2-(1,3-Dithian-2-yl)-1,3-dithiane-2-carbaldehyde

The asymmetric unit of the title compound, C9H14OS4, comprises two crystallographically independent molecules with similar conformations. In each molecule, an intramolecular C—H⋯O hydrogen bond generates a six-membered ring, producing an S(6) ring motif. All of the six-membered dithiacyclohexane rings adopt chair conformations. The crystal structure is stabilized by four intermolecular C—H⋯O and one C—H⋯S interaction.

The asymmetric unit of the title compound, C 9 H 14 OS 4 , comprises two crystallographically independent molecules with similar conformations. In each molecule, an intramolecular C-HÁ Á ÁO hydrogen bond generates a sixmembered ring, producing an S(6) ring motif. All of the sixmembered dithiacyclohexane rings adopt chair conformations. The crystal structure is stabilized by four intermolecular C-HÁ Á ÁO and one C-HÁ Á ÁS interaction.

Comment
Vicinal tricarbonyl compounds are powerful electrophiles with widespread applications in organic synthesis (Rubin & Gleiter, 2000;Wasserman & Parr, 2004). They act as useful precursors to synthesis of elaborate heterocylic compounds and numerous novel biologically important substances such as FK-506, rapamycin and related immunosuppressants. They are also used to develop protease inhibitors derived from peptide carboxylic acids. Thioacetalization of carbonyl compounds (Goswami & Maity, 2008) plays an important role in organic synthesis. Dithioacetals have become widely used tools for C-C bond formation. Here we reported the first synthesis of 2,2'-bis(1,3-dithianyl)-2-carbaldehyde from the smallest vicinal tricarbonyl compound, 2-oxo-1,3-propandial.

Experimental
To a stirred solution of 2-oxo-1,3-propandial (250 mg, 0.34 mmol) and boron trifluoride etherate (0.5 mL) in dichloromethane (50 mL) cooled at 0 °C is added propane dithiol (450 mg, 4.1 mmol) dropwise over 15 min with stirring. The mixture is stirred at room temperature for 3h. The progress of the reaction is monitored by TLC. After completion of the reaction, NaHCO 3 solution is added slowly and carefully to neutralize the mixture at room temperature, which is then extracted with dichloromethane. The organic layer is dried (anhydrous Na 2 SO 4 ) and then the solvent is removed under reduced pressure.

Refinement
All of the hydrogen atoms were positioned geometrically with C-H = 0.93-0.98 Å and refined in the riding model approximation, with U iso (H) = 1.2U eq (C).

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.