The preparation of 8-bromokalafungin (20) which is a key intermediate for the
synthesis of C-glycoside containing pyranonaphthoquinone
antibiotics related to medermycin (6) is described. Although attempts to
selectively monobrominate kalafungin (1) at C8 were unsuccessful,
8,10-dibromokalafungin (21) was prepared by using excess
N-bromosuccinimide in chloroform. Selective bromination
at C6 on a naphthalene ring was achieved upon treatment of naphthol (9) with
N-bromosuccinimide (1 equiv.). Conversion of naphthol
(9) into naphthoquinone (15) was effected by methylation,
Baeyer–Villiger oxidation, acetylation via a Fries rearrangement and
oxidation with ceric ammonium nitrate. Conversion of the 7-bromo quinone (15)
into 8-bromokalafungin (20) proceeded through subsequent addition of
2-trimethylsilyloxyfuran (16) followed by oxidative rearrangement of the
resultant furonaphthofuran (17) to furonaphthopyran (18). After reduction of
the lactol (18) to cis ether (19), demethylation and epimerization at C5 with
boron tribromide afforded 8-bromokalafungin (20). 8-Bromokalafungin (20)
failed to undergo Pd(0)-mediated cross-coupling reactions with the stannyl
glucal (22).
8-bromokalafungin (20)是合成含吡喃萘醌的 C-糖苷的关键中间体。
8-bromokalafungin (20) 的制备方法。
8-bromokalafungin (20) 的制备方法。虽然试图
在 C8 处选择性地单溴化卡拉非菌素(1)的尝试并不成功、
通过使用过量的
在氯仿中使用过量的 N-溴代丁二酰亚胺制备出了 8,10-二溴卡拉非因(21)。选择性溴化
用 N-溴琥珀酰亚胺处理萘酚(9)时,在萘环 C6 处实现了选择性溴化。
N-溴代丁二酰亚胺(1 个等量物)。萘酚
(9) 通过甲基化作用转化为萘醌 (15)、
萘酚 (9) 通过甲基化、拜尔-维利格氧化、通过弗里斯重排进行乙酰化和
硝酸铈铵氧化。将 7-溴醌 (15)
转化为 8-bromokalafungin (20)。
2-三甲基硅氧基呋喃(16),然后将生成的呋喃萘醌氧化重排
呋喃(17)氧化重排为呋喃并吡喃(18)。将
内酯(18)还原成顺式醚(19)后,用三溴化硼在 C5 处进行去甲基化和表
三溴化硼进行脱甲基和 C5 处的环化反应,得到 8-溴木犀草素(20)。8-溴卡拉芬净(20)
未能发生钯(0)介导的与丹宁基
葡萄糖醛(22)发生交叉偶联反应。