Pummerer reaction, silicon induced

The use of chiral sulfoxides to transfer the chirality from sulfur to the a carbon has been investigated, and the high asymmetric induction observed in chiral acyclic sulfoxides using a silicon-induced Pummerer-typc reaction is noteworthy [249]. 

The Pummerer reaction, whose key step is a [2,3]-sigmatropic rearrangement, has never been observed to lead to efficient transfer of chirality starting from chiral sulfoxides in the presence of acetic anhydride [1632, 1633], A modification via silyloxysulfides, generated with O-methyl-OTBDMS ketene acetal at 65°C, allows asymmetric silicon-induced Pummerer reaction from chiral sulfoxides 10.29 with a high chirality transfer [1634] (Figure 10.11). The ( S)-sulfoxides generate the (5)-secondary ethers and vice-versa. 

The second set of examples involves the use of thionium ions as electrophiles in inter- and intramolecular processes to obtain a-substituted sulfides (see 24 25, Scheme 20.7T which is the most common type of Pummerer reaction. Applications of this classical Pummerer rearrangement are exemplified in the synthesis of trans-solamin, the synthesis of indolizidine alkaloids, and the synthesis of the CDE ring of erinacine E. The first exanple fScheme 20.10 uses Pummerer chemistry in the generation of a thionium ion, which reacts in an intermolecular tin-mediated ene reaction the second one fScheme 20.11 uses Pummerer chemistry to introduce a nitrogen-containing heterocycle by intramolecular addition to form the coniceine core and the third example fScheme 20.12 is an intramolecular silicon-induced Pummerer reaction with oxygenated nucleophiles applied to the synthesis of a precursor of erinacine. Details of these Pummerer-based strategies are discussed below. 

Additionally, Scheme 20.11 shows the versatility of Pummerer products. Under classical Pummerer conditions, heterocyclic conpounds behave as good nucleophiles, including for the production of tertiary bicyclic lactams 44. Kuhakarn et used a silicon-induced Pummerer reaction (this activation method will be explained in more detail later in this section) to generate the thionium intermediate, which was intramolecularly trapped by the lactamic nitrogen. Even when a mixture of S-stereoisomers was used, the reaction proved to be diastereoselective, and the lactam sulfide 44 was obtained as 5 1 mixture of trans cis products. Subsequent transformations led to the synthesis of a variety of bicyclic alkaloids 45-48 starting from the same sulfide lactam intermediate 44. 

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