2-Isopropyl-2-(6-methoxy-1,3-benzothiazol-2-yl)-5,5-dimethyl-1,3-thiazolidin-4-one

The title compound, C16H20N2O2S2, crystallizes with two enantiomers (A and B) in the asymmetric unit. The most noticeable difference between these two molecules is the relative orientation of the benzothiazole rings, with S—C—C—S torsion angles of −19.4 (2) (molecule A) and 100.6 (1)° (molecule B). The amide structure of the thiazolidinone rings leads to intermolecular hydrogen-bonded dimers of the R and S enantiomers.


Comment
Luciferin, especially the class which is produced by the firefly Photinus pyralis, is of particular interest because of its bioluminescence and chemoluminescence properties (Naumov et al., 2009). Dimethyloxyluciferin, one prominent derivative which is known for its ability to emit visible light in the red region (Branchini et al., 2002), was further investigated in our group focusing on the modification on the 4-position of the thiazoline ring (Würfel, 2012). An extension of the chromophore should give rise to new dimethylluciferin derivatives with altered absorption and emission properties. The nucleophilic attack of isopropylmagnesium bromide with dimethyloxyluciferin should lead to a tertiary alcohol at the 4-position of the thiazoline ring. Subsequent dehydration reaction should form a 2-propylene substructure, thus representing a carbon extended luciferin derivative.
However, the dimethyloxyluciferin derivative did not react in this expected manner. The strong carbon nucleophile exclusively attacked the 2-position of the thiazoline ring leading, after aqueous work-up, to a racemic mixture of R,Sthiazolidines. C8 became an sp 3 carbon (C8A-C1A = 1.524 (2) Å and C8B-C1B = 1.520 (2) Å), which results in the loss of the conjugation with the benzothiazole parent moiety. The thiazolidine rings are almost coplanar, with a dihedral angle of 10.32 (4)°, due to a dimer formation of the (R)-and (S)-enantiomers in the asymmetric unit. The dimer structure results from two hydrogen bonds between the amide moieties of the thiazolidine rings [N(A)-H···O(B) = 2.942 (2) Å and N(B)-H···O(A) = 2.802 (2) Å] from the two symmetry-independent molecules A and B. The most noticeable difference between these two molecules is the relative orientation of the benzothiazole moiety due to rotation around the C1-C8 bond. The resulting torsion angles S1-C1-C8-S2 are -19.4 (2)° (molecule A) and 100.6 (1)° (molecule B).

Experimental
All chemicals were synthesized according to given literature or purchased from commercial sources. All solvents were purified and dried according to Armarego & Chai (2009). 215 mg (8.85 mmol) of magnesium turnings in 20 ml of dry diethylether and a catalytic amount of iodine are placed in a 100 ml two-necked round-bottomed flask. 0.9 ml (9.60 mmol) 2-bromopropane was added. After a slight exothermic reaction, the mixture was refluxed for 1 h then cooled to room temperature. To that mixture 1.73 g (5.90 mmol) of 2-(6-methoxybenzothiazol-2-yl)-5,5-dimethylthiazolin-4-one in 20 ml of dry THF was added. The mixture was refluxed for 2 h, cooled to room temperature and hydrolysed with 10 g of ice and 10 ml of saturated NH 4 Cl solution, then extracted with ethyl acetate (3 × 20 ml). The extract was dried over MgSO 4 , filtered and distilled off. The remaining solid was purified by crystallization from n-heptane/ethyl acetate, yield: 70%, 1.43 g (4.25 mmol). 2-(6-Methoxybenzothiazol-2-yl)-5,5-dimethylthiazolin-4-one was synthesized from 2-cyano-6-methoxybenzothiazole and ethyl 2-mercapto-2-methylpropanoate according to Würfel (2012). Light-yellow single crystals were obtained by dissolving the title compound at reflux temperature in n-heptane/ethyl acetate and, after cooling to room temperature, left alone in a closed vessel for several days. Elemental analysis, calculated for C 16 H 20 N 2 O 2 S 2 : C 57.11, H 5.99, N 8.33, S 19.06%; found: C 57.25, H 6.06, N 8.46, S 19.14.

Refinement
All H atoms were located from difference Fourier maps and freely refined.   Crystal packing, viewed along a axis, showing hydrogen bonding between molecules A and B drawn as dotted lines.

Figure 3
The formation of the title compound.

Crystal data
Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.