Protection of functional groups

Sulfonium ylides may be added to C—N double bonds to yield aziridines in a formal [1+2]-cycloaddition. Alkyl azides are decomposed upon heating or irradiating to yield nitrones, which may also undergo [l + 2]-cycloaddition reactions to yield highly strained hetero-cycles (A.G. Hortmann, 1972). 

CONH CONBoc B0C2O, EtsN, DMAP, CH2CI2 830 269 

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E. Gross and J. Meinenhofer, Eds., The Peptides Analysis. Synthesis, Biology Vol. 3 Protection of Functional Groups in Peptide Synthesis, Academic Press, New York, 1981. 

The direct use of dioxygen in the catalytic oxidation of nonactivated C-H bonds under mild conditions without using a coreductant was reported by using a K2Pt04/CuCl2 catalytic system to give the selective functionalization of a-amino acids in water.10 The aqueous medium avoided the need for protection of functional groups (Eq. 2.3). 

Hydroamination of alkynes offers a straightforward preparation of a variety of amines, enamines, and imines.79 Numerous reports have appeared in the literature on this process. However, almost all these reactions have been carried out in organic solvents, which usually require the protection of functional groups or harsh conditions. Recently, Marinelli et al. have reported an Au(III)-catalyzed hydroamination of alkynes in... 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

Enzymatic synthesis relying on the use of aldolases offers several advantages. As opposed to chemical aldolization, aldolases usually catalyze a stereoselective aldol reaction under mild conditions there is no need for protection of functional groups and no cofactors are required. Moreover, whereas high specificity is reported for the donor substrate, broad flexibility toward the acceptor is generally observed. Finally, aldolases herein discussed do not use phosphorylated substrates, contrary to phosphoenolpyruvate-dependent aldolases involved in vivo in the biosynthetic pathway, such as KDO synthetase or DAHP synthetase [18,19]. 

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