Iodomethylation carbonyl compounds

In the case of the allenyl copper intermediate 724, its alkylation with (iodomethyl)zinc iodide and carbonyl compounds afforded dienes 7251032 (Scheme 189). 

A(, -Dimethyl(methylene)iminium salts have been the most widely used class of preformed iminium salts, mainly due to their applications in the synthesis of a,p-unsaturated carbonyl compounds, normally accomplished by subjecting the /V, -dimethyl Mannich base to quatemarization followed by base-induced elimination. Table 3 outlines various counterion forms of /V, -dimethyl(methylene)iminium salts that have been used in Mannich reactions as well as their synthetic precursors. The crystalline iodide (30), known also as Eschenmoser s salt , has seen the most widespread use and is prepared by thermal fragmentation of (iodomethyl)trimethylammonium iodide or, more conveniently, by a variant of the... 

---