Stereoselective synthesis catalysis

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

Figure 10.46 Application of ThrA catalysis for the stereoselective synthesis of dihydroxyprolines from glyceraldehyde, and an adenylamino acid for RNA mimics (a). ThrA based preparation of precursors to the immunosuppressive lipid mycestericin and the antibiotic thiamphenicol (b).

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

Monographs J. D. Morrison, ed., Asymmetric Synthesis, Vol. 5, Academic Press, New York, 1985 B. Bosnich, Asymmetric Catalysis, Martinus Nijhoff, Dordrecht, 1986 M. Ndgradi, Stereoselective Synthesis, Verlag Chemie, Weinheim, 1987 I. Ojima, ed., Catalytic Asymmetric Synthesis, VCH, Weinheim, 1993. 

Stereoselective synthesis of /1-amino esters via asymmetric aldol-type and aza-Diels-Alder reactions has been reviewed.81 Siliranes react cleanly with benzaldehyde to produce oxasilacyclopentanes—with inversion—under conditions of Bu OK catalysis enolizable aldehydes yield silyl enol ethers.82... 

A highly regio- and stereoselective synthesis of 3-arylidene-l,4-benzodioxepin-5-ones has been achieved with palladium-copper catalysis (Scheme 7) <2000J(P1)775>. 

An enantioselective Strecker reaction involving Brpnsted acid catalysis uses a BINOL-phosphoric acid, which affords ees up to 93% in hydrocyanations of aromatic aldimines in toluene at -40 °C.67 The asymmetric induction processes in the stereoselective synthesis of both optically active cis- and trans-l-amino-2-hydroxycyclohexane-l -carboxylic acids via a Strecker reaction have been investigated.68 A 2-pyridylsulfonyl group has been used as a novel stereocontroller in a Strecker-type process ees up to 94% are suggested to arise from the ability of a chiral Lewis acid to coordinate to one of the sulfonyl (g)... 

In principle, three approaches may be adopted for obtaining an enantio-merically pure compound. These are resolution of a racemic mixture, stereoselective synthesis starting from a chiral building block, and conversion of a prochiral substrate into a chiral product by asymmetric catalysis. The last approach, since it is catalytic, means an amplification of chirality that is, one molecule of a chiral catalyst produces several hundred or a thousand molecules of the chiral product from a starting material that is optically inactive In the past two decades this strategy has proved to be extremely useful for the commercial manufacture of a number of intermediates for biologically active compounds. A few recent examples are given in Table 9.1. 

Like in other fields of asymmetric synthesis, catalysis is in focus. Catalysts for stereoselective synthesis utilizing chiral lithium amides are being developed to make such synthesis more useful in the laboratory as well as in industry. The progress made is also reviewed in detail below. 

Enders D (1993) Enzymemimetic C-C and C-N bond formations. In Stereoselective synthesis. Springer-Verlag, Berlin Heidelberg New York, p 63 Enders D, Balensiefer T (2004) Nucleophilic carbenes in asymmetric organo-catalysis. Acc Chem Res 37 534—541... 

O Donnell MJ, Bennett WD, Wu S (1989) The stereoselective synthesis of a-amino acids by phase-transfer catalysis. J Am Chem Soc 111 2353... 

Zeitler K (2006) Stereoselective synthesis of (E)-afi—unsaturated esters via carbene-catalyzed redox esterification. Qrg Lett 8 637-640 Zeitler K, Mager I (2007) An efficient and versatile approach for the immobilization of carbene precursors via copper-catalyzed [3+2]-cycloaddition and their catalytic apphcation. Adv Synth Cat 349 1851-1857 Zhao GL, Cordova A (2007) A one-pot combination of amine and heterocyclic carbene catalysis direct asymmetric synthesis of fi-hydroxy and fS-malonale esters from a,fS-unsaturated aldehydes. Tetrahedron Lett 48 5976-5980 Zhou ZZ, Ji FQ, Cao M, Yang GF (2006) An efficient intramolecular Stetter reaction in room temperature ionic liquids promoted by microwave irradiation. Adv Synth Cat 348 1826-1830... 

General reference G. Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann (eds.), Houben-Weyt Methods of Organic Chemistry, Vol. E 21, Stereoselective Synthesis, Thieme, Stuttgart, 1995. See also R. Noyori, Asymmetric Catalysis in Organic Synthesis, John Wiley, Chichester, 1994 M. Nogradi, Stereoselective Synthesis, VCH, Wein-heim, 1995 R. S. Atkinson, Stereoselective Synthesis. John Wiley, Chichester, 1995 G. Procter, Asymmetric Synthesis, OUP, Oxford, 1996. 

Considerable effort has been directed to the stereoselective synthesis of aimulated cyclopentadienyl complexes of the group 4 transition metals. Ci asymmetric and C2 synunetric aimulated cyclopentadienyl ligands have been used to prepare chiral organotitanium complexes, which are now being actively studied as catalysts for asymmetric synthesis and olefin polymerization see Asymmetric Synthesis by Homogeneous Catalysis). 

The stereoselective synthesis of (+)-trichodiene was accomplished by K.E. Harding and co-workers. The synthesis of this natural product posed a challenge, since it contains two adjacent quaternary stereocenters. For this reason, they chose a stereospecific electrocyclic reaction, the Nazarov cyclization, as the key ring-forming step to control the stereochemistry. The cyclization precursor was prepared by the Friedel-Crafts acylation of 1,4-dimethyl-1-cyclohexene with the appropriate acid chloride using SnCU as the catalyst. The Nazarov cyclization was not efficient under protic acid catalysis (e.g., TFA), but in the presence of excess boron trifluoride etherate high yield of the cyclized products was obtained. It is important to note that the mildness of the reaction conditions accounts for the fact that both of the products had an intact stereocenter at C2. Under harsher conditions, the formation of the C2-C3 enone was also observed. 

An analogous approach was used in a stereoselective synthesis of /Tamino-a-hydroxy phosphonates [33]. Reduction of corresponding a-keto phosphonate substrates with borane-dimethylsulfide complex aided by oxazaborolidine catalysis afforded a mixture of diastereomers, but significant diastereoselectiv-ity was achieved using catecholborane as the reductant in toluene at -60 °C. 

The use of cydophanes as chiral auxiliaries for stereoselective synthesis or as chiral ligands in asymmetric catalysis are even more promising future applica-... 

One of the most fascinating aspects in the history of asymmetric catalysis with its countless successful applications in the stereoselective synthesis of a broad variety of functional groups is the structural variety of the complexes which are able to be used as catalysts [1,2]. Many catalysts have been developed based on different ideas and concepts of mechanistic effect. However, in spite of the abundance of such catalysts which have been successfully applied in asymmetric catalysis, not a handful of them possess multifunctional abilities catalyzing different type of enantioselective reactions. The development of such a type of chiral catalyst, the catalytic effect of which is not limited to one reaction but to different types of asymmetric synthetic organic transformations, remained an attractive challenge for a long time. 

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