Pummerer intermediate

DFIT-induced fluoro-Pummerer reactions of N-substituted-2-(phenylsul-fanyl)acetamides have also been reported (Scheme 37) [107]. With some amides, cyclization of the Pummerer intermediate was competitive with fluorination, and in other cases, fluorination was preempted by sulfoxide formation. 

Participation by aromatic rings is also possible and there are now several examples of electro M]ic aromatic substitution involving Pummerer intermediates. Equation (20), the alkylation of benzene with dimethyl sulfoxide in trifluoroacetic anhydride, illustrates the process in its inq>lest foim. As widi al-kenes, reaction with aromatics has been more widely exploit in intramolecular versions for the construction of carbocycles and heterocycles. In many cases the sulfoxide precursor is of the keto variety, thus ensuring regiospecificity in the point of cyclization. Equation (21) (formation of a six-monber carbocycle), equation (22) (formation of a six-membered sulfur heterocycle), equation (23) (formation of a six-membered nitrogen heterocycle) and equation (24) (formation of a seven-membered nitrogen, sulfur heterocycle) provide illustrations of the versatility of diis form of intramolecular aromatic alkylation. 

Use of trimethylsilyl triflate to bring about Piunmeier rearrangement requires the presence of a base such as a tertiary amine (vide supra equations 15 and 26). In some instances, involving attempts to alkylate Pummerer intermediates with silyl enol ethers under such conditions, the base has been found to compete as a nucleophile. In the absence of the silyl enol ether, amine addition can be very efficient. For example, treatment of methallyl phenyl sulfoxide with diisopropylethylamine and trimethylsilyl inflate in dichloromethane (equation 29) at 0 C yields the ammonium triflate indicated in 91% yield. Other tertiary amines which undergo this reaction include niethylamine and Af,Af-diethyltrimethylsiI-amine. In the latter case with allyl phenyl sulfoxide as the substrate and a mildly acidic wotk-up, the Mannich derivative shown in equation (30) can be obtained in 90% yield. ... 

In the context of natural product synthesis, sulfide 931 was oxidized with Stang s reagent to a Pummerer intermediate that underwent sequential cyclizations to the tetracyclic product 932 (Scheme 225). The latter was further transformed into the sponge metabolite dibromophakellstatin 933 under oxidative hydrolysis conditions <20050L929>. 

Isomunchnones can also be generated from the Pummerer intermediate 98 which was derived from imidosulfoxide 97. This methodology has been utilized in a formal synthesis of anagyrine (Scheme 22) <1999JOC2038>. 

Under similar conditions a-phenylthio esters 9 afford fluorides 10 (Scheme 3.5) [25]. The mechanism of this Pummerer-type reaction involves initial nucleophilic addition of the sulfur atom to the electrophilic iodine center to form the iodosulfonium salt 11. The liberated fluoride anion acts as a base with resultant formation of the classical Pummerer intermediate 12. Subsequent trapping of cation 12 with fluoride anion yields the final product 10 (Scheme 3.5) [25]. 

The literature on Pummerer chemistry is quite extensive, and multiple variants are described. However, looking deeply into these alternatives, it becomes apparent that most of them are based on the same principle, and the only variation is the nucleophile source. One example of this is the so-called fluoro-Pummerer reaction, which is in fact a fluoride addition to the normal Pummerer intermediate. The distinguishing feature is the generation of the thionium intermediate, using (diethylamino)sulfur trifluoride (DAST) in combination with metals (Scheme 20.391. DAST is known as a source of nucleophilic fluoride and has been widely used for this purpose. A review on the fluoro-Pummerer reaction was published by Haufe and coworkers in 2012. ... 

Pummerer intermediates obtained from sulphoxides on treatment with trifluroacetic anhydride/trifluoroacetic acid were trapped by... 

C protonation generates eiectro-phiie (Pummerer intermediate)... 

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