Cyclobutanones, asymmetric Baeyer-Villiger reaction

The use of a chiral hydroperoxide as oxidant in the asymmetric Baeyer-Villiger reaction was also described by Aoki and Seebach, who tested the asymmetric induction of their TADOOH hydroperoxide in this kind of reaction98. Besides epoxidation and sulfoxidation, for which they found high enantioselectivities with TADOOH (60), this oxidant is also able to induce high asymmetry in Baeyer-Villiger oxidations of racemic cyclobutanone derivatives in the presence of DBU as a base and LiCl as additive (Scheme 174). The yields and ee values (in parentheses) of ketones and lactones are given in Scheme 174 as... 

Katsuki successfully appHed two different metal salen complexes in asymmetric Baeyer-Villiger reactions. Cationic cobalt complex 51 catalyzes the reaction between cyclobutanone 50 and UHP as oxidant to give lactone 46 with 77% ee in 72% yield [349], In the same transformation zirconium salen complex 52 affords the product with higher enantioselectivity (87% ee) (Scheme 9.5). Over 99% ee could be achieved in the conversion of tricyclic ketone 47 [350],... 

Another method for the asymmetric version of the Baeyer-Villiger reaction was presented by Lopp and coworkers in 1996 . By employing overstoichiometric quantities of Ti(OPr-t)4/DET/TBHP (1.5 eq./1.8 eq./1.5 eq.), racemic andprochiral cyclobutanones were converted to enantiomerically enriched lactones with ee values up to 75% and moderate conversions up to 40% (Scheme 171). Bolm and Beckmann used a combination of axially chiral C2-symmetric diols of the BINOL type as ligands in the zirconium-mediated Baeyer-Villiger reaction of cyclobutanone derivatives in the presence of TBHP (or CHP) as oxidant (Scheme 172) . With the in situ formed catalysts 233a-d the regioisomeric lactones were produced with moderate asymmetric inductions (6-84%). The main drawback of this method is the need of stoichiometric amounts of zirconium catalyst. 

Chiral a-methylene-y-butyrolactones An asymmetric synthesis of a-methylene- y-butyrolactones such as 7 utilizes the benzoate (2) of (1R,2S)-1, which on reaction with 1,1-dibromohexane and Zn-TiCL TMEDA forms the (Z)-enol ether 3. Thus this product reacts with dichloroketene to form the cyclobutanone (—)-4. Baeyer-Villiger oxidation of 4 followed by oxidation with chromium(Il) perchlorate provides the a-chlorobutenolide (+)-5. Hydrogenation and oxidation provides the 8-carboxy-y-butyrolactone (-) 6. The final step involves introduction of a methylene group to provide (-)-7, methylenolactocin, a natural antitumor antibiotic. 

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