Allyl oxonium intermediate

Whereas these transformations require stoichiometric gold compounds, catalytic amounts of both gold and palladium are sufficient for the cycloisomerization of allyl allenoates to allyl-substituted butenolides. Blum and co-workers reported this tandem C-O/C-C bond formation, which is initiated by activation of the distal allenic double bond with PhaPAuOTf (Scheme 4-107). This induces cyclization to an allyl oxonium intermediate, which undergoes deallylation in the presence of Pd2dba3. Nucleophilic attack of the resulting a-vinylgold intermediate at the ti-allylpalladium species and reductive elimination furnish the allylated butenolide and regenerate both catalysts. 

On the other hand, 3-benzyloxy-l,6-enynes of type LVIII lead to LIX and LX via formal C-H insertion on intermediate LV (Scheme 1.37). This reaction proceeds via proton abstraction from the CH2Ar group in LV to form the t/ -allyl gold intermediate LXI, which reacts at Cl or C3 with the oxonium cation to give LIX or LX, respectively. 

In addition to the A and B rings, Williams et al. also used an interesting cationic process to build the D ring contained in phorboxazole A (Scheme 18) [40]. The transformation involved formation of the triflate from 57 followed by expulsion of triflic acid to generate an aUyhc carbocation. Subsequent internal trapping of the transoid allylic cation intermediate by the C22 methoxymethyl ether and concomitant dealkylation of the resultant oxonium species provided THP 58 in moderate yield (55 %) as the sole diastereomer. 

Gold-catalysed ring closure of 1,5-enyne eontaining a silyl ether at the allylic position has been reported to induce a skeletal rearrangement to form an oxonium intermediate, which undergoes intra- and inter-molecular allylation (Scheme 119). ... 

The relationship between 9 and its predecessor 10 is close. Oxidation of the allylic C-3 methylene group in 10 and elimination of the methoxy group could furnish enone 9. Retrosynthetic disassembly of ring E in 10 furnishes tertiary alcohol 11 as a viable precursor. That treatment of 11 with a catalytic amount of acid will induce the formation of a transient oxonium ion at C-12 which is then intercepted by the appropriately placed C-4 tertiary hydroxyl group is a very reasonable proposition. As we will see, the introduction of the requisite C-4 hydroxyl group is straightforward from intermediate 12. 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Overman reported the synthesis of highly enantiopure 3-acyltetrahydrofurans with C5 substituents from formaldehyde acetals of allylic diols <00TL9431>. An example of Overman s procedure is depicted below, which involves the generation of a formaldehyde oxonium ion intermediate 92 before the cyclization. 

A closely related reactive oxonium ion can be prepared by Lewis-acid-catalysed breakdown of the corresponding acetal. Alternatively, especially if the acetal is at least partly a silyl acetal, the same oxonium ion can be produced in situ using yet more silicon in the form of TMSOTf as the Lewis acid catalyst. All these intermediate oxonium ions act as powerful electrophiles towards allyl silanes producing homoallylic alcohols or ethers. 

The second Prins reaction goes through the oxonium ion 223 to give the final product. Again the nucleophile is an allyl silane 223 and the second intermediate is a cation stabilised by the silicon p-effect. The relative stereochemistry is decided in the second reaction and 220 has the favoured diequatorial conformation. 

Enantiomerically pure l,3-dihydrobenzo[c]furan derivatives were recently obtained from o-phthaldehyde and 1,2-0-isopropylidene-a-D-xylofuranose <01TA4995>. An oxonium ion was proposed as an intermediate in the Amberlyst 15E promoted reaction of the cyclic allylic lactol ether shown in the following scheme. A (2,5)oxonium-ene process was believed to be the subsequent route from which aldehydes were generated <01TL6859>. 

When the aldol reaction furnishes an intermediate oxonium ion, a Prins cydization may ensue in the case where a suitable internal nucleophile is present that intercepts this oxonium ion. Rychnovsky et al. have developed this strategy into a powerful tool for the straightforward synthesis of tetrahydropyrans [7]. Thus, enol ether 15 attached to an allyl silane reacted with various aldehydes under BFj activation to produce 2,6-cis-substituted-4-methylene tetrahydropyrans 16 in good to very good yields (Table 8.2). 

Cyclohexenes and cycloheptenes containing an acyloxy group at the allylic or homoallylic positions give regiospecific addition products in polar solvents due to formation of intermediate oxonium ions of different stabilities. For example, methyl-3-cyclohexene-1-carboxylate 17 in acetic acid on benzene-sensitized irradiation gives methyl trans-4-acetoxy cyclohexane carboxylate 18 as major product along with small amount of methyl /rans-3-acetoxy cyclohexane carboxylate 19 [24]. 

Oxasteroids arising from tandem P-fragmentation and cyclization are products in the photolysis of the hypoiodites generated from 3-hydroxy-A -steroids and lead tetraacetate-iodine, as outlined in Scheme 84. The path of the formation of the oxasteroids is outlined in Scheme 85 one-electron oxidation of the intermediate radicals arising from the cyclization of allylic radicals generates the oxonium ions, which trap acetic acid to give the products. 

The intramolecular cyclization strategy is applied to efficient synthesis of oxa-cycles starting with trimethylsiloxy-containing allylic silanes (Scheme 5.8). Treatment of 30 with benzaldehyde in the presence of a catalytic amount of McjSiOTf and PrOSiMej gives tetrahydropyran 32. An oxonium ion intermediate (31) is considered to be generated first and then undergo intramolecular nucleophilic attack by an allylsilane part [15]. When ortholactones are employed in lieu of aldehydes, spiroketals are readily prepared. 

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