Oxocarbenium ions Subject

From a strictly chemical point of view, the synthesis of glycosides still presents a formidable challenge to synthetic chemists in spite of major advances in the area [1], Unlike peptidic bonds, the formation of the glycosidic linkage is subject to various factors that include, among others, electronic, stereoelectronic, conformational, substituent, and reactivity effects generally associated with incipient oxocarbenium ions derived from carbohydrates. 

Figure 9.32 adds the information of how enol ethers are normally produced, i.e., enol ethers with no conjugation between the C=C- and the neighboring C=0 double bond 0,0-Acetals are subjected to an acid-catalyzed elimination of one equivalent of alcohol, via an El mechanism, that is, via an oxocarbenium ion intermediate that is deprotonated to give the respective enol ether (i.e., the product presented in the first line of Figure 9.32) or dienol ether (the product shown in the second line of Figure 9.32). Among other things, enol ethers are required for the Mukaiyama aldol addition (example Figure 12.23).

The strategy employed to discern the conformational preference of the less substituted oxocarbenium ions described above was not suitable for studying oxocarbenium ions with alkoxy groups at C2. When the lactone precursors 70 and 71 (Fig. 4.25) were subjected to alkylation conditions, the dioxocarbenium ion products 74 and 75 were not observed (Fig. 4.27). Instead, the reaction yielded a... 

The dramatic requirement for the specifically placed benzylidene has been subjected to elegant analysis by Bols [75], who noted that the stereoselective synthesis employs, as its key element, donors that have been disarmed by locking their conformation. .. The purpose of the lock, Bols contends, is to provide an antiperiplanar relationship between the 06-C6 and C5-05 bonds, an arrangement that facilitates electron withdrawal that stabilizes the oxocarbenium ion intermediate. 

Overall it appears increasingly likely to the authors of this chapter that, as the editor s cryptic Nobel prize winner stated, half of carbohydrate chemistry is the stabilization of the anomeric cation by the ring oxygen. What our common mentor failed to state is that it is the many subtleties of the chemistry of the glycosyl oxocarbenium ion that make the subject so fascinating and instmctive. Paradoxically, however, it is important to note that an actual glycosyl oxocarbenium ion has yet to be observed [71] ... 

The acid-base reaction of a ketone or aldehyde with an acid will give an oxocarbenium ion (44), which is resonance stabilized and subject to attack by a nucleophile at the acyl carbon. If ethanol reacts with 44, the product is 45, an unstable species that is in equilibrium with 44. Intermediate 45 is more stable than 42 because a C-OH unit replaces the alkoxide unit (C-0 ) found in 42. 

The concept of TSAS was further proven to be vahd by the following experiments. Chi-oxa monomer was subjected to polymerization by enzymatic catalysis. The hydrolysis enzymes used were chitinase (family 18) involving an oxazohnium intermediate and lysozyme involving an oxocarbenium ion intermediate (Fig. 13). With the former enzyme, synthetic chitin was quantitatively obtained after 50 h at pH 10.6, whereas with the latter no chitin was produced after 165 h of reaction [69]. This imphes that the oxazoline monomer could not be a substrate for the lysozyme enzyme. 

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