BF3-induced cyclobutane-opening of verbenone and its deconjugate homolog. Efficient preparation of o-mentha-1,8-dien-3-one and o-menth-1-en-3-one in optically active forms

Starting with (+)-verbenone, readily obtainable from (+)-nopinone, enantios elective preparation of (S)-(+)4-isopropenyl-, (S)-(-)-4-isopropyl- and (R) -(+)-4-(1-acetoxy-1-methylethyl)-3-methyl-2-cyclohexen-1-ones was accomplis hed with little loss of stereochemical integrity via BF3-induced cyclobutan e-opening of (+)4-(methylene)nopinone. As we have developed an efficient ch emical transformation of (+)-nopinone into (-)-verbenone, the present synth eses of the above cyclohexenones are formal syntheses of their enantiomers from (+)-nopinone. (C) 1999 Elsevier Science Ltd. All rights reserved.