(1R,2R,3R,4S,5S)-3-Methyl-8-oxabicyclo[3.2.1]oct-6-ene-2,4-diyl diacetate

The molecule of the title compound, C12H16O5, has crystallographically imposed mirror symmetry with the mirror plane passing through the endocyclic O atom and the mid-point of the double bond. In the crystal, molecules are linked by C—H⋯O hydrogen bonds, forming chains running along the a axis.

Determination of the relative stereochemistry of compound (4) (Fig. 2) by NMR methods was ambiguous so recourse was made to X-ray crystallography for which purpose the crystalline diacetate (I) was synthesized.
Molecule (I) has crystallographically-imposed mirror symmetry with the mirror plane, m, passing through atoms C3, C8, the endocyclic oxygen O8 and the midpoint of the double bond C6/C6 i (i: x,1.5 -y, z). The 6-membered ring of the molecule adopts a chair conformation, with atoms O8 and C3 displaced out of plane defined by the atoms C2/C2 i /C1/C1 i (plane 1) by 0.856 (2) and -0.525 (2) Å, respectively. The carbon atom of the methyl-group and atoms C6,C6 i (double bond) are displaced out of plane 1 by -2.028 (2) Å and -1.326 (2) Å respectively. The molecules (I) are linked by means of weak C-H···O hydrogen bonds to form chains running along a axis.

supplementary materials sup-2
To a solution of 0.4 g (2.1 mmol) of diketone (2) in 4 ml of glacial acetic acid was added 0.23 g of Zn-powder. After the beginning of heating-up the mixture was stirred at 298 K for 0.5 h and 20% aqueous NaOH was added dropwise until solid formed. The solid was dissolved by addition of 1 ml 0.1 N HCl, the mixture was extracted with CH 2 Cl 2 (5*20 ml), the combined organic layers were dried over Na 2 SO 4 and evaporated under reduced pressure to yield 0.26 g of compound (3) as a pale yellow crystalline solid, (compound 3) mp 359-361 K.
To a solution of 200 mg (1.3 mmol) of diketone (3) in 15 ml of MeOH 340 mg (8.9 mmol) of NaBH 4 was added portionwise for 3 h, during which time the reaction temperature was kept between 293-303 K. The mixture was stirred at 298 K for 2 h, then 0.4 g of crystalline NH 4 Cl was added followed by 0.5 ml of 1 N HCl. The solvents were evaporated under reduced pressure, the residue was flashed by EtOAc/Me 2 CO (1:1) through a silica gel column to give 0.2 g of a yellow oil.
The 1 H and 13 C NMR spectra showed that the oil contained several isomers so it was purified by column chromatography on silica gel eluting with a EtOAc/Me 2 CO (5:2) mixture to afford 95 mg of the major isomer (4) as a colorless oil, yield 46%; Rf = 0.5 (Et 2 O/Me 2 CO = 3:1).

) (I).
Compound (4) (95 mg, 0.61 mmol) was dissolved in 5 ml of py and 3 ml of Ac 2 O was added. The mixture was stirred for 24 h at room temperature and then concentrated under reduced pressure (1 mm Hg) to give a thick brown oil. Diacetate (I) was separated by flash chromatography on silica gel (CH 2 Cl 2 ) as colorless crystals, yield 134 mg (92%), mp 428-430 K, Rf = 0.4 (CH 2 Cl 2 ). Crystals suitable for diffraction analysis were obtained by slow evaporation of solvents from a dichloromethane-hexane solution.