Oxymercuriation–demercuriation of the methyl esters of communic acids. X-Ray molecular structure of methyl (8R,12R)-8,12-epoxyisopimar-15-en-19-oate

The oxymercuriation-demercuriation (OM–DM) of methyl (E)-communate with mercury(II) acetate yielded mainly the hydration product of the 14,15-double bond. The isopimarane oxide methyl (8R,12R)-epoxyisopimar-15-en-19-oate, the endoperoxide methyl (8S,12R,13S)-13,17-epidioxy-8,12-epoxylabd-14-en-19-oate and the diorganomercurial bis-(4β-methoxycarbonyl-19-norlabda- 8,12E-14-trien-17-yl)mercury were also obtained when NaBH4–NaOH was used as the reducing agent. The organomercurial regenerated methyl (E)-communate on prolonged treatment with NaBH4 in excess. The OM–DM of methyl (E)-communate using Na(Hg) as the reducing agent achieved partial isomerization of the 8(17)-double bond, giving methyl 1 4-hydroxylabda-8,1 2E-dien- 1 9-oate and methyl labda-8,12E,14-trien-19-oate; the oxide methyl (8S,12R)-8,12-epoxylabd-13E-en-19-oate, instead of the aforementioned isopimarene oxide, endoperoxide, and diorganomercurial; relatively important amounts of the product of stereoselective 1,4-addition, methyl (12R)-12-hydroxylabda-8(17),13E-dien-19-oate, and the hydration product of the 8(17)-double bond on the (a priori) most hindered, β side, methyl (8S)-8-hydroxylabda-12E,14-dien-19-oate. Besides these, small amounts of the doublehydration product methyl (8S)-8,14-dihydroxylabd-12E-en-19-oate were also obtained. Finally, methyl (Z)-communate 2 showed similar behaviour to its E-isomer in OM–DM reactions.