Assisted removal

Three selective methods to remove protective groups are receiving much attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl ether group (eq. 4) a /3-haloethoxy (eq. 5) or a /3-silylethoxy (eq. 6) derivative is cleaved by attack at the /3-substituent and a stable o-nitro-phenyl derivative can be reduced to the o-amino compound, which undergoes cleavage by nucleophilic displacement (eq. 7) ° ... 

The design of new protective groups that are cleaved by assisted removal is a challenging and rewarding undertaking. 

Monochloroacetamides are cleaved (by assisted removal ) by reagents that contain two nucleophilic groups (e.g., o-phenylenediamine, thiourea, 1-piperi-dinethiocarboxamide, 3-nitropyridine-2-thione, 2-aminothiophenol ) ... 

Other routes include the high-temperature halogenation of metal oxides, sometimes in the presence of carbon, to assist removal of oxygen the source of halogen can be X2, a volatile metal halide CX4 or another organic halide. A few examples of the many reactions that have been used industrially or for laboratory scale preparations are ... 

As already remarked, in compounds wer-IrX3 (PMe2Ph)3, the most labile ligand is the group X (X = H, Me, halogen) trans to a phosphine. The Ag+-assisted removal of chloride in (I) yields [IrCl2(H20)(PMe2Ph)3]+ (XII) where the H20 is very weakly bound (Ir—O 2.189 A, compare Ir(H20) + 2.041 A) and readily replaced. 

Waldmann et al. developed a second exo-linker following a new approach [43-44] which makes use of a safety-catch linker. It is based on the enzymatic cleavage of a functional group embodied in the linker. In this way an intermediate is generated, which subsequently cyclizes intramolecularly according to the principle of assisted removal [54—58] and thereby releases the desired target compounds (Scheme 10.11). 

Assistant Remove the phlegm regulate the Qi, promote blood circulation and therefore calm the mind strengthen and stabilize the weak organs and calm the mind strengthen the Middle-Jiao and protect the Stomach so as to reduce the side effect of the herbs that strongly sedate the Shen... 

When methanol is used, the solvent is removed under vacuum, leaving a creamy white solid or a yellow oil. The oil may be converted to the solid by dissolving it in methanol and then removing the methanol. The product obtained at this point is mainly (PnAO 2HC1). This material is slurried with anhydrous benzene to assist removal of undesired by-products, collected on a filter, and dried in vacuo at 70°C. 

Lastly, silver-assisted removal of halide has been used to facilitate a Favorskii rearrangement in the key step of the synthesis of the natural sesquiterpene ( )-sirenin (equation 190)989. Fused bicyclic cyclopropane derivatives have also been made by a similar... 

Assisted removal, 2-3 of carbamates, to form amines, 341-345 of chloroacetamides, to form amines, 352 of substituted amides, to form amines, 356-357... 

Three selective methods to remove protective groups have received attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl... 

Olefins add hydrogen fluoride very readily, but polymerization of the olefins also occurs as a side reaction. Traces of acid and water, or temperatures of about 70°, lead to loss of HF from the alkyl fluorides produced. Preparative importance attaches to addition of HF to chlorinated olefins, whose tendency to polymerize in presence of HF is less the more Cl atoms are attached to the doubly bonded carbon atoms. HF adds readily to olefins with unsymmetrical halogen substitution, such as CH2=CHX, RCH=CX2, or RCX=CH2, but with greater difficulty when halogen is attached to both the doubly bonded carbon atoms. BF3, which assists removal of H+ from HF by complex formation, accelerates the reaction but also the exchange of halogen for fluorine and the resin formation by polymerization that occur as parallel reactions.169... 

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