The scope of the segment-coupling Prins cyclization has been investigated. The method is outlined in Scheme 1 and involves esterification of a homoallylic alcohol (1), reductive acetylation to give the alpha-acetoxy ether (3), and cyclization on treatment with a Lewis acid to produce a tetrahydropyran (4). Alkene geometries dictate the product configurations, with E-alkenes leading to equatorial substituents and Z-alkenes leading to axial substituents (Table 1). Not unexpectedly, applying the method to allylic alcohols leads to fragmentation rather than a disfavored 5-endo-trig cyclization. Dienols in which one alkene is allylic and the other alkene is homoallylic cyclize efficiently and produce the tetrahydropyrans 49-54, Table 3. Dienols with two homoallylic alkenes cyclize with modest to high regioselectively, generating tetrahydropyrans 40-45, Table 2. The relative rates for cyclization decrease in the order of vinyl > Z-alkene > E-alkene > alkyne. The configurations of the products are consistent with cyclization via a chair conformation, Figure 1. The 2-oxonia Cope rearrangement may be a factor in the regioselectivity of diene cyclizations and in the erosion of stereoselectivity with Z-alkenes. This investigation establishes the stereoselectivity and regioselectivity for a number of synthetically useful segment-coupling Prins cyclizations.
Dendritic effects on both the enantioselectivity and diastereoselectivity of the direct aldol reaction were observed for pyridine-2,6-dicarboxamide dendrons terminated with L-prolinamides.
Isomeric “compact” and “expanded” dendrimers functionalized with L‐prolinamide catalytic units at the periphery were compared as catalysts to monomer controls in the organocatalytic direct aldol condensation. A positive dendritic effect that amplifies the stereoselectivity of the direct aldol condensation was observed for dendrimers 3 and 4, compared with lower molecular weight catalysts L‐prolinani‐lide 1 and G1 dendron 2. The difference in the compactness between 3 and 4 appears to have less impact on the stereoselectivity than the preorganized multivalency of the dendritic catalysts.
A method amenable to the multigram-scale preparation of 4-chloropyridine-2,6-dicarbonyl dichloride is described. The key transformation is the deoxygenative chlorination of the pyridine Noxide with oxalyl choride.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.