Building planar-chiral

Enantioenriched alcohols and amines are valuable building blocks for the synthesis of bioactive compounds. While some of them are available from nature s chiral pool , the large majority is accessible only by asymmetric synthesis or resolution of a racemic mixture. Similarly to DMAP, 64b is readily acylated by acetic anhydride to form a positively charged planar chiral acylpyridinium species [64b-Ac] (Fig. 43). The latter preferentially reacts with one enantiomer of a racemic alcohol by acyl-transfer thereby regenerating the free catalyst. For this type of reaction, the CsPhs-derivatives 64b/d have been found superior. 

We were able to synthesize chiral dendri- rations. In addition, the circular dichrograms mers with stable planar-chiral building blocks clearly indicate that chiral dendrimers based to avoid racemisation. [47] In contrast to the on derivatives of [2.2]paracyclophanes can results of Seebach et al. these dendrimers be employed for complexation of certain show increasing chirality with inreasing gene- metal cations. 

An enantioselective total synthesis of (+)-ptilocaulin (79), a marine alkaloid with high antimicrobial and cytotoxic activity, was reported by Schmalz (Scheme 14) [40, 41]. The synthesis starts from anisole-Cr(CO)3, which is converted to the planar-chiral building block 74 with >99% ee by enantioselective deprotonation/silylation [42] and subsequent recrystallization. After attachment of a 2-butenyl side-chain ( 75) [43], nucleophilic addition of 2-lithio-l,3-... 

This synthesis is remarkable because mayor parts of it are carried out at the Cr(CO)3-complexed ligand and several new bonds are formed with high diaster-eoselectivity under the stereochemical influence of the planar-chiral n-complex moiety. The chiral building block 80 is obtained in enantiomerically pure form either by resolution (via imine formation with 1-valinol) or by diastereoselective complexation of a chiral cyclic aminal according to the procedure of Alexakis... 

Recently, Uemura and coworkers reported a synthesis of the A-B ring system of vancomycin (Scheme 20) [58]. The planar-chiral building block 106 is pre-... 

In conclusion, q -arene transition metal complexes have demonstrated their unique potential for organic synthesis. In particular, planar-chiral q -arene-Cr(CO)3 complexes are valuable building blocks for the diastereo- and enanti-oselective synthesis of complex natural products and related bioactive compounds. Highly original and competitive overall syntheses of various classes of natural products have been developed. The expenditure spent for the introduction of the metal fragment pays off especially in those cases, where the various chemical and stereochemical effects of the metal unit can be exploited in several subsequent transformations. Besides arene-Cr(CO)3 complexes, cationic arene-RuCp complexes have also been applied in synthesis, especially as they allow for efficient arylether formation under mild conditions. 

Planar chiral cyclopentadienyhnetal complexes (M=Zr, Ti, Co, Fe, Mo) are stereoselective catalysts, which have received attention in a number of as3munetric synthetic applications [42]. The conpling of l-(p-tolyl-snlfinyl)naphthalene-2-carboxylate ester 50 with indenyl lithium 51 and fluorenyl Uthinm 52 provides access to l-(2 -methyl-3 -indenyl)naphthalene (53) and 9-(l -naphthyl)fluorene (54), respectively, as single rotamers (Scheme 8.12) [43-45]. The atropisomer-ism arises fiom hindered rotation abont the bond between the cyclopentadienyl ring and the naphthalene moiety. Ligands 53 and 54 are building blocks for the enantiospecific synthesis of planar chiral bidentate ansa zirconocene complexes 55 and 56 [46,47]. 

Tomooka, K., Akiyama, T, Man, P., and Suzuki, M. (2008) Asymmetric synthesis of (—)- and (-l-)-kainic acid using a planar chiral amide as a chiral building block. Tetrahedron Lett, 49, 6327-6329. 

However, in the case of the ligands with backbone chirality the asymmetric environment reverses when changing from a Rh(I) square planar to a Rh(III) octahedral complex. This is demonstrated in Figure 1.18 (bottom left), showing that in the octahedral complex the area above the "quasi"-equatorial phenyl becomes more attractive for building a chelate cycle. ... 

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