(9-beta)-Pimara-7,15-diene (3), a proposed intermediate in the biosynthesis of the momilactone phytoalexins (1 and 2) from rice, and its C-13 epimer, (9-beta)-isopimara-7,15-diene (4), were synthesized from methyl pimara- and isopimara-8,15-dien-18-oates (8b and 8a, respectively). Allylic oxidation of 8a and 8b as well as the derived diterpene hydrocarbons 15a and 15b with chromium trioxide-dipyridine complex afforded 8,15-dien-7-ones 9a, 9b, 16a, and 16b (35-54%). Lithium-ammonia reduction of 9a, 16a, and 16b gave predominantly trans,anti,trans-isopimara- and -pimara-15-en-7-ones 10, 17a, and 17b. In contrast, catecholborane reduction of the tosylhydrazones of 9a and 9b provided methyl (9-beta)-isopimara- and (9-beta)-pimara-7,15-dien-20-oates (23a and 23b) having the 9,10-syn stereochemistry. The parent diterpenes, 3 and 4, were obtained by carboxyl-to-methyl conversions. In a collaborative investigation 3 was tentatively identified as one of five diterpene hydrocarbons produced upon incubation of (E,E,E)-geranylgeranyl pyrophosphate with a crude enzyme extract from UV-treated rice plants.
(9-β)-坝沙烯-7,15-二烯(3)及其C-13表式异构体(4)是从
水稻中产生的植保素类木菠萝烯(1和2)
生物合成中的一个中间体。3和4从甲基
巴沙和异
巴沙-8,15-二烯-
18酸(8b和8a)中合成。
铬酸-二
吡啶配合物对8a和8b以及它们的二烯烃15a和15b进行邻位氧化得到8,15-二烯-7-
酮类化合物-9a、9b、16a和16b(35-54%产率)。用液
氨-
锂对9a、16a和16b进行还原,主要产率获得trans,anti,trans-异
巴沙和
巴沙-15-烯-7-酮(10,17a和17b)。相比之下,使用
邻苯二酚硼烷还原9a和9b的犀角腙,得到具有9,10-顺式立体结构的甲基(9-β)-异
巴沙-7,15-二烯-20酸和甲基(9-β)-
巴沙-7,15-二烯-20酸(23a和23b)。通过
羧酸基团转化为甲基基团得到这两种二烯烃的亲本化合物。合作研究发现,3是用紫外处理过的
水稻粗酶提取液培养(E,E,E)-瑞香
龙脑二磷酸酯后产生的五种二烯烃中的一种。