Tt-Facial selection

An interesting phenomenon has been observed in the high pressure Diels-Alder reactions of the l-oxa[4.4.4]propella-5,7-diene (117) with 1,4-naphthoquinone, maleic anhydride and N-phenylmaleimide, where the diene 117 undergoes a rearrangement to the diene isomer 118 which, although thermodynamically less favored, exhibits a greater reactivity [40]. The reactivities of the three dienophiles differed since maleic anhydride and N-phenylmaleimide reacted only in the presence of diisopropylethylamine (DIEA) and camphorsulfonic acid (CSA), respectively (Scheme 5.15). The distribution of the adduct pairs shows that the oxygen atom does not exert a consistent oriental dominance on TT-facial selectivity. 

It becomes intriguing to inquire what leads to the observed contrasteric reactivity. Intensive studies to disclose the origin of Tt-facial selectivity examined various dienes having unsymmetrical 7t-plane, since their reactions potentially generate five or more consecutive stereocenters with one operation. In this chapter, we review the theories to disclose the origin of 7t-facial selectivity in Diels-Alder reactions of the substrates having unsymmetrical 7t-planes. Recent works are discussed. 

In contrast, 5-chlorocyclopentadiene 2 gave syn/anti mixture and 5-bromo- and 5-iodocyclopentadienes 11 and 12 reacted with anti tt-facial selectivity [11, 12], In these cases, repulsive interaction between the substituents and dienophiles cannot be excluded (Scheme 7). 

Overman, Hehre and coworkers also reported anti tt-facial selectivity in Diels-Alder reactions of the vinylcyclopentenes 73,74 and 4,5-dihydro-3-ethynylthiophen 5 -oxide 75. They attributed the selectivity to destabilizing electronic interaction between the allylic heteroatom and dienophile in the syn attack transition state (Scheme 43) [38],... 

TT-Facial selectivity in the Diels-Alder reactions of thiophen 1-oxides has recently attracted keen attention (Scheme 49). Fallis and coworkers reported in situ generated 2,5-dimethylthiophene 1-oxide 98 reacted with various electron-deficient dienophiles exclusively at the syn face with respect to sulfoxide oxygen [57],... 

Anti TT-facial selectivity with respect to the sterically demanded substituent in the Diels-Alder reactions of dienes having unsymmetrical tt-plane has been straightforwardly explained and predicted on the basis of the repulsive interaction between the substituent and a dienophile. However, there have been many counter examples, which have prompted many chemists to develop new theories on the origin of 7t-facial selectivity. We have reviewed some theories in this chapter. Most of them successfully explained the stereochemical feature of particular reactions. We believe that the orbital theory will give us a powerful way of understanding and designing of organic reactions. 

This modification is based on the consideration that such bidentate dienophiles would form rigid complexes with a chiral Lewis acid, resulting in high reactivity and a good level of TT-facial selectivity during the cycloaddition reaction. 

TT-Facial Selectivity in Reaction of Carbonyls A Computational Approach... 

Entries 1, 2, 8, or 9). On the other hand, the silyl ethers 90-94 provided the desired endo-adduct A predominantly (Entries 3-7). In the case of the bulkier silyl ethers 93 and 94, the Tt-facial selectivity significantly decreased (Entries 6 and 7 compared to 3-5). As anticipated, by employing the silyl ethers 90-92 for the IMDA reaction, the desired cycloadduct A for the syntheses of mniopetal F and kuehneromycin A was obtained stereoselectively. 

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