(2R,4S,5S)-5-Methoxy-4-methyl-3-oxohept-6-en-2-yl benzoate

The title compound was synthesized in the course of the total synthesis of fusaequisin A in order to verify and confirm the configurations of the stereogenic centers and to exclude the possibility of epimerization during the methylation process. The absolute configuration was determined by anomalous dispersion and agrees with the configuration of the allylic alcohol used in the synthesis.

The title compound, C 16 H 20 O 4 , was synthesized in the course of the total synthesis of fusaequisin A in order to verify and confirm the configurations of the stereogenic centers and to exclude the possibility of epimerization during the methylation process. The crystal structure of the title compound at 100 K has orthorhombic (P2 1 2 1 2 1 ) symmetry. The absolute configuration was determined by anomalous dispersion and agrees with the configuration of the allylic alcohol used in the synthesis.

Structure description
The title compound, C 16 H 20 O 4 (Fig. 1), was obtained during the synthesis of the Western fragment of fusaequisin A. Background to fusaequisin A is given by Shiono et al. (2013). The asymmetric synthesis of the Western fragment is based on Paterson's anti aldol chemistry (Paterson et al., 1994;Paterson, 1998). In the course of the total synthesis of curvicollide C (Che et al., 2004) the precursor of the title compound (I) was prepared (von Kiedrowski et al., 2017) and provided potential for further investigations regarding the total synthesis of fusaequisin A. The methylation process is shown in Fig. 2.
The title compound crystallizes in the orthorhombic space group P2 1 2 1 2 1 with four molecules in the unit cell with H1A and H3A almost in plane (H1A-C1Á Á ÁC3-H3A pseudo torsion angle = À1 ) and H2A and H3A in an antiperiplanar arrangement (H2A-C2-C3-H3A = 179 ), which minimizes 1,3-allylic strain. Furthermore, the C8 methyl group and the O1 atom of the ether group are also in an antiperiplanar arrangement with a C8-C4-C3-O1 torsion angle of 177.32 (10) . The ester moiety shows the most stable and expected s-cis-conformation. In the crystal, a weak C-HÁ Á ÁO interaction arising from the aromatic C-H grouping para to the side chain links the molecules into C(10) chains propagating in the [010] direction (Table 1).

data reports Synthesis and crystallization
The reaction (Fig. 3) was carried out under an argon atmosphere. To an ice-cooled solution of the allylic alcohol (C 15 H 18 O 4 , 262.31 g mol À1 , 300 mg, 1.10 mmol, 1 equiv.) in CH 2 Cl 2 were successively added dried (0.1 mbar, 250 C, 2 h) 3 Å molecular sieves (200 mg), 1,8-bis(dimethylamino)naphthalene (proton sponge 1 , C 14 H 18 N 2 , 214.31 g mol À1 , 943 mg, 4.40 mmol, 4 equiv.) and trimethyloxonium tetrafluoroborate (Me 3 OBF 4 , C 3 H 9 BF 4 O, 147.91 g mol À1 , 651 mg, 4.40 mmol, 4 equiv.). The opaque, orange solution was warmed to room temperature. The reaction mixture was stirred at room temperature for 4 h and was then diluted by the addition of aqueous phosphate pH 7 buffer. The phases were separated and the aqueous layer was extracted three times with CH 2 Cl 2 . The combined organic layers were dried (MgSO 4 ) and all volatiles were removed under reduced pressure. The light yellow residue was purified by flash chromatography (cyclohexane-ethyl acetate, 20:1 to 10:1) to afford the title methyl ether (I) (C 16 H 20 O 4 , 276.33 g mol À1 , 238 mg, 0.86 mmol, 78%) as a white solid. Colourless crystals of I suitable for X-ray crystallographic analysis were obtained under air by slow evaporation from the mixed solvents of diethyl ether and npentane.  Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
Reaction conditions for the methylation of the allylic alcohol.

Figure 2
Methylation of O-desmethylfusaequisin A.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
The TU Dortmund and the DFG are gratefully acknowledged for financial support.