Protection/deprotection reactions

Protection-deprotection reaction sequences are an integral part of organic syntheses such as the preparation of monomers, fine chemicals, and reaction intermediates or precursors for pharmaceuticals. These reactions often involve use of acidic, basic, or hazardous reagents and toxic metal salts [31]. The solvent-free MW-accelerated protection/deprotection of functional groups, developed during the last decade, is an attractive alternative to conventional cleavage reactions. 

Hamelin and coworkers protected aldehydes and ketones as acetals and dioxolanes by using orthoformates, 1,2-ethanedithiol, or 2,2-dimethyl-l,3-dioxolane. This acid-catalyzed reaction proceeds in the presence of p-toluenesulfonic acid (PTSA) 

2 Microivave-accelerated Solvent-free Organic Reactions 365 

Acetic anhydride-pyridine on basic alumina has been used to conduct acetylations of hydroxy, thiol and amino groups under microwave irradiation conditions [35]. This rapid, safe, and eco-friendly technique can be applied for a broad variety of N, O, and S acetylations. 

In an extremely simple approach, an unmodified household microwave oven has been used in this study with excellent results. The generation of higher temperatures is avoided simply by intermittent irradiation [38]. 

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The coupling and protection-deprotection reactions to yield Taxol were fully licensed by BMS from FSU as part of the original licensing agreement (Scheme 7.3). This so-called metal alkoxide process was patented by Holton in 1992 [13]. 

Nafion-H has been shown to be effective in a variety of protection-deprotection reactions including (9-trialkylsilylation of alcohols, phenols, and carboxylic acids, as well as the preparation and methanolysis of tetrahydropyranyl (THP) ethers.672 However, when compared, for example, with HBF4-silica or Nafion nanocomposites,... 

Since its first description in 1971 [35], gel-phase NMR was applied to peptide chemistry by Manatt and coworkers [36, 37], These authors used 13C NMR to determine the extent of chloromethylation of crosslinked polymers and 19F NMR to monitor protection-deprotection reactions. These two nuclei are the most commonly used in these types of studies, mainly because of their significant chemical shift dispersion, which can alleviate in part the resolution loss due to the non ideal linewidth obtained in the gel state. Apart from restricted molecular motion, that shortens T2 because of an efficient transverse relaxation, other sources of line-broadening derive from magnetic susceptibility variations within the sample (due to the physical heterogeneity of the system) and residual dipolar couplings. 

Glycopeptides are more difficult to synthesize than the conventional peptides, because common protection-deprotection reactions used in the peptide synthesis can cause serious problems to the protective groups used for carbohydrates. Many of these problems have been solved by recent technical innovations [21]. The combination of 9-fluorenylmethyloxycarbonyl (Fmoc) group for N-protection and pentafluo-rophenyl (PFP) group as the activating group for the carboxylic acid allows GlcNAc-Asn... 

In Banwell s de novo synthesis of Neu5Ac [136] cis-1,2-dihydrocatechol 228, a product of microbial oxidation of chlorobenzene, has been converted into a protected form of Neu5Ac via a fifteen steps reaction sequence (Scheme 50). Synthesis started from azido alcohol 229, obtained from catechol 228 by an established procedure [137]. This was subjected to ozonolytic cleavage and a reductive work-up to afford diol 230. Protection-deprotection reaction sequence led to alcohol 232 which was then oxidized to D-mannosamine derivative 233 using the Swem protocol. Condensation with the organozinc reagent derived from... 

Three methods have been developed to introduce functionality at the macromolecular level. These are (a) by the use of cx ganic side groups that have "soft" functionality, (b) by the reactions of "unreactive" polyphosphazenes with aggressive reagents, and (c) via protection-deprotection reactions. These will be considered in turn. 

Although inherently wasteful, protection-deprotection reaction sequences constitute an integral part of organic syntheses such as the preparation of monomers, fine chemicals, and precursors for pharmaceuticals. These reactions often involve... 

The protection-deprotection reaction is an integral part of organic synthesis namely in the preparation of monomer building blocks, fine diemicals and precursors... 

Our original strategy for one-pot methods utilized common anomeric leaving groups and a common activator. We chose p-methylphenyl thioglycosides (STol) as our donor species not only because of their easy preparation and stability toward carbohydrate manipulation, such as standard protection/deprotection reactions, but also for three reasons ... 

Rare earth metal triflates have been employed as mild Lewis acid in many organic transformations including Mukaiyama-aldol, Michael addition, Diels-Alder, aziridination, oxidation, reduction, rearrangement, and protection/deprotection reactions [102]. Several methods for the immobilization of these Lewis acids have been developed [102, 103]. Janda and coworkers developed a new method for the immobilization of Yb(OTf)3 on the cross-linked pyridine/styrene copolymers [104]. Since rare earth metal triflates are known to be catalytically active in the presence of bases, the cross-linked pyridine/styrene polymers could serve as suitable... 

There is also ample literature on the use of elemental irxiine - either in stoichiometric or catalytic amounts - in organic synthesis. This chemistry has been covered in two reviews from 2006 and 2011 [86-88], and a multitude of further cases of iodine-mediated or iodine catalyzed transformations have been published since then [89-104]. Lately, two more specialized reviews on the use of iodine in protection/deprotection reactions [105] and in reactions involving oxygen-containing functional groups [88] have appeared. 

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