Selectivity shielding

Murray and colleagues199 developed some 2,5-diketopiperazines as new chiral auxiliaries and examined their asymmetric induction in the Diels-Alder reactions of their A-acryloyl derivatives with several dienes. Some of their results with dienophile 320 have been summarized in Table 19 (equation 89). When the benzyl group on 320 was substituted by an isopropyl or /-butyl group, the diastereofacial selectivity dropped dramatically. It was proposed that tv-tt stacking between the phenyl group and the electron-poor double bond provided a more selective shielding of one face of the double bond in this special case. 

Aliphatic enamine derivatives 198 linked to a chiral oxazolidone allow an almost totally diastereoselective [2 + 2] photocycloaddition of 102 (Scheme 53). This efficiency was attributed to a selective shielding of the lower face by the R substituents in the preferred conformation of the chiral oxazilidone enecarbamate. Interestingly, the de of 199 did not depend on the configuration of the stereocenter on the exocyclic appendage [122]. 

The stereochemical outcome of the cyclizations is explained by assuming a preferred transition state with a planar radical center, conjugated with the carboxylic group of the ester or amide. In this conformation an oxygen of the camphor sultam or the phenyl ring of the 8-phenylmcn-thyl auxiliary selectively shields one face of the radical center. 

The Spaceship shall be designed to accommodate the radiation environment specified in the Environmental Requirements Document (982-00029). Ref 1-8) Solar, galactic and Jovian radiation sources, coupled with reactor radiation, must be considered during electronics selection, shielding trades and material evaluation. 

The external radiation environment is described in detail in the Environmental Requirements Document (982-00029). The internal radiation environment will be influenced by position relative to the reactor and shield, by local shielding, and by the external radiation levels. Solar, galactic and Jovian radiation sources, coupled with reactor radiation, must be considered during electronics selection, shielding trades and material evaluation. A detailed description of the internal radiation environments relative to the different zones of the reactor module can be found in Section 10.5,... 

These selective shieldings are expressed using the parameter Ad, which is defined as the difference between the chemical shift of a certain proton in the (, -MTPA ester and the chemical shift of the same proton in the (7 -MTPA derivative. All the protons shielded in the (. -MTPA will present a positive Ad value, whereas those shielded in the (S)-MTPA derivative will present a negative Ad value. 

Despite the fact that the ap conformer is less abundant, it is considered to be the most important conformer, in terms of NMR shifts, and its use in the correlation model shown in Figure 60 is justified by the results of NMR studies at low temperature and by aromatic shielding calculations. A second aspect that must be considered concerns the fact that in neither of the two conformers can the aromatic ring produce selective shielding on just one of the substituents (R1/R2), which is a situation that is contrary to that observed for the AMAA derivatives. In the case in question here, both substituents are moderately shielded in the ap conformer, whereas in the sp conformer, they are only slightly shielded. 

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