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

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).