d-Galactal 1 reacts in THF in the presence of catalytic amounts of concentrated sulfuric acid to give (2R)-2-hydroxy-6,8-dioxabicyclo[3.2.1]oct-3-ene (4) in a Ferrier-type rearrangement in 40% yield. When 4 is treated with diethylaminosulfur trifluoride (DAST) under certain reaction conditions, a novel intramolecular second order allylic rearrangement follows leading to previously unknown diastereomeric monofluoro derivatives 7 and 8. Direct α-substitution of 4 by DAST affords the 2-monofluoro derivative 6 under kinetic control. When DAST is used with dimethylformamide as solvent an apparent [1,3] sigmatropic migration of the 2-hydroxy group of 4 gives (4S)-4-hydroxy-6,8-dioxabicyclo[3.2.1]oct-2-ene (9). The corresponding 4-fluoro-substituted product was not obtained from the reaction of 9 with DAST. Oxidation of 4 and repeated treatment with DAST gives a novel 2,2-difluoro compound 11. Electrophilic addition of bromine to protected 4 afforded the 1,6-anhydromonobromo- and -dibromohexopyranoses 15-17. Monohalo and monopseudohalo derivatives (-F, -Cl, -Br, -N3) 22-25 were prepared by nucleophilic oxirane ring opening of the easily available endo-epoxides 18-20.
d-半
乳糖1在
四氢呋喃(THF)中,存在催化量浓
硫酸的情况下反应,得到(2R)-2-羟基-6,8-二氧双环[3.2.1]八-3-烯(4),产率为40%。当4在特定反应条件下与二乙
氨基
硫三
氟化物(
DAST)反应时,出现了一种新颖的分子内二级顺式重排,生成之前未知的非对映体单
氟衍
生物7和8。4与
DAST的直接α取代在动力学控制下生成2-单
氟衍
生物6。当
DAST与二甲基甲酰胺作为溶剂使用时,4的2-羟基明显发生[1,3] σ热迁移,得到(4S)-
4-羟基-6,8-二氧双环[3.2.1]八-2-烯(9)。从9与
DAST的反应中未获得相应的4-
氟取代产物。4的氧化以及重复与
DAST的处理生成一种新颖的2,2-二
氟化合物11。
溴对保护的4的亲电加成反应得到1,6-脱
水单
溴-和双
溴六糖(15-17)。通过易得的内端
环氧化物18-20的亲核开环反应制备了单卤和单伪卤衍
生物(-F, -Cl, -Br, -N3)(22-25)。