Regio- and stereo-selective bromo(alkoxylation)s of (E )-α-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxymethylene) carbonyl compounds. A route to near-stereopure α-bromo α-dioxymethyl carbonyl compounds †
作者:M. Sum Idris、David S. Larsen、Robin G. Pritchard、Anthony Schofield、Richard J. Stoodley、Peter D. Tiffin
DOI:10.1039/b002749i
日期:——
(E)-4-Methoxymethoxy-3-methylbut-3-en-2-one 17b reacts with NBS in propan-1-ol in a highly regio- and anti-stereo-selective manner to give (3R*,4R*)-3-bromo-4-methoxymethoxy-3-methyl-4-propoxybutan-2-one 18. Compound 10, a relative of the butenone 17b in which the methoxymethyl group is replaced by the 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl unit, undergoes an analogous bromo(propoxylation) reaction with reasonable facial selectivity to give an 86â¶14 mixture of (3R,4R)-3-bromo-3-methyl-4-propoxy-4-(2â²,3â²,4â²,6â²-tetra-O-acetyl-β-D-glucopyranosyloxy)butan-2-one 11c and its (3S,4S)-diastereomer 12c. The major bromo(propoxy) derivative, isolable in 57% yield by fractional crystallisation, is assigned the stereostructure 11c by single-crystal X-ray crystallographic analysis. Other primary alcohols and methanol participate in the reaction of compound 10 with NBS, leading predominantly (with selectivities ranging from 75â¶25 to 89â¶11) to bromo(alkoxy) products of type 11 which are usually separable from their diastereomers of type 12 by fractional crystallisation (in yields ranging from 41 to 64%). A model to account for the role of the 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl unit in the stereoinduction process is proposed.
Related bromo(propoxylation)s are observed with the vinylogous esters 24, 25a and 25b, leading to the isolation of the major products, 28, 30a and 30b (in yields ranging from 39 to 55%), and with the vinylogous carbonates 32a, 32c, 37a and 37b, providing access to the major products 33a, 33c, 38a and 38b (in yields ranging from 52 to 73%).
In the presence of trifluoroacetic acid and ethane-1,2-diol, the bromo(propoxy) derivatives 11c, 28, 30b and 33c undergo transacetalisation to give the ethylene glycol acetals 40a, 40b, 41 and 40c with ees of 94â98%, in yields ranging from 56 to 67%.
p
(E)-4-甲氧基甲氧基-3-甲基丁-3-烯-2-酮 17b 在丙-1-醇中与 NBS 反应,以高度的区域选择性和反式立体选择性方式生成 (3R*,4R*)-3-溴-4-甲氧基甲氧基-3-甲基-4-丙氧基丁-2-酮 18。化合物 10 是丁烯酮 17b 的类似物,其中甲氧基甲基被 2,3,4,6-四-O-乙酰基-β-D-吡喃葡糖基单元取代,在适当的面对选择性下经历类似的溴(丙氧基化)反应,得到 (3R,4R)-3-溴-3-甲基-4-丙氧基-4-(2′,3′,4′,6′-四-O-乙酰基-β-D-吡喃葡糖氧基)丁-2-酮 11c 和其 (3S,4S) 非对映异构体 12c 的 86∶14 混合物。主要溴(丙氧基)衍生物通过分步结晶可分离得到 57% 产率,并通过单晶 X 射线晶体学分析确定其立体结构为 11c。其他一级醇和甲醇参与化合物 10 与 NBS 的反应,主要生成类型 11 的溴(烷氧基)产物,通常可通过分步结晶从类型 12 的非对映异构体中分离(产率范围为 41% 至 64%),选择性从 75∶25 到 89∶11 不等。提出了一个模型来解释 2,3,4,6-四-O-乙酰基-β-D-吡喃葡糖基单元在立体诱导过程中的作用。类似的溴(丙氧基化)反应在乙烯基酯 24、25a 和 25b 中也观察到,主要产物 28、30a 和 30b 被分离(产率范围为 39% 至 55%),乙烯基碳酸酯 32a、32c、37a 和 37b 也观察到类似反应,主要产物 33a、33c、38a 和 38b 被分离(产率范围为 52% 至 73%)。
在三氟乙酸和乙二醇存在下,溴(丙氧基)衍生物 11c、28、30b 和 33c 经历转缩醛反应,生成乙二醇缩酮 40a、40b、41 和 40c,对映体过量为 94% 至 98%,产率范围为 56% 至 67%。