作者:Patrick G. Steel、Eric J. Thomas
DOI:10.1039/a605892b
日期:——
Treatment of 2-methylpropanal with the
(E)-but-2-enyl(diisopinocampheyl)borane 9 prepared
from (+)-α-pinene gives the anti- and
syn-products 10 and 11, ratio 88â¶12, from which
the major anti-isomer 10 is separated by preparative
GLC. Hydroborationâoxidation of its
tert-butyldimethylsilyl ether 14 gives the primary
alcohol 15 which has been converted into the bromide 16 and iodide 17.
The propenyl(diisopinocampheyl)borane 23 prepared from
(-)-α-pinene reacts with the aldehyde 22 prepared from
(S)-malic acid to give the anti- and
syn-1,3-diol derivatives 24 and 25, ratio 86â¶14,
and the anti-product 24 has been taken through to the
epoxide 31. Sequential alkylation of 1,3-dithiane with the iodide 17 and
the epoxide 31 gives the 2,2-dialkyl-1,3-dithiane 33 which is converted
into the spiroacetal 4 by treatment with dilute aqueous hydrogen
fluoride. After protection, ozonolysis gives the aldehyde 43 which has
been condensed with the ylide 34 to give the α,β-unsaturated
ester 44. This has been reduced and converted into the iodide 46 which
has been used to alkylate the chiral oxazolidinone 39 to give the
required C(11)âC(25) fragment 48 of milbemycin E 1 after reductive
removal of the chiral auxiliary. This has been converted into the
phosphonium salt 2 ready for Wittig coupling with the hydroxybutenolide
3 for the assembly of the milbemycin nucleus.
将2-甲基丙醛与由(+)-α-松节油制备的(E)-丁-2-烯基(二异匹诺卡啉)硼烷9反应,得到反式和顺式产物10和11,比例为88∶12,其中主要的反式异构体10通过制备气相色谱法分离。其叔丁基二甲基氟硅醚14经过氢硼化–氧化反应得到初级醇15,进一步转化为溴化物16和碘化物17。由(-)-α-松节油制备的丙烯基(二异匹诺卡啉)硼烷23与由(S)-苹果酸制备的醛22反应,得到反式和顺式1,3-二醇衍生物24和25,比例为86∶14,反式产物24进一步转化为环氧化物31。1,3-二硫烷与碘化物17和环氧化物31进行顺序烷基化,得到2,2-二烷基-1,3-二硫烷33,通过稀氢氟酸处理转化为螺环醚4。保护后,臭氧解离反应得到醛43,随后与醇盐34缩合得到α,β-不饱和酯44。该酯经过还原反应转化为碘化物46,并用于烷基化手性噁唑烷酮39,得到米尔比霉素E 1所需的C(11)–C(25)片段48,经过还原去除手性辅助基后转化为磷盐2,为与羟基丁内酯3进行韦蒂格耦合以组装米尔比霉素核做好准备。