Endocyclic oxygen

The replacement of the endocyclic oxygen of the sugar with a different element is another approach exploited to modify the anomeric reactivity. The carbon atom is the most obvious element to be used, the carbocycle so generated (cyclitol) completely loses the anomeric reactivity. The chemistry of cyclitols will not be described in this chapter as it is treated in another chapter of the same book. 

Another interesting class of compounds is that in which the endocyclic oxygen is replaced by nitrogen. 

Replacement of the endocyclic oxygen with a nitrogen imino sugars... 

The replacement of the endocyclic oxygen of the sugar with a nitrogen atom generates a class of compounds defined imino sugars. 

The endocyclic oxygen of carbohydrates has been replaced also with a phosphono group (Fig. 48).80 However these compounds have not found applications. 

In most cases, 1-azasugars present a piperidine or pyrrolidine skeleton, in other words a methylene group is positioned in place of the endocyclic oxygen, like in the case of compound 122. In other cases the endocyclic oxygen is maintained, like in the case of compound 123, or is replaced by a nitrogen (124). 

The three hydrogen bonds to Glu 35 stabilize the observed conformation in which Hei interacts with the endocyclic oxygen and has little contact with the glycosidic oxygen, O4. ... 

Until the middle of the 1980s, it was accepted that the mechanism for the cationic ROP initiated by alkylating agents was based on the reaction of the cation with the endocyclic oxygen, followed by cleavage of the acyl-oxygen bond (Fig. 16). 

Cationic polymerization of lactones is achieved with the range of initiators used for cyclic ethers (Sec. 7-2b) [Hofman et al., 1987a,b Kricheldorf and Sumbel, 1988a,b Kricheldorf et al., 1986, 1987a,b Kubisa, 1996 Penczek and Slomkowski, 1989a,b]. Initiation was formerly thought to involve attack of a positive species on the endocyclic oxygen to form an... 

Figure 1.11 The anomeric effect interaction of the endocyclic oxygen electron lone pair with the nonbonding orbital in an axially substituted compound.

Mono O-debenzylation can be achieved with Lewis-acid-mediated conditions such as with TiCLt of SnCU- Good yields are obtained when the substrate has three continuous cri-orientated metal-chelating groups (Scheme 2.3b). Thus, in case of a 1,6-anhydro-mannose derivative chelation takes place between the exocyclic anomeric oxygen and the 2-0-and 3-O-benzyl groups. On the other hand, 1,6-anhydrogIucose derivatives are chelated between endocyclic oxygen and the functionalities on... 

FIGURE 1.10 The anomeric effect, (a) The n-o interaction stabilizes the a anomer. (b) The P anomer experiences unfavorable dipole-dipole interaction that is reduced in the a anomer. (c) Greater electrostatic repulsion between the lone-pair electrons of the endocyclic oxygen and the electronegative anomeric substituent in the (1 anomer. 

The 2D-NMR NOESY correlations determined the absolute configuration (3S) for spirodioxocin 641a, which could adopt different conformations whose relative populations depended on steric factors, anomeric, and related effects and intramolecular //-bonding. In the preferred conformation of 641a, both endocyclic oxygen atoms were disposed in equatorial position relative to the other cycle and, therefore, such a structure was not stabilized by a conformational anomeric effect. The conformation (3Y)-641 a that disposed both endocyclic oxygen atoms in an axial position, which was stabilized by a double conformational anomeric effect, would imply chair-chair conformation for... 

Even carboxylate ions can serve as active centres of / -propiolactone polymerization [316]. Cationic polymerization is characterized by the formation of an oxonium transition salt generated by the reaction of an active centre with an exo- or endocyclic oxygen atom. The reaction mode depends on the kind of initiator and monomer [317]... 

---