Diastereoselectivity achiral olefins

Another achiral olefinic precmsor, methyl sorbate, was chosen to exercise formal synthesis of L-daunosamine through enzymatic chiral induction of the C-4, C-5 centers and the diastereoselective conjugated addition of benzylamine to the resulting a,fi-imsaturated ester. Thus, intermediate ace-tonide (116) gave upon benzylamine addition compoimd 117, which after deprotection-acylation underwent lactonization to 118, completing formal synthesis, since the compound was earlier converted to NTFA-daunosamine (Scheme 21) [84,85]. 

Thus, in the example of the chiral olefin 3, there is simple diastereoselectivity of (m4a + m4b)/(m4c + m4d) and induced diastereoselectivities of m4a/m4b and m4c/m4d. It is not strictly necessary, but conversely there is no harm, in applying the term simple diastcrcosclcc-tivity to the first case, i.e., to a diastereoselective reaction of achiral reactants. In this volume the presentation of a given reaction type always begins with simple diastereoselectivity of achiral reactants. 

Fig. 11.15. Analysis of the overall stereoselectivity of a Still—Gennari olefination such as the one in Figure 11.13 simple diastereoselectivity of the formation of the alkoxide intermediate from the achiral phosphonate A and the achiral aldehyde B. For both reagents the terms "back face" and "front face" refer to the selected projection.

Fig. 11.16. Analysis of the simple diastereoselectivity of a Still-Gennari olefination that starts from the enantiomeri-cally pure phosphonate A and the achiral aldehyde B.

It is remarkable and impressive to find that stereochemistry in the conjugate addition of a radical species to an activated olefin is controlled by the chiral bisox-azoline ligand 98 to give a conjugate addition product 100 from 99 in quite high ee and diastereoselectivity (Scheme 12) [58]. Radical trapping by hydrogen abstraction was also shown to be possible in the reaction of 99 to 103, which was controlled by the combination of a chiral alcohol 101 and achiral oxazolidinone 102 [59]. 

The preparation of vicinal polyol triads requires the placement of oxygen functionality at the y-position of the allylic stannane. The Lewis acid-promoted reaction of y-alkoxyallylstannanes with achiral aldehydes was first reported by Koree-da (Scheme 10-60) [98]. The reactions proceed in moderate to high yield and with good diastereoselectivity to produce the homoallylic alcohols. As in the case of simple ( )- and (Z)-138, the reactions are stereoconvergent giving rise to predominantly the syn diastereomer independently of olefin geometry. It was speculated that the reaction proceeds via an acyclic transition structure. 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization of 175, which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid (see Eq. (13.55)). The enantioselective variant simply utilizes an achiral template 188 in conjunction with a bulky chiral binol-derived aluminum Lewis acid 189 (Eq. (13.59)) [75]. Once again the steric bulk of the chiral aluminum Lewis acid complex favors the s-trans rotamer of the acceptor olefin. Facial selectivity of the radical addition can then be controlled by the chiral Lewis acid. The highest selectivity (48% ee) was achieved with 4 equivalents of chiral Lewis acid, providing a yield of 63%. 

Dr Reddy s Laboratories claimed the diastereoselective hydroformylation of an enantiopure bicyclic lactam by means of a Rh[(/ ,/ )-Kelliphite] catalyst (Scheme 4.59) [18]. The olefinic substrate that is produced on a multi-ton scale by Chirotech gives after hydroformylation and a single crystallization step the almost pure aldehyde. Noteworthy, (S,S)-Kelliphite or other ligands, such as (/J,/ ,S)-Bisdiazaphos or (achiral) BIPHEPHOS, induced mainly the formation of the undesired regioisomeric aldehyde. The reaction has been upscaled to 15 g of substrate and used eventually for the production of multifunctionalized... 

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