A Practical Improvement of Crystallization‐Induced Asymmetric Transformation of Allene‐1,3‐dicarboxylates.

Abstract

Allenes P 0025A Practical Improvement of Crystallization-Induced Asymmetric Transformation of Allene-1,3-dicarboxylates. -Diastereomeric mixtures of allenes (I) bearing naturally abundant chiral alcohols undergo an Et3N-catalyzed epimerization-crystallization between the (R)-and (S)-isomers of the allene moiety in non-polar pentane or hexane solvent to give the pure (R)-allene epimers by repeating the procedure. Pure isomers (II) undergo Diels-Alder cycloaddition reactions with cyclic dienes to give the corresponding bicyclic products as single isomers. -(KATOH, T.; NOGUCHI, C.; KIMURA, H.; FUJIWARA, T.; ICHIHASHI, S.; NISHIDE, K.; KAJIMOTO, T.; NODE*, M.; Tetrahedron: Asymmetry 17 (2006) 20, 2943-2951; Dep. Pharm. Manuf.

Reactivity for the Diels–Alder Reaction of Cumulenes: A Distortion-Interaction Analysis along the Reaction Pathway

“…Cumulenes, such as allene, ketenimine, and ketene, are characterized by an unusual allenic or heteroallenic bond structure, which is the origin of their distinctive reactivity. The highest occupied molecular orbital (HOMO) of a cumulene is located perpendicular to the lowest unoccupied molecular orbital (LUMO). − As shown in Figure , with a 1,2-dipolar resonance structure, the cumulenes can react with alkenes to form the corresponding [2 + 2] cycloaddition product via either 1,2-addition or hetero-1,2-addition. − Moreover, cumulenes can act as dienophiles in a Diels–Alder reaction. − In previous studies, both concerted and stepwise mechanisms are possible for this type of reaction. As shown in Figure , if the X group of cumulene is either NH or O, there are two plausible products of [4 + 2] cycloaddition.…”