Zr[bis(salicylidene)ethylenediaminato]-mediated Baeyer–Villiger oxidation: Stereospecific synthesis of abnormal and normal lactones

Abstract

Baeyer-Villiger oxidation of racemic bicyclic cyclobutanones with Zr[bis(salicylidene)ethylenediaminato] (salen) complex 1 as catalyst in the presence of a urea-hydrogen peroxide adduct was found to proceed enantiospecifically. The enantiotopos selection in the oxidation was governed primarily by the Zr(salen) catalyst, although migratory aptitude (methine > methylene > methyl) in Baeyer-Villiger oxidation affected the selection to a varied extent, depending on the substrate structures; one enantiomer of cyclobutanones gave exclusively a normal lactone expected from the migratory aptitude, and the other enantiomer gave an abnormal lactone preferentially, the formation of which is counter to the migratory aptitude. Furthermore, the rates of abnormal lactone formation were found to be faster than those of normal lactone formation in most of the oxidations examined. For example, the enantiomer of racemic bicyclo[3.2.0]heptan-6-one giving an abnormal lactone reacted 2.2 times faster than the other enantiomer giving a normal lactone. To our knowledge, this example of chemocatalytic Baeyer-Villiger oxidation giving an abnormal lactone in preference to a normal lactone has been previously unreported. This unusual behavior is likely to be attributable to strict control of stereoelectronic demand in Baeyer-Villiger oxidation and chiral recognition by complex 1. Baeyer-Villiger (B-V) oxidation, oxidative transformation of carbonyl to ester (or lactone), has high synthetic value and has been widely used in various organic syntheses. In particular, asymmetric B-V oxidation of racemic or prochiral cyclic ketones is a useful tool for the synthesis of optically active lactones. Several enzymes, so-called Baeyer-Villigerase, are known to catalyze highly enantiospecific B-V oxidations (regiodivergent parallel kinetic resolution) (1, 2) of racemic cyclic ketones, although the stereochemistry of the oxidations depends on the enzyme used (3-9). Some of them promote B-V oxidation of only one enantiomer, leaving the other enantiomer intact, whereas some other enzymes promote B-V oxidation of both enantiomers in an enantiospecific and topos-selective manner; that is, one enantiomer is converted to a normal lactone (NL) and the other is converted to an abnormal lactone (AL). [A lactone generated in accord with the migratory aptitude of the carbonyl substituent in B-V oxidation (tertiary Ͼ secondary Ͼ primary) is called a NL, and a lactone generated in contravention of the aptitude is called an AL (8).] On the other hand, chemical versions of B-V oxidation of racemic ketones are still limited in number and are inferior to biocatalyzed B-V oxidation in terms of enantiospecificity and topos selectivity. In 1994, Bolm et al.(10) reported copper-catalyzed asymmetric B-V oxidation. In the same year, Strukul and coworkers (11) reported platinumcatalyzed asymmetric B-V oxidation. Since then, several catalysts have been introduced for asymmetric B-V oxidation (12-16). Although stoichiometric enantiospecific B-V oxidation has recently been repor...

On the influence of oxygen and cell concentration in an SFPR whole cell biocatalytic Baeyer–Villiger oxidation process

“…In order to make this reaction applicable at industrial level, the replacement of shock-sensitive and sacrificial oxidants by oxygen and the move from achiral stoichiometric reagents to asymmetric catalysis of Baeyer-Villiger oxidation is desirable. Moderately good results were recently achieved using organometallic catalysts by Watanabe et al (2004) or using biomimetic bisflavine compounds by Murahashi et al (2002). BaeyerVilliger oxidations catalyzed by monooxygenases, that is, Baeyer-Villiger monooxygenases (BVMOs) have shown a clear advantage with respect to both the nature of the oxidant and solvent as well as enantioselectivity (Kamerbeek et al, 2003;Mihovilovic et al, 2002;Roberts and Wan, 1998).…”

Mechanistic Investigation of Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer–Villiger Reaction of 3‐Substituted Cyclobutanones with H2O2 as the Oxidant

“…[29,30] Chemical catalysts offer an attractive alternative, especially reactions using environmentally benign oxidants such as H 2 O 2 , but they are still limited in number and are often less enantioselective than their biological counterparts. [5] Since the first reports on metal-catalyzed asymmetric B-V reactions in 1994, [31] various chiral metal catalysts based on Cu II , [32] Pt II , [33] Ti IV , [34] Co III , [35] Zr IV , [36] Mg II , [37] Al III , [38] and Pd II [39] have been developed in recent years for this transformation. However, so far, very few of them could attain enantioselectivity at a level comparable to enzymes, and wide substrate generality has not yet been achieved in each case.…”

Access to a Key Building Block for the Prostaglandin Family via Stereocontrolled Organocatalytic Baeyer–Villiger Oxidation

“…[13] Specifically,amixture of the desired normal lactone (NL) and undesired abnormal lactone (AL) were formed, while rates of AL formation were found to be faster than those of NL (Scheme 2b). [14,15] Moreover,t he separation of these mixtures is difficult. Major breakthroughs in this area were made by Dingsgroup using aBrønsted acid catalysis strategy,a chieving asymmetric B-V oxidation of prochiral cyclobutanones in good yield and enantioselectivity.…”