Formaldehyde Dialkylhydrazones as Neutral Formyl Anion and Cyanide Equivalents: Nucleophilic Addition to Conjugated Enones
作者:Elena Díez、Rosario Fernández、Consolación Gasch、José M. Lassaletta、José M. Llera、Eloísa Martín-Zamora、Juan Vázquez
DOI:10.1021/jo970481d
日期:1997.7.1
A versatile methodology for the nucleophilic formylation and cyanation of conjugated enones is reported. The procedure is based on the use of formaldehyde dimethylhydrazone, which, acting as a neutral formyl anion equivalent, adds to preformed trialkylsilyl-enone complexes. Both 4-(silyloxy)-3-enal hydrazones 3 or deprotected 4-oxo aldehyde monohydrazones 4 can be obtained as products depending on quenching conditions. In full analogy, an asymmetric version of the reaction using chiral formaldehyde SAMP-hydrazone as a neutral synthon of the chiral formyl anion has been developed, giving rise to the corresponding adducts 5 and 6 in good yields and with excellent diastereoselectivities (de 85-greater than or equal to 98%). Ozonolysis or HCl-mediated hydrolysis of adducts 4 and 6 readily affords racemic and optically enriched 4-oxo aldehydes 7, respectively. Additionally, high-yielding MMPP-oxidative cleavage of 4-oxo hydrazones 4 and 6 has been performed to obtain Li-oxo nitriles 8 in racemic and optically enriched forms, respectively. In this way, interesting chiral bifunctional building blocks, some of them bearing newly created stereogenic quaternary centers, have been efficiently synthesized.