Oxygen-containing bicyclic monoterpenes. 1H, 13C and 17O NMR Spectroscopic and X-ray diffraction studies of seven oxidation products of (+)-3-carene

Abstract

Seven oxidation products of (1S,6R)-(+)-3,7,7-trimethylbicyclo[4.1.0]hept-3-ene [naturally abundant terpene, (+)-3-carene)]1; (1S,6R)-3,7,7-trimethylbicyclo[4.1.0]hept-3-en-2-one (3-caren-2-one)2, (1S,6R)-3,7,7-trimethylbicyclo[4.1.0]hept-3-en-5-one (3-caren-5-one)3, (1S,6R)-3,7,7-tri-methylbicyclo[4.1.0]hept-3-ene-2,5-dione (3-carene-2,5-dione)4, (1S,3S,4R,6R)-(+)-3,7,7-trimethylbicyclo[4.1.0]heptene 3,4-trans-oxide (trans-3,4-epoxy-3-carene)5, (1S,3R,4R,6R)-3,7,7-trimethylbicyclo[4.1.0]heptane-3-exo-4-endo-diol (carane-3-exo-4-endo-diol)6 and (1S,2R,4R,5R)-1-methyl-4-exo-(1-hydroxy-1-methylethyl)bicyclo[3.1.0]hexan-2-endo-ol 7 and (1S,6R)-3-endo-7,7-trimethylbicyclo[4.1.0]heptan-4-one (trans-4-caranone)8 have been obtained by oxidation with tert-butylchromate, selenium dioxide, hydrogen peroxide and peracetic acid. The ¹H, ¹³C and ¹⁷O NMR spectra of the purified oxidation products have been recorded and assigned. In addition to C,H-COSY spectra, the ¹J(C,H) coupling constants were especially useful in ¹³C NMR spectral assignment. The differentiation between isomeric ketones 2 and 3 is based on a clear difference in the ¹³C NMR shifts of the double bond methyl. The stereochemical structure elucidation of oxide 5 is based on lanthanide shift reagent [Eu(dpm)₃] induced effects. For ketone 8, molecular mechanics (MM) calculations and comparison of experimental and theoretical ³J(H,H) coupling constants are needed for a final structure elucidation. The assignment of ¹⁷O NMR lines of diols 6 and 7 is based on literature values. The crystal structures and absolute configurations of pure enantiomers of diols 6 and 7 have been determined by X-ray diffraction. Crystal data: a= 7.659(2), b= 10.804(3), c= 25.509(4)Å, orthorhombic, space group C 222₁, Z= 8 (6) and a= 8.076(4), b= 8.836(2), c= 12.487(3)Å, orthorhombic, space group P2₁2₁2₁, Z= 4 (7).