Asymmetric synthesis of (+)-loline, a pyrrolizidine alkaloid from rye grass and tall fescue
作者:Paul R. Blakemore、Sung-Kee Kim、Volker K. Schulze、James D. White、Alexandre F. T. Yokochi
DOI:10.1039/b103936a
日期:——
(+)-Loline (1) was synthesized via a pathway that employed intramolecular [4 + 2] cycloaddition of an acylnitrosodiene, 25 or 26, as a key step. The acylnitrosodienes, which were used in situ, were obtained by oxidation of the corresponding hydroxamic acids, 17 and 24, and these were prepared from either glucose via aldehyde 9 or more directly from (S)-malic acid (18). The endo dihydrooxazines 27 and 29, obtained in a mixture with their exo stereoisomer, were transformed by reductive N–O bond cleavage and reannulation
into pyrrolizines 34 and 35. The latter was subjected to Sharpless aminohydroxylation in the presence of (DHQD)2PHAL to give 50 along with its regioisomer 51. N-Methylation of tosyl amide 50, followed by mesylation of alcohol 52 and reduction of the γ-lactam 53 with borane, afforded pyrrolizidine 54. Cleavage of the p-methoxybenzyl ether and subsequent thermal treatment of 55 resulted in intramolecular etherification to yield N-tosylloline (57). Final reductive cleavage of the N-tosyl residue produced (+)-loline,
characterized as its dihydrochloride.
(+)-Loline(1)是通过分子内[4 + 2]环加成酰基亚硝基二烯(25 或 26)这一关键步骤合成的。原位使用的酰基亚硝基二烯是通过氧化相应的羟肟酸(17 和 24)获得的,这些羟肟酸可以通过醛 9 从葡萄糖中制备,也可以更直接地从(S)-苹果酸(18)中制备。内型二氢恶嗪 27 和 29 与外型立体异构体混合后,通过还原 N-O 键裂解和再annulation 转化为吡咯烷 34 和 35。后者在 (DHQD)2PHAL 存在下进行 Sharpless 氨基羟化反应,得到 50 及其外立体异构体 51。对甲酰基酰胺 50 的 N-甲基化、醇 52 的甲磺化以及γ-内酰胺 53 与硼烷的还原反应得到了吡咯烷 54。对甲氧基苄基醚的裂解和随后对 55 的热处理导致分子内醚化反应,生成 N-甲基吡咯烷酮(57)。最后对 N-对甲基苯磺酰残基进行还原裂解生成 (+)-loline,其特征为二盐酸盐。