Chiral intermediate

The chiral intermediate (S)-l-(2 -bromo-4 -fluorophenyl) ethanol was prepared by the enantioselective microbial reduction of 2-bromo-4-fiuoroacetophenone [lObj. Organisms from genus Candida, Hansmula, Pichia, Rhodotcnda, Saccharomyces, Sphingomonas, and baker s yeast reduced the ketone to the corresponding alcohol in 

This chiral intermediate, when carried through the reaction sequence in Scheme 13.30, generated the enantiomer of natural longifolene. Thus D-proline would have to be used to generate the natural enantiomer. 

The key chiral intermediate 4, highly functionalized hydroisoquinoline, was obtained by a Diels-Alder reaction between siloxydiene 5 and chiral dienophile 6 which was prepared from Z-serine [4]. Ruche reduction of enone 4 gave allyl alco- 

Fig. 5. Synthesis of Evan s chiral oxa2ohdinone auxiUary and use of this auxiUary for preparation of a chiral intermediate in the synthesis of mtamycin B.

The study concerns the desymmetrization of the prochiral dinitrile (16) with preferential formation of the (Ji)-17, which was known to be a chiral intermediate in the synthesis of the cholesterol-lowering therapeutic drug (18) (Lipitor, Sortis, Torvast, etc.) as shown in Scheme 2.3. 

Hetereochiral dimer 134 Figure 8-8. Association of chiral intermediates. 

Two RCM reactions were employed in a new and efficient route to a key chiral intermediate, isoquinuclidine 150, in the synthesis of alkaloid (-F)-catharanthine <06AG(I)5334>. The first RCM makes use of chiral enone 151, derived from L-serine, to generate a chiral dihydropyridinone 152. Intramolecular alkene metathesis of dialkenyl piperidine 153 generates 150, which represents the first example of the use of RCM in the generation of an azabicyclo[2.2.2]alkene system. 

A furanose-fused c/.,(3-unsaturated 8-lactone was also prepared from ester 11 as a key chiral intermediate for the synthesis of the enantiomer of (+ )altholactone, a natural product with cytotoxic and antitumor activities.18 A Reformatsky reaction with ethylbromoacetate or a Wittig 

Takasago International Co. commercialized this hydrogenation method for the production of a chiral intermediate for the synthesis of carbapenem antibiotics (Equation (8)).29 

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

Enanlioselectivc hydrogenation of a-kelocsiers Pt/C, Pl/Al,0, a-hydroxyesters Chiral intermediates 

Kalaitzakis, D., Rozzell, J.D., Kambourakis, S. and Smonou, I., Synthesis of valuable chiral intermediates by isolated ketoreductases application in the s3mthesis of -alkyl—hydroxy ketones and 1,3-diols. Adv. Synth. Catal, 2006, 348, 1958-1969. 

Acyloxybutenolides obtained by / -elimination from ribonolactone (see Section IX.2) have served as appropriate chiral intermediates in several synthesis of antibiotics. Barrett and Sheth (205) reported a seven-step synthesis of racemic terI-butyl-8-0-ter/-butyldimethylsilylnonactate, a monomeric moiety of the antibiotic nonactin, from 157. Also, (4,S,6[Pg.189]

Synthetic applications of the asymmetric Birch reduction and reduction-alkylation are reported. Synthetically useful chiral Intermediates have been obtained from chiral 2-alkoxy-, 2-alkyl-, 2-aryl- and 2-trialkylsllyl-benzamides I and the pyrrolobenzodlazeplne-5,ll-diones II. The availability of a wide range of substituents on the precursor benzoic acid derivative, the uniformly high degree of dlastereoselection in the chiral enolate alkylation step, and the opportunity for further development of stereogenic centers by way of olefin addition reactions make this method unusually versatile for the asymmetric synthesis of natural products and related materials. 

The use of a stereogenic carbon centre allowed an efficient asymmetric induction in the benzannulation reaction towards axial-chiral intermediates in the synthesis of configurationally stable ring-C-functionalised derivatives of al-locolchicinoids [51]. The benzannulation of carbene complex 52 with 1-pen-tyne followed by oxidative demetalation afforded a single diastereomer 53 (Scheme 33). 

Methods in asymmetric synthesis, particularly those which employ transition metal catalysis, have and continue to contribute enormously to the synthetic chemists armoury and, providing both enantiomers of the catalyst ligand are available, one can synthesise either enantiomer of a chiral intermediate at will. 

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