“Synthetic Enzymes”. Highly Stereoselective Epoxidation of Chalcone in a Triphasic Toluene‐Water‐Poly[(S)‐alanine] System

Abstract

thus obtainable only as a chloroform solution with 2-4% of (2) and 10-15% of tert-butyl alcohol.Although the hydrocarbon (1) is a highly reactive compound, which polymerizes in condensed phase even at -78"C, its solution in chloroform containing a small amount of hydroquinone can be kept several days at -30 "C. Its spectroscopic resemble those of cyclopropane and cyclopropene. In particular the I3C-H spin-spin coupling constants (158 and 225 Hz) compared to the coupling constants observed for cyclopropane and 3,3-dimethylcyclopropene (respectively 161 and 220 Hz)@I, suggest no difference in the states of hybridization of the corresponding C-H bonds. This would indicate that (1) is a normal spiro compound, and that the interaction between the cyclopropane Walsh orbitals and the cyclopropene m orbitals (pseu-doconj~gation)~'~, if at all present, is not important. Confirmation by UV and PE spectroscopy has so far been precluded by the impurities present in (I).The high strain of (1) is revealed by its unusual chemical reactivity compared with that of 3,3-disubstituted cyclopropenes. As expected, the cyclopropene double bond is very

Recent Advances in Asymmetric CC and C‐Heteroatom Bond Forming Reactions using Polymer‐Bound Catalysts

“…Weitz±Scheffer epoxidation catalyzed by polymeric amino acids reported by Julia ¬ and Colonna. [73] Table 5. HKR of racemic epichlorhydrin (46) catalyzed by salen complex 45 (for details see Jacobsen et al [77] Figure 14.…”

“…The copper complexes of 69a ± 72a and the corresponding monomeric species were used as catalysts in cyclopropanation reactions of substituted styrenes and ethyl diazoacetate (73) to give cis-and trans-2-arylcyclopropanecarboxylic acid ethyl esters (74) (Scheme 10). [41] The cis/trans-ratio depended on the R 1 substituent (Table 12).…”

“…[72] In 1980, Julia ¬ and Colonna reported that polymeric amino acids catalyze the Weitz±Scheffer epoxidation of chalcone in a highly enantioselective and intriguingly simple manner (Scheme 5). [73] Julia ¬ and Colonna could show good yield and a high enantioselectivity from 93% ee.…”

Pursuing Chemical Efficiency by Using Supported Organocatalysts for Asymmetric Reactions under Aqueous Conditions

“…The Juliá-Colonna epoxidation is one of the first examples of polypeptide-catalyzed reactions in an aqueous system. Thus, excellent enantioselectivities have been obtained using polyamino acid derivatives such as poly-L-alanine or poly-L-leucine in an organic solvent and an aqueous layer containing H 2 O 2 and NaOH [162,163]. However, this transformation has several limitations, such as the narrow substrate scope and the long reaction times.…”

Enantioselective Total Syntheses of Several Bioactive Natural Products Based on the Development of Practical Asymmetric Catalysis

“…Catalytic asymmetric epoxidations of allylic alcohols using a Ti-tartrate complex 121,122) and unfunctionalized olefins using salen-manganese complexes 123) are well established. On the other hand, since the initial report by Juliá and coworkers, 124) the catalytic asymmetric epoxidation of enones has been studied using several other methodologies such as ligand-metal catalysts, phase-transfer catalysts, and polyamino acid catalysts. 119,120) Previously, Shibasaki et al reported a general catalytic asymmetric epoxidation of enones using alkali-metal free lanthanide-BINOL complexes.…”