133604-82-7Relevant articles and documents
A hetero Diels-Alder approach to the synthesis of chromans (3,4-dihydrobenzopyrans) using oxonium ion chemistry: The oxa-Povarov reaction
Taylor, Rivka R. R.,Batey, Robert A.
, p. 1404 - 1420 (2013/03/28)
An oxa analogue of the well-known Povarov reaction has been developed for the synthesis of 3,4-dihydrobenzopyrans (chromans). The reaction involves the formal inverse electron demand [4 + 2] cycloaddition reaction of in situ-generated cationic aryl 2-oxadiene oxocarbenium ions with alkenes. The oxonium ion intermediates are generated through Lewis acid (SnCl 4)-promoted reactions of phenol-derived Rychnovsky-type mixed acetals. The yield and diastereoselectivity of the chroman products are found to depend upon the substitution pattern of the precursor alkene (i.e., monosubstituted, trans- or cis-disubstituted and cyclic alkenes). Generally, the reactions afford the endo-diastereomers as the major products, except for the reactions of trans-β-methylstyrene, which afford exo-chromans. A comparison of the product distributions from the reactions of trans- and cis-β-methylstyrene reveal that the reaction proceeds, at least in part, by a nonconcerted ionic pathway. Just as for the aza-Povarov reaction, there are two potential mechanisms for the reaction. The first mechanism involves a direct asynchronous [4 + 2] cycloaddition pathway, while the second occurs through the stepwise Prins addition of the alkene to the aryl 2-oxadiene oxonium ion, followed by an intramolecular aromatic substitution reaction of the resultant cation (i.e., a domino Prins/intramolecular Friedel-Crafts reaction).
Degenerate Transesterification of 3,5-Dimethylphenolate/3,5-Dimethylphenyl Esters in Weakly Polar, Aprotic Solvents. Reactions of Aggregates and Complex-Induced Proximity Effects
Jackman,Petrei,Smith
, p. 3451 - 3458 (2007/10/02)
The rates of exchange of the 3,5-dimethylphenolate ion between lithium 3,5-dimethylphenolate-d6 and a series of 3,5-dimethylphenyl esters have been determined in the weakly polar, aprotic solvents dioxolane, dimethoxyethane (DME), tetrahydrofuran (THF), and pyridine. The esters include the propionate, butyrate, methoxyacctate, β-methoxypropionate, 4-methoxybutyrate, 2-tetrahydrofuroate, 2-furoate, (N,N-dimethylamino)acetate, (methylthio)acetate, 2- and 4-pyridine-carboxylates, 2-pyridylacetate, 4-pyridylacetate, phenylacetate, andp-methoxy-,p-chloro-, and p-(trifluoromethyl)phenylacetates. The rates and kinetic orders of the reactions of 3,5-dimethylphenyl propionate in various solvents at 35°C gave the following second-order rate constants (104k2, L mol-1 sec-1) for the following major aggregate species: THF tetramer, 6.5; DME tetramer, 3.3 (40°C); dioxolane, 13, hexamer, 71; pyridine tetramer, 2.2, dimer, 29. For 3,5-dimethylphenyl β-methoxypropionate, the order of reactivity is dioxolane > DME > THF. These results are interpreted in terms of a preequilibrium in which a solvent on lithium in the tetramer is replaced by the ester. The rates of transesterification have been compared with the rates of hydrolysis in 30% aqueous ethanol for the above series of esters. Those esters that have a second Lewis base center proximal to the ester function show significantly increased reactivity in transesterification, which is attributed to a complex-induced proximity effect.
