Chiral induction in photochemical reactions. Part 4. Conformational analysis and crystal structure of (–)-8-phenylmenthyl [(–)-(1R,2S,5R)-2-(1-methyl-1-phenylethyl)-5-methylcyclohexyl] phenylglyoxylates: their face-differentiating effect in photochemical oxetane formation

Abstract

Depending on the solvent, the observed differences in the ¹³C n.m.r. chemical shifts of the 8,9-methyl groups: Δδ8/9 in (–)-(1R,2S,5R)-2-(1-methyl-1-phenylethyl)-5-methylcyclohexyl esters are used to gain information about the population P_A of conformation A in which the phenyl and ester groups point towards each other. The combination of ¹H n.m.r. high-field shifts with these P_A values provides valuable information about structural details. The generally accepted conformation A results from dipole–dipole-induced interactions. The ester residue is oriented so as to allow for maximum dipole–dipole-induced interaction. If there is no such interaction, one observes conformers B or C. The combined ¹³C and ¹H n.m.r. method described here is generally applicable for 8-phenylmenthyl esters. It is suited to explore the absolute configuration of the product oxetanes as well as the factors responsible for the face-differentiating effects of the chiral alcohol moiety. The crystal structure of (–)-(1R,2S,5R)-2-(1-methyl-1-phenylethyl)-5-methylcyclohexyl phenylglyoxylate has been determined by single-crystal X-ray diffraction techniques using diffractometer data. The crystals are monoclinic with space group P2₁ and lattice parameters a= 11.516(1), b= 10.190(5), c= 9.886(1)Å, β= 114.12(1)°, final R= 0.040. The structure reveals that in this compound the phenylmenthyl moiety is frozen into conformation B. The bond lengths and angles are normal. The torsion angle for the two carbonyl groups is 103.3°.