Synthesis of nucleosides and their related compounds. XII. Menthyl 2,4-dioxo-1,3-oxazine-5-carboxylates: New dienophiles for the asymmetric Diels-Alder reaction directed towards synthesis of carbocyclic C-nucleosides.
作者:Nobuya KATAGIRI、Masatoshi HIROSE、Masayuki SATO、Chikara KANEKO
DOI:10.1248/cpb.37.933
日期:——
2, 4-Dioxo-1, 3-oxazine-5-carboxylic acid esters prepared by thermal reaction of 4-oxo-1, 3-dioxin-5-carboxylates with isocyanates were found to react with cyclopentadiene either in the presence or absence of suitable Lewis acid catalysts under quite mild conditions. Thus, reaction of methyl 3-substituted 2, 4-dioxo-1, 3-oxazine-5-carboxylates with cyclopentadiene either at room temperature or in the presence of a catalytic amount of titanium tetrachloride at -15°C gave the corresponding adducts in almost quantitative yield. The 5-phenyl derivative was converted to a carbocyclic C-uncleoside precursor through reductive retrograde aldol reaction (RRA reaction) as a key step. This titanium tetrachloride-catalyzed Diels-Alder reaction, when applied to l-8-phenylmenthyl 2, 4-dioxo-1, 3-oxazine-5-carboxylate, afforded the corresponding adduct in high diastereomeric excess (95% for the major endo adduct).
通过热反应将4-氧代-1, 3-二噁烯-5-羧酸酯与异氰酸酯制得的2, 4-二氧代-1, 3-恶嗪-5-羧酸酯,在适当的路易斯酸催化剂存在或缺失的情况下,发现能够在较温和的条件下与环戊二烯反应。因此,甲基3取代的2, 4-二氧代-1, 3-恶嗪-5-羧酸酯与环戊二烯的反应,无论是在室温下还是在-15°C下存在催化量的四氯化钛的情况下,几乎定量地生成相应的加成物。5-苯基衍生物经过还原逆向醛缩反应(RRA反应)转化为一个碳环C-核苷前体,这一步骤是关键。当将这种以四氯化钛催化的达尔斯-阿尔德反应应用于1-8-苯基薄荷基2, 4-二氧代-1, 3-恶嗪-5-羧酸酯时,获得了相应的加成物,显示出较高的立体异构体过量(主要内端加成物为95%)。