Synthesis of Acyclic Ketones by Catalytic, Bidirectional Homologation of Formaldehyde with Nonstabilized Diazoalkanes. Application of a Chiral Diazomethyl(pyrrolidine) in Total Syntheses of Erythroxylon Alkaloids

This work offers a catalytic approach to convergent ketone assembly based upon formal and tandem C–H insertion of diazoalkanes in the presence of limiting amounts of monomeric formaldehyde, which is easily generated as a gas by thermolysis of the inexpensive and abundant paraformaldehyde (∼30 USD/kg). The method forms di-, tri-, and even tetrasubstituted acetones with high efficiency, and it has streamlined a synthesis of (−)-dihydro­cuscohygrine in which the absolute stereochemistry of a proline-based starting material is preserved. Assisted by the advent of new protocols for hydrazone oxidation, we also provide full details on handling non-carbonyl-stabilized diazo compounds.