Protection-deprotection method

Caution Carry out all procedures in a well-ventilated hood, and wear disposable vinyl or latex gloves, and chemical-resistant safety goggles. 

Synthesis of the reactive branch cell Reagent 1-methyl- 

To a round-bottomed flask (500 mL) equipped with a Dean-Stark trap fitted with a reflux condenser and a magnetic stirrer bar, add pentaerythritol (27.2 g, 200 mmol), triethyl orthoacetate (32.44 g, 36.6 mL, 200 mmol), PPTS (1 g, 4 mmol), and 200 mL of dioctylphthalate, 

Attach the condenser to the dual manifold using a gas-inlet adapter, and place the system under a nitrogen atmosphere, Heat the reaction mixture on an oil-bath, with stirring at 140°C for 2-3 h under nitrogen until quantitative recovery of ethanol (32 mL theoretical) is obtained. Replace the nitrogen line with an aspirator vacuum ( 25 mmHg) to remove the residual ethanol. 

Replace the trap with a large Dewar condenser containing ice-water and evacuate the mixture at 0.1 mmHg. 

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Denkewalter et al. [90] reported the synthesis of monodendron polyamides through generation 10. The scheme involved protection/deprotection methods commonly used by polypeptide chemists. Dendron construction started with benzhydrylamine as the initiator core and an activated ester of amino-protected L-lysine as the branching reagent (Scheme 4). 

The use of protection/deprotection methods or highly differentiated, branch cell reagents allows selective coupling without bridging or looping side reactions. Table 4 summarizes just a few of the branch reagents used successfully so far. 

The second part deals with the grafting of amines or alcohols with protection/ deprotection methods and the third one presents the grafting of alkenes bearing free hydroxyl or acid functions. Finally, a table summarizing all experiments is also presented. 

It is known that pyridinium tribromide converts olefins with specific r/7 t/-dibromina-tion. Therefore, the v/c-dibromides thus obtained can be cathodically reduced to the original olefins without geometrical changes of molecular structure [218]. This stereospecific bromination-debromination may be useful as an olefin protection-deprotection method. 

Calix[n]arenes have attracted great attention as ionophoric receptors [1-4] and potential enzyme mimics [5] in host-guest chemistry. Shinkai et al. have reported the preparation and ionophoric properties of four conformers of tetra-terf-butyl-tetrakis[(ethoxycarbonyl)methoxy]calix[4]arene [6-8]. Cone and partial-cone conformers are obtained by the metal template effect using sodium and cesium ions, respectively, but the 1,2-altemate and 1,3-altemate conformers were synthesized by the protection-deprotection method [8-10]. They also found that the cone conformer shows a selectivity for sodium [6] and the other conformers show a selectivity for potassium [8]. 

Although use of automated oligonucleotide synthesis is widespread, work continues on the optimization of protecting groups, coupling conditions, and deprotection methods, as well as on the automated devices.56... 

Microwave-assisted epoxide ring-openings of l,5 2,3-dianhydro-4,6-0-benzyl-idene-D-allitol with nucleobases have been reported [218], Various rapid microwave-assisted protection and deprotection methods in the area of carbohydrate chemistry are known [210], and two general review articles on microwave-assisted carbohydrate chemistry were published in 2004 [219, 220]. 

The synthesis of hyperbranched polymers can often be simplified compared to that of dendrimers as it does not require the use of protection/deprotection steps. This is due to the fact that hyperbranched polymers are allowed to contain some linearly incorporated A B monomers. The most common synthesis route follows a one-pot procedure where A B monomers are condensed in the presence of a catalyst. Another method using a core molecule and an A B monomer has also been described. 

An alternative method for dialkyl peroxide synthesis is the nucleophilic addition of an alkyl hydroperoxide to an alkene under acid catalysis reported by Davies and coworkers (Scheme 31, path B) ". A similar reaction is the nucleophilic addition of alkylhy-droperoxides to vinyl ethers under acid catalysis, producing perketals. Perketals can be deprotected under mild conditions (THF/water/acetic acid) and this hydroperoxide protection-deprotection sequence has been used by Dussault and Porter as a means for the resolution of racemic hydroperoxides (see also Section II.A.2) . In this respect more detailed studies were carried out with the perketals 75, which were prepared via reaction of alkyl hydroperoxides with vinyl ethers (Scheme 33). Weissermel and Lederer reported that in the presence of teri-butyl hypochlorite, a-chlorodialkyl peroxides can be formed in yields between 12% and 45% (Scheme 31, path C)". a-Alkoxydialkyl peroxides and diperoxyacetals were prepared by Rieche and coworkers via acid catalyzed reaction of one or two equivalents of alkyl hydroperoxides with acetals, ketals or aldehydes (Scheme 31, path D)" or by methylation of the corresponding a-alkoxy hydroperoxides with diazomethane (yields 11%, 27%)" . The diperoxyacetals 76 were isolated in yields ranging from 39 to 77%. 

This enantioselective preparation of allylic alcohols has been applied to the synthesis of the side chain of prostaglandins . The addition to functionalized aldehydes, such as 483, allows the synthesis of C2-symmetrical 1,4-diols, such as 484, with excellent diastereoselectivity and enantioselectivity . An extension of this method allows the synthesis of C3-symmetrical dioF . Aldol-type products result from the catalytic enantioselective addition of functionalized dialkylzincs to 3-TIPSO-substituted aldehydes, such as 485, followed by a protection-deprotection and oxidation sequence affording 486 in 70% yield and 91% ee (Scheme 118) . The addition to a-alkoxyaldehydes provides a... 

Clearly, an improved synthesis of L-ascorbic acid would result from the direct oxidation of L-sorbose (25) to L-xy/o-2-hexulosonie acid (28), thus eliminating the protecting-deprotecting steps currently required in the Reichstein-Griissner synthesis (see Scheme 4). Efforts to perform this oxidation may be divided into two categories, namely, chemical and fermentative. The results of each method will be summarized. 

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