Dioxocarbenium ion

Figure 7.23. (a) Pseudo-equatorial and pseudo-axial conformers of the dioxocarbenium ions, (b) ORTEP representations of the X-ray structures obtained for carbenium ion 49 and 50. (Taken from ref. 127.)... 

The formation of orthoester may be explained tiy the preferential addition of tlie ketene acetal to the most reactive alcohol function (primary hydroxyl g,roup) giving thc f non - i so 1 a t ed) acyclic orthoester whicti is attacked by the neighbouring OH-4 with subsequent elimination of methanol. The partial hydrolysis of the diacetate is assumed to proceed through protonation of the methoxyl group (7), via the dioxocarbenium ion 8 and the orthoacid 9. collapse of 9 by either [lath b or path a accor-ding to the mechanism generally proposed (see, for instance, ref. 29 and refs. cited therein) affords the compounds 5 or respectively. 

In fact, these reactions likely involve 13-dioxocarbenium ions, which can also be prepared from pyranosyl chloride and reduced to acetals thus endo- and exo-1,2-0-ethylidene-a-D-allapyranoses have been prepared from penta-O-acetyl-p-D-allopyranosyl chloride (reaction with sodium borohydride) [88]. These types of 1,2-acetoxonium ions ate also known to react with dialkylcadmium to give 1,3-diacetals [89]. 

Trimethylorthoformate forms a stabilized dioxocarbenium ion which activates the lactone towards attack by methanol with Sn2 inversion at the methyl-substituted carbon. 

Giusti s studies on ring-opening of a series of spiro ketal derivatives showed that, on acid hydrolysis (Table 10), ring-opening usually occurs at the site of the less-substituted carbon. The preference shown by cyclopropanone ethylene ketals for path b in Scheme 18 may be attributed to (i) the reversibility of path a due to rapid intramolecular ketalization at the incipient C(l)-carbenium ion (45) (R = CH2CH2OH) and (ii) the stability of the intermediate dioxocarbenium ion (46) generated in path b. 

Acid catalyzed reaction Dioxocarbenium ion activation of the lactone carbonyl group followed by attack of methanol (8 2 inversion) at the methyl-substituted y-carbon. 

Control experiments indicate that these reactions proceed via free oxocarbenium ions and not contact ion pairs [23]. This frans-selective outcome can be understood by considering that the alkoxy-substituted oxocarbenium ion favors the axial conformer 72, in accord with the structure 77 of the dioxocarbenium ion 8 (Fig. 4.3). The product selectivity is in accord with computational predictions (Fig. 4.1) [8,11]. Nucleophilic addition to the lower energy conformer 72 (Fig. 4.4) through the stereoelectronically preferred chair-like transition stmcture leads to the observed trans product [24-26]. The reaction of the corresponding alkyl-substituted acetate 77 (X = Me) in the presence of a Lewis acid afforded 1,4-cis product 70, consistent with an equatorial preference for the substituent at C4 (viz. 77) followed by the stereoelectronically controlled nucleophilic addition [23, 24, 26]. 

To study the conformational equilibria of the mannosyl oxocarbenium ion in solution, Yang and Woerpel [93] prepared a series of monosubstituted and multiply substituted tetrahydropyran dioxocarbenium ions and analyzed their conformational preferences. This approach was employed successfully for determining that the C4 alkoxy-substituted dioxocarbenium ion preferred the pseudoaxial conformation [17]. 

Yang and Woerpel [93] prepared a series of lactones which were then converted into dioxocarbenium ions by alkylation with Meerwein salts [91, 92]. 

Fig. 4.14 Theoretical and experimental J-values for C3 alkoxy dioxocarbenium ion (B3LYP/6-31G )...

The conclusion from spectroscopic evidence that dioxocarbenium ion with an alkoxymethyl group at C5 adopts a pseudoequatorial conformation is consistent with reactivity of the related oxocarbenium ion. Lewis acid-catalyzed nucleophilic substitution of acetate 45 afforded the 1,5-trans product 46 with high diastereos-electivity (Fig. 4.18). This result is inconsistent with the product ratio (70 30) previously reported for this reaction [84] (Fig. 4.18). In that paper, a minor product observed in the unpurified reaction mixture was assumed to be a diastereomer of 46. It was later found that the minor product formed in the reaction mixture is not the cis isomer. This result can be explained by stereoelectronically controlled addition to the half-chair conformer analogous to 38 (or even 40) in which the alkoxymethyl... 

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... 

Chamberland S, Ziller JW, Woerpel KA (2005) Structural evidence that alkoxy substituents adopt electronically preferred pseudoaxial orientations in six-membered ring dioxocarbenium ions. J Am Chem Soc 127 5322-5323... 

Fig. 6 Possible non-stabilized oxocarbenium ion I and stabilized dioxocarbenium ion J as intemediates in glycosylation with 87 and with related donors...

Fig. 10 Possible non-stabilized oxocarbenium ion O and stabilized dioxocarbenium ion P as intermediates in fucosylations with donors bearing 4-O-acyl groups...

The initial steps of acid-catalyzed hydrolysis of the following orthoester can give two different dioxocarbenium ion... 

Spectroscopic studies that illuminate the three-dimensional structures of highly substituted tetrahydropyran dioxocarbenium ions have been carried out by Yang and Worpel. By comparing the NMR couplings of both mono and multiple substituted dioxocarbenium ions with those predicted by computational methods, the authors estabhshed the conformational preferences of these compounds and came to the conclusion that in the absence of severe steric interactions electrostatic forces dictate the conformational preferences. 

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