Reactions of Styrylmalonates with Aromatic Aldehydes: Detailed Synthetic and Mechanistic Studies

Abstract

Reactions of styrylmalonates with aromatic aldehydes in the presence of Lewis and Brønsted acids and their regularities have been studied in detail. Approaches to the synthesis of various polysubstituted 5,6-dihydropyran-2-ones, indenes, aryl-containing dienes and trienes, cyclopentadienes, and polycyclic lactones have been developed with chemo-, regio-, stereo-, and diastereoselectivity control. The mechanisms of these reactions were studied in depth using multinuclear NMR experiments, monitoring the reactions in the NMR tube, and 18O isotope labeling.

BF3 Mediated [1,5]‐Hydride Shift Triggered Cyclization: Thioethers Join the Game

“…To confirm the formation of chelate complexes, we examined the reaction mixtures by heteronuclear NMR (see SI, parts 9 and 12). Previously, for monoalkylmalonate compounds, this method has already been used to confirm the BF 2 bridge formation [30] . In the case of compound 2 a , we did not observe changes in the 11 B and 19 F NMR spectra in comparison with pure BF 3 ⋅ Et 2 O solution.…”

“…Previously, for monoalkylmalonate compounds, this method has already been used to confirm the BF 2 bridge formation. [30] In the case of compound 2 a, we did not observe changes in the 11 B and 19 F NMR spectra in comparison with pure BF 3 • Et 2 O solution. This result correlates well with the calculations performed -the chelate complexes should not form in significant amounts in this case.…”

A Cost-Effective Way to Produce Gram-Scale 18O-Labeled Aromatic Aldehydes

“…It has also been generally accepted that the isotope tracing method 11 is an effective way to provide some paramount information regarding to the possible reaction pathway. To date, the procedures for preparing the isotopically 18 O-labeled aromatic aldehydes are very limited, such as (1) the nucleophilic substitution of benzyl chloride with H 2 18 O and then oxidation with Dess-Martin periodinane, 12 (2) synthesis of 2-formylpyrroles from pyridinium iodide salts by using H 2 18 O as an oxygen source, 13 and (3) the physically dynamic exchange between 18 O and 16 O by aldehydes with H 2 18 O. 14 Otherwise, some carbonyl compounds are obtained by oxidation of sp 3 -hybridized atoms with an 18 O 2 15 or H 2 18 O 16 oxygen source.…”

“…To date, the procedures for preparing the isotopically 18 O-labeled aromatic aldehydes are very limited, such as (1) the nucleophilic substitution of benzyl chloride with H 2 18 O and then oxidation with Dess-Martin periodinane, 12 (2) synthesis of 2-formylpyrroles from pyridinium iodide salts by using H 2 18 O as an oxygen source, 13 and (3) the physically dynamic exchange between 18 O and 16 O by aldehydes with H 2 18 O. 14 Otherwise, some carbonyl compounds are obtained by oxidation of sp 3 -hybridized atoms with an 18 O 2 15 or H 2 18 O 16 oxygen source. Obviously, these procedures suffer from disadvantages such as more complicated workup procedures and also expensive material cost because excess H 2…”

Natural and Synthetic 6-Aryl Pyrones. An Unexpected Reaction of Dihydropyrones with NBS Leads to the First Synthesis of Allantopyrone C

“…7 Attempts to oxidize 6 using DDQ, trityl tetrafluoborate, or elemental sulfur returned recovered starting material. 9 Reactions employed to cleave benzyl esters (DDQ, MeOH) also returned starting material. 10 Allylic bromination using excess NBS is reported to afford only 5-bromo pyrones.…”