(3S*,4S*,E)-tert-Butyl 3,4-dibromo-5-oxocyclooct-1-enecarboxylate

The title compound, C13H18Br2O3, was prepared by a bromination reaction of (1E,3Z)-methyl 5-oxocycloocta-1,3-dienecarboxylate, which was obtained by an epoxydation reaction of tert-butyl cyclooct-1,3-dienecarboxylate. The crystal structure confirms unequivocally the absolute configuration of both chiral centres to be S. In the crystal, C—H⋯O interactions link the molecules into chains running along the c axis.

The title compound, C 13 H 18 Br 2 O 3 , was prepared by a bromination reaction of (1E,3Z)-methyl 5-oxocycloocta-1,3dienecarboxylate, which was obtained by an epoxydation reaction of tert-butyl cyclooct-1,3-dienecarboxylate. The crystal structure confirms unequivocally the absolute configuration of both chiral centres to be S. In the crystal, C-HÁ Á ÁO interactions link the molecules into chains running along the c axis.

Comment
In our research group there has been an enormous interest in the synthesis of conjugated unsaturated esters used as starting material in the Michael addition of enantiomerically pure lithium amides (Davies et al., 2005) as a base tool in the asymmetric synthesis of β-amino acids and alkaloids because of their interest and value in the development of biologically active compounds for the pharmacology industry (Fülöp et al., 2001). Considering that the reactivity of the cycloocta-1,5diene is very peculiar, highlighting its trend in basic medium to conjugate its double bonds (Huber et al., 1969) because of the greater thermodynamic stability (Huber et al., 1970), is necessary and important to establish the exact structure in this class of unsaturated rings. This conjugation was determined previously for an isomer of compound 1 (tert-butyl cyclooct-1,7-dienecarboxylate) (Garrido et al., 2008) and herein for compound 1 (tert-butylcyclooct-1,3-dienecarboxylate) by R-X spectroscopy of compound 6 which confirms unequivocally its configuration and structure. The crystal was afforded by epoxydation reaction of compound 1 with MCPBA and bromination reaction of compound 4 (Fig. 1).
The crystal contains an unique molecule in the asymmetric unit. The title molecule consists of a ring cyclooctene with two bromine atoms, a carbonyl group and a tert-butoxycarbonyl group as susbtituents. All the bond lengths and angles are within the normal ranges. The Br1-C3 and Br2-C4 bond lengths are 1.956 (4) Å and 1.946 (4) Å, respectively. The bromine atoms at C3 and C4 are nearly coplanar with the cycloctene ring being the Br1-C3-C4-C5 and Br2-C4-C3-C2 torsion angles of 173.6 (3)° and -173.0 (1)°, respectively. In the case of the tert-butoxycarbonyl group at C1 is also coplanar with the cycloctene ring being the O2-C9-O1-C1 torsion angle of 178.2 (7)°. The carbonyl group at atom C5 is twisted with the cycloctene ring being the O3-C5-C4-C3 torsion angle of 123.3 (8)°.
In the crystal structure, molecules are connected by intermolecular C-H···O interactions to form infinite chains running along [001] direction, which seems to be effective in the stabilization of the structure (Table 1).
Synthesis of (3R*,4R*,E)-tert-butyl 3,4-dibromo-5-oxocycloocta-1-enecarboxylate 6. Compound 4 (27.00 mg, 0.12 mmol) was dissolved in CCl 4 (10 ml) and the reaction system was stirred and cooled down at 0°C. After, Br 2 (0.01 ml, 31 mmol) was added and stirred for 30 min, the ice bath was removed and stirred for 4 h at r.t. The reaction mixture was dissolved in DCM (20 ml), washed with HCl 2 N., NaHCO 3 (sat.), H 2 O and NaCl (sat.); dried over Na 2 SO 4 , filtered and concentrated in vacuo. It afforded tert-butyl 3,4-dibromo-5-oxocyclooct-1-enecarboxylate 6 (43.00 mg, 91%) which crys-     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 Rfactors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )