Enantiopure building-blocks

Enzymatic desymmetrization of prochiral or meso-alcohols to yield enantiopure building blocks is a powerful tool in the synthesis of natural products. For example, a synthesis ofconagenin, an immunomodulator isolated from a Streptomyces, involved two enzymatic desymmetrizations [149]. The syn-syn triad of the add moiety was prepared via a stereoselective acylation of a meso-diol, whereas the amine fragment was obtained by the PLE-catalyzed hydrolysis of a prochiral malonate (Figure 6.56). 

To be presented in the sequel, are a series of prototype reaction channels leading from simple, tautomerically fixed D-glucose derivatives to enantiopure building blocks along preparatively useful, practical protocols, followed by their utilization for the straightforward total synthesis of a series of natural and nonnatural products in optically active, enantiomerically homogeneous form. 

The exemplary reaction channels described in this account, leading from simple sugars to versatile enantiopure building blocks, provide a conceptual... 

Chiral Assemblies Comprising Enantiopure Building Blocks... 

Figure 5. (jeneral structural formulae of the a-, p -, p - and / -peptides derived from proteinogenic L-amino acids (top). Retrosynthetic presentation for the preparation of enantiopure building blocks required for the synthesis of p- and /-peptides (bottom). - The oligtxners (with n > 10 for a-, n = 6 for p- and n = 4 for /-peptides) form helices in methanol solution when we go from a- to P -to p - to / -peptides the helicity is P (right-handed), M (left-handed), P, and P, respeetively [12],... 

The linear synthesis of the title compound started from an enantiopure building block with one stereogenic center. The other six stereogenic centers were introduced by two reagent-controlled Evans aldol reactions, an unselective 1,3-dipolar cycloaddition with subsequent separation of the diastereomers, and a substrate-controlled epoxidation step. 

Lichtenthaler FW (1992) Enantiopure building blocks from sugars and their utilization in natural product synthesis. In Scheffold R (ed) Modern Synthetic Methods. VHCA, Basel, p 273... 

F. W. Lichtenthaler, U. Klaeres, M. Lergenmueller, and S. Schwidetzky, Enantiopure building blocks from sugars. 14. Various glycosyl donors with a ketone or oxime function next to the anomeric center facile preparation and evaluation of their selectivities in glycosidations, Synthesis, (1992) 179-184. 

In this context, however, asymmetric transformations with chiral metal complexes or enzymes are particularly important. The demand for enantiopure building blocks in the fine chemical and pharmaceutical industries is still hampered by simple asymmetric processes which can be scaled up, as well as by the stability, recyclability, and hence the price of most chiral catalysts (refer also to the chapter by K.-U. Schoning in this volume). Immobilization of effective and robust catalytic systems and their application in continuous flow reactors is regarded as a key for success in this field. 

The elaborated synthetic route is shown in Scheme 7 [25, 26]. Treatment of the enantiopure building block 34 (obtained from 17 by oxidation) with an excess of isopropenyl lithium results in diastereoselective nucleophilic addition to... 

Xiong and coworkers used an off-the-shelf antimalarial alkaloid, quinine derived enantiopure building block, HQA (6-methoxyl-(85, 9R)-cmchonan-9-ol-3-carboxylic acid, L,), to synthesize a homochiral MOPM, [CdCQA) ] (7, Scheme 1) with an open channel 3D structure, which crystallizes in a distorted octahedral geometry with two... 

Domino metathesis reactions of the dienynes 97 and 99 catalyzed by the phosphine-free ruthenium catalyst 4 in the presence of ethylene allowed a rapid access to the naturally occurring marine trisnorsesquiterpenes (—)-clavukerin A (98) and (—)-isoclavukerin A (100), respectively, by preferential initiation at the disubstituted alkene (Scheme 2.36) [18i]. As the dienyne substrates 97 and 99 were easily available from (S)- and (J )-citronellal, respectively, in only three high yielding operations, the 1,3-dienes 98 and 100 now offer themselves as enantiopure building blocks for the preparation of structurally more complex hydroazulene targets. 

J )-Pulegone, readily available from pennyroyal oil, is an important enantiopure building block for organic syntheses. Propose a mechanism for each step in this transformation of pulegone. 

Manoj, K.M., Archelas, A., Baratti, J. and Furstoss, R. (2001) Microbiological transformations. Part 45. A green chemistry preparative scale synthesis of enantiopure building blocks of Eliprodil elaboration of a high substrate concentration epoxide hydrolase-catalyzed hydrolytic kinetic resolution process. Tetrahedron, 57,695-701. 

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