Asymmetric epoxides

Dioos, B. M. L. Jacobs P. A. (2005) Impregnation of dimeric Crlll(salen) on silica and its application in epoxides asymmetric ring opening reactions.. Applied Catalysis A General, 282 181-188. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

We began these studies with the intention of applying this tandem asymmetric epoxidation/asymmetric allylboration sequence towards the synthesis of D-olivose derivative 63 (refer to Figure 18). As the foregoing discussion indicates, our research has moved somewhat away from this goal and we have not yet had the opportunity to undertake this synthesis. This, as well as the synthesis of the olivomycin CDE trisaccharide, remain as problems for future exploration. Because it is the enantioselectivity of the tartrate ester allylboronates that has limited the success of the mismatched double asymmetric reactions discussed here, as well as in several other cases published from our laboratorythe focus of our work on chiral allyiboronate chemistry has shifted away from synthetic applications and towards the development of a more highly enantioselective chiral auxiliary. One such auxiliary has been developed, as described below. 

Asymmetric Epoxidation. Asymmetric epoxidation of nonfunctionalized alkenes manifests a great synthetic challenge. The most successful method of asymmetric epoxidation, developed by Katsuki and Sharpless,332 employs a Ti(IV) alkoxide [usually Ti(OisoPr)4], an optically active dialkyl tartrate, and tert-BuOOH. This procedure, however, was designed to convert allylic alcohols to epoxy alcohols, and the hydroxyl group plays a decisive role in attaining high degree of enantiofa-cial selectivity.333,334 Without such function, the asymmetric epoxidation of simple olefins has been only moderately successful 335... 

In addition, solid-phase bound short-chain peptides have been recently found by the Ber-kessel group to act as highly efficient catalysts in asymmetric epoxidation reactions [17]. In the early 1980s, Julia and Colonna reported that chalcone 11 can be epoxidized asymmetrically by akaline hydrogen peroxide in the presence of poly-amino acids as catalysts [18, 19], The work by Berkessel et al. revealed that in fact as little as five I-Leu residues are sufficient for the epoxidation of the enone 11 with 96-98% ee (Scheme 8). 

EPOXIDATION, ASYMMETRIC (- )-Benzylquininium chloride. f-Butyl hydroperoxide-Vanadyl acetoacetate. Hydrogen peroxide-1,1,3,3-Tetrachloro-acetone. (S)-(2-Hydroxy-N,N-dimethyl-propanamide-0,0 )oxodiperoxymolyb-denum(VI). 

JULI A-COLONNA Asymmetric Epoxidation Asymmetric epoxidation of electron-poor olefins catalyzed by poly-a amino acids. 

Epoxidation. Asymmetric epoxidation has been an active area of research, and many... 

There are many possible answers here. What we had in mind was some sort of asymmetric Diels-Alder reaction for the first, an asymmetric aldol for the second or else opening an epoxide made by Sharpless epoxidation, asymmetric dihydroxylation for the third, and perhaps asymmetric dihydroxylation of a 2-alkene for the fourth. Of course you might have used resolution or asymmetric hydrogenation, or the chiral pool, or any other strategy from chapter 41. 

In the total synthesis, the most noteworthy aspect is the philosophy of the approach. Also, the important point is not just the total yield, but, primarily the completion of the synthesis, as artists never exhibit their unfinished work and since a synthesis cannot be an Unfinished Symphony. Considering the many kinds of useful reactions which have been developed to serve as synthetic key steps, such as asymmetric epoxidation, asymmetric reduction and enantiospeci-fic aldol reaction using chiral reagents, and, considering the many kinds of enantiomerically pure materials such as carbohydrates which are available, any complex antibiotic may be synthesized. Consequently, a philosophy of synthesis is more urgently required than ever before. 

The original asymmetric synthesis has been extensively improved in modem organic synthesis, such as in the Sharpless Epoxidation, asymmetric hydrogenation, and enzymatic hydrolysis. 

A number of insect pheromones are epoxides. Asymmetric hydrolysis of meso-diacetate 3 (Figure 24.2) witii pig pancreatic lipase (PPL) gives (2S,3P)-4-acetoxy-2, 3-epoxy-l-butanol (4, 90% ee) in 71% yield [4], Various epoxide pheromones were synthesized from (2S,3K)-4, which could be purified to give enantiopure material by recrystallizing tiie corresponding 3,5-dinitrobenzoate. 

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