. 

The hidden carbonyl group is more apparent after the removal of the MEM protecting group 94 and formation of the acetal 95 in acid solution. One diastereoisomer of the spirocyclic compound 95 is formed in 71% yield. 

Important examples include MeOCH2a, available commercially (but a dangerous carcinogen) and compound (15) used by Corey to introduce his MEM protecting group for alcohols (see Chapter 9). The synthesis of alcohol... 

It is more surprising that acetals can be used to protect simple alcohols as well as diols. Two of the best are the THP (6) and MEM (7) derivatives. These are often preferred to ethers (Table 9.1) because they can be removed under such mild conditions. The reagent for the MEM protecting group is discussed in Chapter 6. 

In an attempt to metalate a MEM-protected phenol with BuLi, the methoxy group was eliminated, forming the vinyloxymethyl ether. This was attributed to intramolecular proton abstraction. ... 

The dibromoalkene S-40 can be prepared from S-ethyl lactate by introduction of the MEM (methoxyethoxymethyl) protecting group, reduction to the O-protected lactaldehyde and Corey-Fuchs carbonyl olefination (Scheme 19). The l -enantiomer of 40 is available analogously from f -isobutyl lactate and serves as the reagent in the enantiomeric series. The lithium carbenoid S-41 is generated from S-40 by treatment with n-butyllithium in diethyl ether and reacted with aliphatic and aromatic aldehydes in tetrahydrofuran. High diastereoselectivities are reached, as shown in Scheme 19 . ... 

Abresoline (66) was prepared recently by Quick and Ramachandra by transesterification of MB-methoxyethoxymethyl (MEM) ether of methyl ferulate with MEM derivative of quinolizidol (63a) and cleavage of protective groups with trifluoroacetic acid (103). 

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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