Cyclic oxonium intermediate

Although not the best catalyst, AgSbF6 led to the rearranged product in 50% yield. This cascade reaction probably started with the well-known cyclization of the ketone to the alkyne on silver coordination, giving a cyclic oxonium intermediate that rearranged to furanone via an alkyl 1,2-migration (Scheme 3.54).82... 

From a mechanistic perspective, the reaction is similar to those described above. Silver-catalyzed cyclization of the ketone to the allene gave a cyclic oxonium intermediate. A [1,2]- or [1,5]-alkyl shift modified the sigma skeleton leading to an alkylsilver intermediate, which on elimination gave a trisubstituted furan. 

Analysis of the x-ray results revealed that all of the stereocenters for compound 134 were as depicted in Scheme 33. The BC-rings were shown to adopt a chair/twist boat conformation to avoid 1,3-diaxial interactions between the methyl group and the benzyloxy unit. A similar conformation was reported for the C-ring in the case of the trans-fvLStd pyranopyran framework [4]. The configuration of the newly formed stereocenter at C in structure 134 is undoubtedly the consequence of a stereoelectronically controlled addition to the cyclic oxonium intermediate, 133. 

According to the proposed mechanism, the Au-catalyst activates a carbon-carbon triple bond of 93 via 95, vhich is then follo ved by a subsequent nucleophilic attack of the carbonyl function to produce a cyclic oxonium intermediate 96. An intermolec-ular nucleophilic addition of an external nucleophile to the activated enone in 96 leads to the furyl-gold species 97. Protiodeauration of the latter produces furan 94 and regenerates the catalyst (Scheme 8.38). 

In synthetic studies of gleosporone, an autoinhibitor of spore germination, Schreiber reported a highly stereoselective Ferrier-type reaction of cyclic enol ether 77 (Scheme 1.10) [29]. It is likely that minimization of A, 3 interactions in the cyclic oxonium intermediate (cf. 79) is the controlling feature in this diastereoselective process. 

Like the un-ionized hydroxyl group, an alkoxy group is a weak nucleophile. Nevertheless, it can operate as a neighboring nucleophile. For example, solvolysis of the isomeric p-bromobenzenesulfonate esters 6 and 7 leads to identical prxKluct nuxtures, suggesting the involvement of a common intermediate. This can be explained by involvement of the cyclic oxonium icai which would result from intramolecular participation. ... 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

The oxygen atom in jl can be replaced by sulfur, and the same prediction is maintained. We have already seen that this is indeed the case for cyclic oxonium and sulfonium ions (Chapter 2). It is essentially for the same reason that lactones, thionolactones (Chapter 3) as well as lactams and their derivatives (Chapter 4) behave in exactly the same manner. Indeed, an axial attack on 12 (X=0, 5, or MR) leads to an intermediate having a chair conformation (12 13) while an equatorial attack necessarily leads to the less favorable boat conformation (12 14). 

The reaction of nucleophiles with the conformationally rigid piperidinium ion lj>, like that with cyclic oxonium ions, can also be controlled by stereoelectronic effects. On that basis, the addition of a nucleophile on the upper face of Jjj must lead to the boat-like intermediate whereas that from the lower face must lead to the chair-like intermediate V7. The transition state leading to must be less favorable than that leading to Yl and product V7 should therefore be favored. 

Cyclic oxonium compounds generated from hemiacetals by a Lewis acid react with 2-silyloxythiophene to produce useful intermediates via C-C bond formation (Scheme 58). This reaction has been extensively used to prepare a large number of synthetic analogs of acetogenins, which are effective antitumor agents (Scheme 59) <2000CSR109>. 

If our hypothesis was correct, the oxonium ion 99, derived from dimethyl acetal 98, would rearrange more readily than oxazoline 100 to give 102. The bond between the 6-carbon and 6 -oxonium carbon in 99 can rotate freely around the axis this would facilitate the approach of the 14-OH group and its overlap with the jt orbital of the carbonyl carbon in 99 to produce the cyclic acetal intermediate 105. Then 105 could rearrange to 102 via the oxonium 106 (Scheme 34). Furthermore, we had shown that the three aforementioned general ketones (TV-benzylpiperidone (78), acetophenone (79), and 4-phenylcyclohexanone (80)) without OH groups... 

The intermediate formation of a 1,6-anhydride bond was used to explain the course of reaction of methyl 2,3,4,6-tetra-0-(chlorosulfonyl)-a-D-glucopyranoside (16) with aluminum chloride.98 The methoxyl group first anomerizes, and then moves from C-l to C-6 by way of the cyclic oxonium ion 17, to give the 6-methyl ether (18). 

The presumed intermediate in all of the reactions mentioned is a cyclic oxonium ion such as 22 which,66,312,313 if acylated at 0-6, may... 

Two independent reports have appeared that detail methodology for the introduction of pyran or oxepene spirocychc moieties onto oxindole scaffolds by means of a Prins cyclization (Scheme 35). In one study by Zhang and Panek, treatment of isatin dimethylketal 134 with sUyl alcohol (S )-135 afforded predominately c -137 or trans-Vi9 depending on reaction time and solvent polarity [78]. A mechanism involving epimerization of the cis-product to the trans adduct was put forward to explain the observed frans-selectivity with increased reaction time in polar solvents. A cyclic transition state involving a (Z)-oxonium intermediate formed via condensation of silyl alcohol 135 with isatin 134 was invoked to rationalize the preferential formation of the cis-spirocycle under kinetic control. Further optimization led to the formation of spirooxindoles 138, e.g., R, = Me, as single... 

Why do we not simply write the isomeric free carbocations as intermediates in the acid-catalyzed ring openings The reason is that the cyclic oxonium ion has an octet sfructure, whereas the carbocation isomer has a carbon with an electron sextet. Indeed, experimentally, inversion is observed when reaction takes place at a stereocenter. Like the reaction of oxacyclopropanes with anionic nucleophiles, the acid-catalyzed process includes backside displacement—in this case, on a highly polarized cyclic alkyloxonium ion. 

Contained within intermediate 25 is an acid-labile mixed acetal group and it was found that treatment of 25 with camphorsulfonic acid (CSA) results in the formation of dioxabicyclo[3.3.0]octane 26 in 77 % yield. Acid-induced cleavage of the mixed cyclic acetal function in 25, with loss of acetone, followed by intramolecular interception of the resultant oxonium ion by the secondary hydroxyl group appended to C leads to the observed product. Intermediate 26 clearly has much in common with the ultimate target molecule. Indeed, the constitution and relative stereochemistry of the dioxabicyclo[3.3.0]octane framework in 26 are identical to the corresponding portion of asteltoxin. 

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