ESCHENMOSER Methylenation Reagent

MANNICH - ESCHENMOSER Methylenation reagent Ammomethylation of activated methyl or methylene groups by in situ formed MeaN -CHR (Mannich) Me2N- -=CH2 X reagent for o methylenation (Eschenmoser)... 

Orthoesters are converted into esters with TMS-I. The dimethyl acetal of formaldehyde, methylal, affords iodomethyl methyl ether in good yield (eq 12) 7a (in presence of alcohols, MOM ethers are formed). 7b a-Acyloxy ethers also furnish the iodo ethers, e.g. the protected 8-acetyl ribofuranoside gave the a-iodide which was used in the synthesis of various nucleosides in good yield (eq 13). Aminals are similarly converted into immonium salts, e.g. Eschenmoser s reagent, Dimethyl(methylene)ammonium Iodide, in good yield. ... 

Scheme 2.12 shows some representative Mannich reactions. Entries 1 and 2 show the preparation of typical Mannich bases from a ketone, formaldehyde, and a dialkylamine following the classical procedure. Alternatively, formaldehyde equivalents may be used, such as l>is-(di methyl ami no)methane in Entry 3. On treatment with trifluoroacetic acid, this aminal generates the iminium trifluoroacetate as a reactive electrophile. lV,A-(Dimethyl)methylene ammonium iodide is commercially available and is known as Eschenmoser s salt.192 This compound is sufficiently electrophilic to react directly with silyl enol ethers in neutral solution.183 The reagent can be added to a solution of an enolate or enolate precursor, which permits the reaction to be carried out under nonacidic conditions. Entries 4 and 5 illustrate the preparation of Mannich bases using Eschenmoser s salt in reactions with preformed enolates. 

Imines are generally less electrophilic than the corresponding aldehyde but this diminished level of activity can be countered by suitable activation with Lewis acids. Formaldehyde imines are generally stable only at low temperatures and are best formed in situ. However, there are exceptions like Eschenmoser s salt 20. This methylene iminium salt is a relatively stable solid that is now commercially available. The utility of this reagent was illustrated in the Mannich reaction of corrin 19 with 20 to generated 21. 

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