Dehydration of the 5-hydroxycyclohexanecarboxylate 13 gives the
exocyclic alkene 14 rather than its endocyclic isomer. However, the
3,4-double bond can be introduced into precursors of milbemycin E 1
using oxidative elimination of phenylselanyl ketones. The
hydroxycyclohexanones 6 and 31 have been converted into the
phenylselanyl ketones 19 and 37, which on oxidative elimination followed
by stereoselective reduction give the 3-methylcyclohex-2-enecarboxylates
23 and 40 together with only 10â15% of the exocyclic alkenes 24
and 42. Interestingly, if the oxidative elimination is carried out on
the alcohol 25, the 5-methylenecyclohexanecarboxylate 24 is the major
product. Conversion of 40 into its benzoate, and oxidation of the furan
ring using singlet oxygen, gives the hydroxybutenolide 43 ready for
incorporation into a milbemycin synthesis. To test the compatibilty of
the cyclohexene double bond with the proposed Wittig reaction, the
alcohol 40 has been converted into the tert-butyldimethylsilyl
ether 44 and the furan oxidised to give the hydroxybutenolide 45.
Condensation with an excess of (2-methylpropylidene)triphenylphosphorane
gives the Wittig product which has been isolated as its methyl ester and
isomerised using a trace of iodine into the
(Z,E)-diene 47.
5-羟基
环己烷羧酸脱
水13生成外环烯烃14,而非其内环异构体。然而,通过氧化消除苯基
硒酮,可以将3,4-双键引入米尔贝霉素E1的前体中。羟基
环己酮6和31已转化为苯基
硒酮19和37,在氧化消除后,通过立体选择性还原,生成3-甲基环己-2-烯
羧酸酯23和40,以及仅10-15%的外环烯烃24和42。有趣的是,如果对醇25进行氧化消除,则5-
亚甲基环己烷羧酸酯24是主要产物。将40转化为
苯甲酸酯,并使用单线态氧氧化
呋喃环,得到羟基
丁烯内酯43,可用于米尔贝霉素的合成。为了测试
环己烯双键与拟议的维蒂格反应的相容性,将醇40转化为叔丁基二甲基
硅烷基醚44,并氧化
呋喃,得到羟基
丁烯内酯45。与过量的(2-甲基丙亚基)
三苯基膦缩合,得到维蒂格产物,将其分离为甲基酯,并使用微量