Zou Wei, Shao Huawu, Wu Shih-Hsiung
Institute for Biological Sciences, National Research Council of Canada, Ottawa, ON, Canada K1A 0R6.
Carbohydr Res. 2004 Oct 20;339(15):2475-85. doi: 10.1016/j.carres.2004.08.008.
Whereas C-2- and 4-ulopyranosyl compounds (C-2- and C-4-ulosides) can be converted to cyclopentenones under base conditions through beta-elimination and ring contraction, base-initiated beta-elimination of C-glycosyl 2'-aldehydes and 2'-ketones results in the formation of acyclic alpha,beta-unsaturated aldehydes or ketones. By combining both molecular features we synthesized 1-C-(4-ulopyranosyl)-2-oxoalkanes 6, 13, and 20 and investigated their reactions when they were treated with base. Both alpha- and beta-anomers of C-(4-ulopyranosyl)acetaldehydes 6 and 13 underwent a fast intramolecular aldol reaction between the C-5 enolate and 2'-aldehyde to form optically pure 8-oxabicyclo[3.2.1]octanones, which further transformed to 8-oxabicyclo[3.2.1]octenones 14 and 15 by beta-elimination. However, this aldol reaction did not occur when 1-C-(4-ulopyranosyl)propan-2-one 20 was treated with base because of steric hindrance exerted by the additional methyl group. Instead, an alternate C-3 enolization led to beta-elimination and further electro-ring opening to form an acyclic enol, which was then converted through a disrotatory intramolecular aldol cyclization to a cis-substituted cyclopentenone 21.
虽然C - 2 - 和4 - 吡喃糖基化合物（C - 2 - 和C - 4 - 苷）在碱性条件下可通过β - 消除和环收缩转化为环戊烯酮，但C - 糖基2'-醛和2'-酮的碱引发β - 消除会导致形成无环α,β - 不饱和醛或酮。通过结合这两种分子特征，我们合成了1 - C - (4 - 吡喃糖基)-2 - 氧代烷烃6、13和20，并研究了它们在碱处理时的反应。C - (4 - 吡喃糖基)乙醛6和13的α - 和β - 异头物在C - 5烯醇盐和2'-醛之间都发生了快速的分子内羟醛反应，形成光学纯的8 - 氧杂双环[3.2.1]辛酮，该酮通过β - 消除进一步转化为8 - 氧杂双环[3.2.1]辛烯酮14和15。然而，当用碱处理1 - C - (4 - 吡喃糖基)丙酮20时，由于额外甲基产生的空间位阻，这种羟醛反应没有发生。相反，另一种C - 3烯醇化导致β - 消除并进一步开环形成无环烯醇，然后通过一个顺旋分子内羟醛环化反应转化为顺式取代的环戊烯酮21。
