Cyclic derivatives, cyclization Chapter

Cyclization may also occur as a result of an intramolecular rearrangement, sometimes unintentionally, as in the cyclization of vitamin Dj in the heated injection port of a gas chromatograph [1], which was evident from the appearance of unexpected extra peaks. Condensation or polymerization reactions may also lead to cyclic derivatives, for example the formation of diketo-piperazines from dipeptide esters or the production of paraldehyde from three molecules of formaldehyde. However, this chapter is mainly concerned with the deliberate formation of a cyclic derivative, most often by a bridging type of reagent ( bidentate ligand ), and the emphasis will be on analysis and quantitation by... 

Cyclizations of alkenic amines and imines using organoiron complexes to generate bicyclic 3-lactams are discussed in Chapter 3.1 of this volume. Examples of heterocyclizations of alkenic NA/-dialkylamine and pyridine derivatives to form cyclic quaternary ammonium salts are cited in the Staninets review.Ic A cyclization of an enol thioether has been used to generate a thiazolidine intermediate used in cephalosporin synthesis (equation 131).262... 

For keto-hexoses, however, the pyranose derivatives are not the major hemiketals that are formed. Cyclization via the OH unit at C5 will lead to furanose derivatives as the preferred structure. Why In Chapter 8, cyclopentane was lower in energy than cyclohexane, but the compounds here are cyclic ethers. Indeed, the tetrahydropyran ring is somewhat more stable than the tet-rahydrofuran ring. Stabilization of the hydroxyl groups and the hydroxymethyl... 

Uchida and co-workers have further exemplified their synthesis (cf. Vol.28, p.233) of cyclic isoureas, e.g. S a-carbatrehazolin 39, by condensing glycosyl isothiocyanates with the cyclitolamine trehazoline, and cyclizing the resulting thiourea adducts (with HgO) for further details see Chapter 19. cis-Fused 1,2-oxazolidine-2-thiones such as 40 were the exclusive products from reactions of 1,2-O-sulfinyl-a-D-gluco-furanose or -pyranose derivatives (i.e. 1,2-cyclic sulfites) with sodium thiocyanate. ... 

Although the number of studies using (radio) labeled substrates in an attempt to confirm the generation of these PIAs in nature is limited, numerous biosynthetic hypotheses occupy the chemical/synthetic literature [34, 35]. These proposals represent all levels of the complexity spectrum for this family, from simple cyclic compounds derived from an oroidin monomer to those that result from dimerization of oroidin and further cyclizations. To prevent this chapter from occupying the whole book, only two examples will be presented in this subsection to serve as a general overview. 

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