Abstract
Anionic Diels–Alder chemistry of electron-deficient cross-conjugated vinylogous alkenones, providing highly stable sodium dienolate ion pairs as electron-rich dienes in the presence of a weak sodium base in THF, has been newly developed, leading to a single Diels–Alder adduct, in racemic form, in moderate to high yields (up to 97%, 37 examples).
Highlights
Diels−Alder cycloaddition reactions remain and continue to be of extreme utility in synthetic organic chemistry in terms of their extraordinary capacity to construct, in one step, fused polycyclic skeletons in a highly regio- and stereoselective manner.[1]
(I) and its α-activated analogue (II) have been well studied in their Diels−Alder chemistry as dienophiles, and several vital conclusions can be derived: (1) Diels−Alder cycloaddition of 2cycloalkenone (I) is a rather poor process; (2) employing Lewis acid as catalyst and/or introducing an additional electronwithdrawing group at its α to ketone position can significantly enhance the dienophilicity of the carbon−carbon double bond; (3) introducing a second double bond into the ring can enhance the secondary effect; (4) C-4 substituent can control the facial selectivity by steric hindrance.[2,3]
In our long-lasting interest in Diels−Alder chemistry of 2cycloalkenones, β-substituted α-activated 2-cyclohexenones (III) are further designed to evaluate whether they are as synthetically useful as their enone counterparts (II). They have experimentally proved to be rather poor dienophiles for Diels−Alder reactions, most likely because of steric hindrance imposed on the β substituent as indicated by many historic cases bearing a similar structure.[4]
Summary
Diels−Alder cycloaddition reactions remain and continue to be of extreme utility in synthetic organic chemistry in terms of their extraordinary capacity to construct, in one step, fused polycyclic skeletons in a highly regio- and stereoselective manner.[1]. To further activate the lithium−enolate ion pair, solvation of lithium cation by HMPA in THF (V/V = 1/4) was examined;[12] reactions resulted in a complex unidentified mixture as observed on TLC (entry 4).
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