Short and Efficient Synthesis of Cyclopentacetaldehyde and Its Dialkylated Product

Recently, another route was reported, where cyclopenta[2,1-b;3,4-b']dithiophen-4-one (2) was reduced according to a Huang-Minlon modification of the Wolff-Kischner procedure (equation 2).4 The reaction condition for the Wolff-Kischner reduction was harsh; treatment of slurry of 2 in ethylene glycol with potassium hydroxide and hydrazin hydrate for 8 h at 180 .deg. C. Due to the slurry nature of the reaction medium and harsh conditions, the reported yield (65%) has not proved reproducible, instead fluctuating in lower values. Furthermore, synthesis of 2 is not straightforward, hampering a large scale synthesis, although some steps have been improved (equation 2). Cyclopentacetaldehyde (CDT) is a key building block for the construction of a low band gap conjugated polymers, which has recently attracted much attention as a base material for heterojunction solar cells.1 Zirconium and titanium complexes of cyclopentadienyl fused with a thiophene unit are also relevant in the development of a homogenous Ziegler catalyst. The first synthesis of CDT was reported four decades ago (equation 1). The synthetic route was rather lengthy and required use of hazardous chemicals such as lithium aluminum hydride and Br2. Furthermore, the overall yield and atom economy for equation 1 were very low (theoretical atom economy, 8.0%; practical atom economy, 1.2%, overall yield, 15%), hampering a large scale synthesis of CDT by this route