Protection alkoxides

Polymerization of ethylene oxide with an acetal-protected alkoxide afforded a-aldehyde-co-methacryloyl PEO macromonomer, 17, after termination with methacrylic anhydride followed by acid hydrolysis [24]. 

The imide proton N-3—H is more acidic than N-1—H and hence this position is more reactive toward electrophiles in a basic medium. Thus hydantoins can be selectively monoalkylated at N-3 by treatment with alkyl haUdes in the presence of alkoxides (2,4). The mono-A/-substituted derivatives (5) can be alkylated at N-1 under harsher conditions, involving the use of sodium hydride in dimethylform amide (35) to yield derivatives (6). Preparation of N-1 monoalkylated derivatives requires previous protection of the imide nitrogen as an aminomethyl derivative (36). Hydantoins with an increased acidity at N-1—H, such as 5-arylmethylene derivatives, can be easily monoalkylated at N-3, but dialkylation is also possible under mild conditions. 

Metal alkoxides are strongly caustic and are decomposed by the humidity of the air or moisture of the skin, requiring the use of protective glasses and gloves. 

Thermolysis is used in the coating of glass and other surfaces with a film of titanium dioxide. When a lower alkoxide, eg, TYZOR TPT, vaporizes in a stream of dry air and is blown onto hot glass botdes above ca 500°C, a thin, transparent protective coating of Ti02 is deposited. 

A benzyl carbonate was prepared in 83% yield from the sodium alkoxide of glycerol and benzyl chloroformate (20°, 24 h). It is cleaved by hydrogenolysis (H2/ Pd-C, EtOH, 20°, 2 h, 2 atm, 76% yield) and electrolytic reduction (-2.7 V, R4N X, DMF, 70% yield). A benzyl carbonate was used to protect the hy-droxyl group in lactic acid during a peptide synthesis. 

Certain functional groups may be protected from reduction by conversion to anions that resist reduction. Such anions include the alkoxides of allylic and benzylic alcohols, phenoxide ions, mercaptide ions, acetylide ions, ketone carbanions, and carboxylate ions. Except for the carboxylate, phenoxide, and mercaptide ions, these anions are sufficiently basic to be proton-ated by an alcohol, so they are useful for protective purposes only in the... 

In this case, the magnesium alkoxide protects the ketal from cleavage. " ... 

One of the more common methods of alcohol protection is by reaction with a chlorotrialkylsilane, CI-S1R3, to yield a trialkylsilyl ether, R -O-SilTj. Chlorotrimethylsilane is often used, and the reaction is carried out in the presence of a base, such as tciethylamine, to help form the alkoxide anion from the alcohol and to remove the HC1 by-product from the reaction. 

Based on these reports, we started investigation of the asymmetric addition of acetylide to pMB protected 5, mainly in the presence of chiral P-amino alcohols. Many types of chiral amines were also screened (e.g., diamines, diethers), and it was soon found that addition of P-amino alkoxides effectively induced enantiose-lectivity on the addition. Since the best result was obtained with a stoichiometric amount of chiral amino alcohols, we focused our screen on readily available chiral P-amino alcohols and the results are summarized in Table 1.2. 

It would be ideal if the asymmetric addition could be done without a protecting group for ketone 36 and if the required amount of acetylene 37 would be closer to 1 equiv. Uthium acetylide is too basic for using the non-protected ketone 36, we need to reduce the nucleophile s basicity to accommodate the acidity of aniline protons in 36. At the same time, we started to understand the mechanism of lithium acetylide addition. As we will discuss in detail later, formation of the cubic dimer of the 1 1 complex of lithium cyclopropylacetylide and lithium alkoxide of the chiral modifier3 was the reason for the high enantiomeric excess. However, due to the nature of the stable and rigid dimeric complex, 2 equiv of lithium acetylide and 2 equiv of the lithium salt of chiral modifier were required for the high enantiomeric excess. Therefore, our requirements for a suitable metal were to provide (i) suitable nucleophilicity (ii) weaker basicity, which would be... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

With careful selection of the combination of starting alkoxides and appropriate synthesis conditions, silica-based sol-gel technology is now widely applied to the conservation of art objects and cultural heritage. For example, a hybrid sol-gel coating protects the fourteenth-century... 

Hybrid polymer silica nanocomposites formed from various combinations of silicon alkoxides and polymers to create a nanoscale admixture of silica and organic polymers constitute a class of composite materials with combined properties of polymers and ceramics. They are finding increasing applications in protective coatings (Figure 7.1), optical devices, photonics, sensors and catalysis.1... 

Bis(tribuyltin) oxide reacts with carboxylic esters in ether at room temperature to give the corresponding tributyltin carboxylate and tributyltin alkoxide, and this reaction is recommended for removing protecting ester groups in the presence of other functional substituents.358-360... 

The synthesis of 4//-pyrano[2,3-d]pyrimidines, a class of compounds important in crop protection, is presented in Scheme 91. Dichlorotriphenyl-phosphorane affords iminophosphorane 247 with ethyl 2-amino-4//-pyran-3-carboxylate (246). Phenyl isocyanate cyclizes under alkoxide migration to afford pyrano[2,3-d]pyrimidines (248) (90LA995). 

Continuing with the diastereomerically pure tricycle 56, an 11-step sequence consisting of redox and protective group chemistry was necessary to generate a / -hydroxy keton (58) suitable for a retro-aldol addition via an intermediate alkoxide to the highly substituted cyclopentanone 52 (Scheme 6). 

The ketone 73 was reduced chemo- and diastereoselectively and protected to provide the silyl ether 74. The ester function was then deprotonated to the corresponding ester enolate (75) that was alkylated with methyl iodide exclusively from the Re face of the enolate to afford the bicycle 76 (Scheme 11). The substrate for the retro-aldol reaction (77) was prepared by a sequence that consists of seven functional and protecting group transformations. The retro-aldol reaction converted the bicyclic yS-hydroxy ketone 77 into the 1,3-diketone 69 via the alkoxide (78) in very good yield. 

Addition of a lithiated secondary amine to an aldehyde both protects the aldehyde from attack by RLi and turns it into an ortholithiation directing group. The best lithioamines for this purpose are A-lithio-A-methylpiperazine 53, iV-lithio-iV,iV,iV -trimethylethylene-diamine 56 and Al-lithio-Al,0-dimethylhydroxylamine 58 , which optimize the opportunity for coordination of BuLi to the intermediate alkoxide (54) (Scheme 27) . ... 

Pyrroles and indoles have one further unique mode of lateral lithiation—deprotonation of an Af-methyl group (also an a lithiation). The reaction works particularly weU with an aldehyde director, temporarily protected as the a-amino alkoxide 565 (Scheme 228)°° . 

Fig. 30 Protected functionalized lactones polymerizable by aluminum and tin alkoxides [15, 114, 136-147]...

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