Synthesis of Furan Derivatives. XL. Synthesis of (5-Nitro-2-furyl)-methylated Polyenals

Abstract

Seven (5-nitro-2-furyl)-methylated polyenals (I, n=2 to 3, n is the number of vinyl or propenyl group) were prepared from aldehyde dimethyl acetal (n=1 and 2) by alternate or repeated condensation with methyl vinyl ether or methyl propenyl ether in the presence of boron trifluoride etherate. Excepting the formation of (5-nitro-2-furyl)-2, 4-dimethyl-2-cis-4-trans-2, 4-pentadienal (V), the chain extension reaction gave higher homologs of methylated polyenal with trans formyl group. The chemical evidence showing V must be above configuration was clearly given by the cyclization of its dimethyl acetal to 1-methoxy-2-(5-nitro-2-furyl)-3, 5-dimethyl-cyclopenta-2, 4-diene. From the van der Waals projection diagram of V, it was suggested that 3-S-cis spatial configuration is more favored than 3-S-trans conformation. Nuclear magnetic resonance (NMR) spectrum of V supported this suggestion from its chemical shift of the proton adjacent to 5-nitrofuran ring. In the presence of HX (X=Cl or Br), V was easily rearranged to five membered conjugated ketone, 2-(5-nitro-2-furyl)-3, 5-dimethyl-cyclopent-2-en-1-one. This acid catalyzed ring closure of the cis dienal is a new type of rearrangement, which promises to have some synthetic utility.