Enantiomerically pure amino acids, amino alcohols, amines, alcohols, and epoxides play an increasingly important role as intermediates in the pharmaceutical industry and agrochemistry, where both a high degree of purity and large quantities of the compounds are required. The chemical industry has primarily relied upon established chemical methods for the synthesis of these intermediates, but is now turning more and more to enzymatic and biotechnological fermentation processes. For the industrial implementation of many transformations alternative methods are available. The advantages of the individual methods will be discussed herein and exemplified by syntheses of relevant compounds.

Enantiomerenreine Aminosäuren, Aminoalkohole, Amine, Alkohole und Epoxide spielen heutzutage als Zwischenprodukte in pharmazeutischer Industrie und Agrochemie eine immer größere Rolle. Dabei sind sowohl ein hoher Reinheitsgrad als auch große Mengen der Verbindungen gefragt. Die chemische Industrie hat sich bei der Synthese dieser Zwischenstufen bislang auf bewährte chemische Verfahren gestützt, greift jedoch zunehmend auf enzymatische und fermentative Prozesse aus der Biotechnologie zurück. Für die industrielle Umsetzung vieler Transformationen stehen alternative Verfahren zur Verfügung. Die Vorteile der einzelnen Methoden werden in diesem Aufsatz anhand der Synthesen wichtiger Beispielverbindungen diskutiert.

Homoallyl alcoholsUsing 1,2-dicyclohexyl-1,2-ethanediol as chiral auxiliary, the enantiomerically pure 2-pentenylboronate 9c was obtained. Its addition to benzaldehyde proceeded with complete transfer of chirality to give the syn-E-homoallyl alcohol 11. The ability of the reagent 9c to create new stereocenters under reagent control of diastereoselectivity was tested in its addition to the chiral aldehydes 15 and 24. This resulted in a short and stereospecific synthesis of invictolide (18), as well as of a C-9/C-15-partial structure 25 of erythronolide A.One strategy for the synthesis of natural products of polyketide-derived biogenesis, especially for the synthesis of polypropionate molecules2), consists in an iterative sequence of chain elongation steps.Stereoselektive Syntbese von Alkoholen, XXXI'? -Stereosdektive C-C-Bindungsbildung mit Hilfe chirakr Z-Pentenylboro&4uter Unter Verwendung von 1,2-Dicyclohexyl-l,2-ethandiol als chiralem Auxiliar wurde der enantiomerenreine Z-Pentenylboronsaureester 9c erhalten. Dessen Addition an Benzaldehyd ergab den syn-E-Homoallylalkohol 11 unter vollstandiger ChiralitatsObertragung. Die Fihigkeit des Reagenz 9c zur Bildung neuer Stereozentren unter Reagenz-Kontrolle der Diastereoselektivitat wurde in der Addition an die chiralen Aldehyde 15 und 24 gepruft. Dies fiihrte zu einer kurzen stereospezifischen Synthese des Invictolids (18) wie der eines C-9/C-15-Bausteins 25 des Erythronolids A.Control of the formation of the individual stereocenters depends on simple diastereoselection during the carboncarbon bond forming steps3) as well as on reagent control of diastereo~electivity~) using chiral reagents exerting high asymmetric induction. We recently reported on cr-methoxy-(E)-crotylboronates (1) as reagents, which allow for the generation of the stereotriades C and D'). Here, we would like to describe the chiral 2-pentenylboronate 2, which should allow for the generation of the stereotriades A and B. Some aspects of this work have already been communicated in preliminary form ' 1.A preceding study on racemic E-and Z-pentenylboronates') demonstrated that only the Z isomer 3 shows high diastereoselectivity on addition to aldehydes. The product 4 with an E-double bond is formed preferentially since the transition state 5 of the competing reaction leading to 6 with a 2-double bond is destabilized by allylic 1,3-strain6). As the Chem.

Enantiopure biphenyl-bridged titanocene and zirconocene complexes were obtained, by an asymmetric thermal transformation of the binaphthol complexes formed from the metallocene racemates and subsequent transformation to the corresponding dichlorides, in practically quantitative yields. Increased rates of this transformation in the presence of O2 gas or TEMPO indicate a radical reaction mechanism. The biphenyl-bridged titanocene enantiomers give rise to an efficient asymmetric catalysis for the hydrogenation of cyclic and noncyclic imines.

A broad selection of olefins undergo hydroborationwith catecholborane in a reaction that is catalyzed by a range of samarium(II1) as well as other lanthanide complexes.The utility of the hydroboration reaction has been significantly extended by the development of transition metal-catalyzed As a consequence of the mechanistic differences between the catalyzed and uncatalyzed processes,3 complementary regio-and diastereoselectivities have been observed. Marks has recently reported a catalyzed hydroboration where the proposed catalytic species is bis(pentamethylcyclopentadieny1)-samarium hydride? The purpose of this paper is to disclose that lanthanide hydroboration catalysis with catecholborane (CB) can be extended beyond lanthanide cyclopentadienyl complexes, to demonstrate that few requirements need be imposed on the catalyst ligand architecture, and to report some of the important mechanistic details of the Sm(II1)-catalyzed process.In the initial set of experiments, various trivalent samarium species were screened as catalysts in the hydroboration of 1-decene with CB to determine whether there are specific ligand requirements for catalyst activity (Table I). Samarium triiodide? (t-BuO)SmI2,6 and (iPr0)3Sm7 were all found to be competent catalysts (entries 1-3) which afforded 1-demo1 in good yield after an 18-h reaction period (25 "C, THF) and subsequent peroxide oxidation. On the other hand, SmBr3, SmCls, and SmF3, as well as Sm(OTf)3,8 failed to catalyze the reaction. In order to evaluate the possibility that samarium is uniquely required for catalysis, other trivalent lanthanides were screened and found to be efficient catalysts (entries 5-8). Regioselectivity for the hydroboration of 1-decene was found to be dependent on both ligand and metal, with ratios of primaryhecondary alcohols ranging from 11-501. Although the results in Table I document a range of catalyst options, SmI3 was selected as the focus of further study.The regioselectivity of the SmI3-catalyzed reaction was discovered to be time-dependent (eq 1). This modulation in regioselectivity during the course of the reaction implies either that formation of the alkylboronate is reversible (vide infra) or that the catalyst is being chemically + Postdodoral Fellow of the Studienstiftung des Deutachen Volkes * Abstract published in Aduance ACS Abstracts, September 1,1993. (1) Burgess, K.; O b e y e r , M. J. Chem. Rev. 1991,91,1179-1191 and (2) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem. SOC. 1992, (3) Evans, D. A.; Fu, G. C.; Anderson, B. A. J. Am. Chem. SOC. 1992, (4) Harrison, K. N.; Marks, T. J. J. Am. Chem. SOC. 1992,114,9220-(5) Imamoto, T.; Ono, M. Chem. Lett. 1987,501-602. (6) Namy, J. L.; Souppe, J.; Collin, J.;Kagan, H. B. J. Org. Chem. 1984, and the BASF AG. references cited therein. 114,6671-6679. 114,6679-6685. 9221. 49,2045-2049. Table I. Hydroboration of 1-Decene with Trivalent Lanthanides. ~ ratio: entry catalyst conv,b % primarrsecondtuy 1 SmIs 2 (t-BuO)SmIz 3 Sm(Oi-Pr)s 98 501 94 46A 98 421 78 261 69 439 92 21:l 84 11:l 92 221...