A simple one-pot synthesis of 2,5-functionalized thieno [2,3-b]thiophenes by the condensation of 1,3-diketones with CS 2 and alkylating agents carrying an electron-withdrawing group such as CO 2 Et, COR or CN in the presence of anhydrous potassium fluoride as the condensation promoter in anhydrous DMF is described. The process is mild, convenient and offers reasonable yields.Thieno[2,3-b]thiophenes, 1,2 structurally related to isoelectronic naphthalene and 10π-pentalene dianion are of potential interest as π-electron donors in several current applications. 3 Of the few syntheses described in the literature for thieno [2,3-b]thiophenes, the majority are multistep, proceed in very poor yield, and being targetoriented, offer little or no generality. 4 On the other hand, a two-step synthesis of thienothiophenes based on the Dieckmann type cyclization of the appropriately activated ketene dithioacetals, in spite of its relatively wider use, suffers from variable yields and complications associated with strongly basic conditions, i.e., MeONa, NH 3 (liq)/Na, NaH, LDA etc., employed for accomplishing these cyclizations. 5 Recently, synthesis of a limited number of thienothiophenes under phase-transfer catalysis in benzene has also been reported. 6 In view of the aforementioned drawbacks, we deemed it desirable to develop a milder and more convenient synthetic approach to thieno-[2,3-b]thiophene systems.Fluoride ions as non-nucleophilic bases have been widely exploited for the generation of carbanions and heteroatom based nucleophiles. 7 Recently, we reported the application of commercially available anhydrous KF as the condensation promoter between CS 2 , active methylene compounds and alkylating agents for a practically useful synthesis of ketene dithioacetals. 8 We envisaged that if in these condensations, the alkylating agent, X-CH 2 -Z is endowed with a suitable carbanion stabilizing group (where Z may be COR, CO 2 R, CN etc.), then the resultant ketene dithioacetal intermediate might directly cyclize under the conditions shown in the Scheme to provide thieno[2,3-b]thiophenes in a single step process. Precedence for analogous cyclization using the KF/DMF system can be found in the context of thiazoline to thiazine ring expansions reported earlier by us. 9Thus, in order to test the validity of the reaction given in the Scheme, we initially investigated a reaction between acetylacetone (1), CS 2 and chloroacetone in anhydrous DMF containing an excess of anhydrous KF at room temperature for 8 hours. The workup of the reaction and purification afforded, though in low yield (ca 11%) a single crystalline compound, unequivocally characterized as the desired 1,5-diacetyl-3,4-dimethylthieno[2,3-b]thiophene (6) by analytical and spectral data. The yield of 6 was enhanced to 46% (Lit. 4a yield: 25%) by applying an elevated temperature of 80-100°C for 2 hours (Table 1). Of the several solvents examined, the reaction proved successful in anhydrous DMF; other solvents such as anhydrous dioxane, THF or MeCN were found to be ineffe...