Induced diastereoselectivity,

Moreover, a dramatic increase of the reaction rate was observed when the coupUng of aromatic imines mediated by samariiun diiodide was carried out in the presence of both water and a tertiary amine or tetramethylethylene-diamine (TMEDA) [29], causing the almost instantaneous formation of the 1,2-diamine, although with undetermined diastereoselectivity. Similarly, the samarium diiodide promoted reductive coupling of iminiiun ions formed in situ by reacting ahphatic aldehydes with secondary amines and benzotriazole occurred at temperatures as low as - 70 °C [30]. Even in this case a mixture of diastereomers with undetermined ratio was obtained nevertheless, the item of diastereoselectivity induced by a chiral amine (auxiliary) is worthy of investigation. 

Almost 50 years ago, Cram outlined a rule (Cram s rule), which proved to be fruitful in understanding, predicting, and controlling diastereoselectivity induced by a remote stereocenter [258,259], Numerous examples of 1,2 induction have confirmed over the time the predictive character of this rule [260], Afterwards, other important contributions of Felkin and coworkers and Anh... 

In another example, the cumulative effect of equatorial attack in prochiral cyclohex-anoneimines with diastereoselectivity induced by a chiral nitrogen substituent allowed the synthesis of spirocyclic oxaziridines with a high induction of axial dissymmetry. The major oxaziridine isomer results from both the favored equatorial attack and oxidation anti to the chiral nitrogen substituent (equation 45)204... 

The diastereoselectivity induced by a chiral auxiliary present in an epoxidized compound has been studied by various groups, albeit often with limited success. 

The menthyl group has often been employed as an easily available chiral auxiliary. 8-Phenylmenthyl 2-amino-3,3-difluorocyclopentenecarboxylate 222 prepared from 219 via 221 was hydrogenated diastereoselectively to form ds-2-aminocarboxylate 223 (see Scheme 9.49) [77]. The higher diastereoselectivity induced by the 8-phenylmenthyl group was also observed in Znl2-catalyzed NaBH4 reduction of open-chain 3-amino-a, 3-unsaturated esters 224. In contrast, the unsubstituted menthyl ester was reduced with no practical diastereoselectivity (see table in Scheme 9.49). 

Although the solid-phase formation of this clinically valuable pharmacophore occurred with high cis selectivity, the diastereoselectivity induced by the asymmetric center of the amino acid was only very moderate. Induction could be achieved, however, with an optically active ketene. By using this approach, a model library of 25 p-lactams of type 111/112 was initially formed in higher than 80% purity, and this chemistry was subsequently used to generate a combinatorial library of thousands of 3,4-bis-substituted 2-azetidinones of sufficient purity to be directly submitted for bioassays. 

The ccnnplete diastereoselectivity, induced and simple, shown by this reaction is not unexpected if one considers the corresponding use of these imines in the preparation of p-lactams. Here, in fact, the formation of the P-amido esters occurs via a closed boatlike transition state that appears to be more stable than the corresponding chair-like one. The aldol intermediate then und goes cyclization via a nucleophilic substitution to give the final aziridine product (Scheme 21). 

Diastereoselectivity induced hy y-substituted allyltins the reaction leads to an excellent syn diastereoselectivity. First reported by Maruyama and coworkers [178], this was explained by proposing an antiperiplanar transition state for the syn selectivity whatever the nature (E or Z) of the crotylstannane. Contrastingly, it was also... 

Diastereoselectivity induced by chiral aldehydes the substrate plays an important role in the facial diastereoselection, particularly when there is an asymmetric center adjacent to the carbonyl group. In the general case, the approach of the allyltin is assumed to follow the Felkin-Anh model giving the syn adduct preferentially. This induction was used for the synthesis of natural products [186] even as complex as Ciguatoxin or Laulimalide [187]. 

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