Ene reactions: evidence that oxabicyclo[3.2.1]octanes, not oxabicyclo-[2.2.2]octanes, are formed from the AlCl3-catalysed reactions of chloral with cyclohexene and the thermal addition of chloral to cyclohexa-1,3-diene

Abstract

The oxabicyclo[3.2.1]octane (3) is formed in the AlCl₃-promoted rearrangement of the ene adduct (1), itself obtained from the AlCl₃-catalysed ene addition of chloral to cyclohexene. This structural assignment, initially made on the basis of mechanistic reasoning, is consistent with the chemical and spectroscopic evidence and is confirmed by an X-ray crystallographic study. Since this compound was previously assigned the oxabicyclo[2.2.2]octane structure (4) on the basis of an ‘unambiguous’ synthesis which utilised the ‘Diels–Alder’ addition of chloral to cyclohexa-1,3-diene, the diene reaction between carbonyl compounds and cyclohexa-1,3-dienes may not be reliable generally for the construction of oxabicyclo[2.2.2]octenes; indeed, the Diels–Alder adduct of chloral and cyclohexa-1,3-diene is obtained only when more than the usual precautions are taken in conducting the (sealed tube) reaction. Mechanisms for the various processes are discussed. Direct methods were employed in the determination of structure (3) from diffractometer data. Crystals are monoclinic, space group P2_1/c with Z= 4 in a unit cell of dimensions: a= 8.527(4), b= 10.244(4), c= 12.176(5)Å, β= 111.12(3)°; the structure was refined by full-matrix least-squares to R 0.0925 for 1 092 independent reflections. The low m.p. of (3) necessitated the mounting of the crystal in a sealed capillary and this, together with its high solubility in most solvents, contributed to the difficulty in obtaining good quality single crystals for the X-ray study.