Effect of steric bulk on the absolute reactivity of allene oxides.
Alpha-alkyl substituted fluorenylidene allene oxides were generated photochemically from appropriately substituted fluorenol derivatives in 2,2,2-trifluoroethanol (TFE) in order to assess the affect of steric bulk on the absolute reactivity of the allene oxides. The absolute reactivities of the allene oxides were measured using nanosecond laser flash photolysis, and were found to change little in TFE upon going from alpha-methyl to alpha-butyl to alpha-t-butyl. However, all three alpha-alkyl substituted allene oxides were considerably more reactive than the alpha-hydrogen derivative, indicating that steric bulk did not result in a decrease in reactivity as is typically found in other allene oxides. The contrary reactivity of the fluorenylidene allene oxides is explained by examining in detail the reactions of these allene oxides, determined from product studies.
Bond angle versus torsional deformation in an overcrowded alkene: 9-(2,2,2-triphenylethylidene)fluorene.
Competition between bond angle and torsional strain in sterically crowded alkenes generally causes twisting in tetrasubstituted alkenes, while most structurally characterized trisubstituted alkenes are planar. To investigate structural effects of steric repulsion between a planar aromatic ring and a vicinal triphenylmethyl (trityl) group, 9-(2,2,2-triphenylethylidene)fluorene (1 a) was synthesized by reaction of 9-bromomethylenefluorene with triphenylmethyllithium. For comparison with a less strained analogue, 9-ethylidenefluorene (1 b) was prepared by reaction of fluorenone with ethylmagnesium bromide. The X-ray crystal structures show that the difference between bond angles at the 9-fluorenyl carbon atom is much larger for 1 a (12.9 degrees) than 1 b (2.6 degrees). Bond angle and torsional deformations were compared theoretically (HF/6-31+G*) with the tert-butyl analogue (1 c), 1,2,2-tri-tert-butylethene (7), and 2,4,4-trimethyl-2-pentene (8) and crystallographically with six known 1,1-diaryl-2-tert-alkylethenes (2). The trisubstituted alkenes formed three groups with 1) large angle distortion with moderate twisting (1 a, 1 b, and 7), 2) moderate bending with a large range of torsional angles (2), and 3) little bending or twisting (1 b and 8). For the entire series, there appears to be a delicate balance between angle and torsional deformation, but twisting appears to produce smaller relief from steric strain than angle bending. In the crystallographically characterized trisubstituted alkenes, the choice between the two is mainly determined by more subtle packing forces.
Absolute configuration of cis-12-oxophytodienoic acid of flaxseed: implications for the mechanism of biosynthesis from the 13(S)-hydroperoxide of linolenic acid.
cis-12-Oxophytodienoic acid (cis-12-oxo-PDA) is a C18 cyclopentenone formed from the 13-(S)-hydroperoxide of linolenic acid in flaxseed and other plant tissues. Although the structure of cis-12-oxo-PDA is well established, the absolute configuration of the side chains has not been determined. We have now measured this important parameter by two independent approaches. The CD spectrum of freshly prepared cis-12-oxo-PDA showed no deviations from base line--implying that the product is racemic. This conclusion was checked by a high-pressure liquid chromatography (HPLC) method capable of resolving the enantiomers; cis-12-oxo-PDA was reduced to two saturated hydroxy analogues which were each converted to (-)-menthoxycarbonyl diastereomers and analyzed by HPLC. Each epimer was resolved as two peaks of equal area, thus confirming that their cis-12-oxo-PDA parent is a racemic mixture, enantiomeric at the ring junctures. We propose that the biosynthesis of racemic cis-12-oxo-PDA proceeds by dehydration of the 13(S)-hydroperoxide to an allene oxide. A major fate of the allene oxide is hydrolysis to an alpha-ketol, which is always formed together with cis-12-oxo-PDA. The allene oxide also opens to a zwitterion, which undergoes charge delocalization to form a planar intermediate; this structure is the achiral precursor of the stable end product of pericyclic ring closure, viz., racemic cis-12-oxo-PDA.