Dienes, attack

The assumed transition state for this reaction is shown in Scheme 5.5. The two bulky t-butoxy groups are expected to locate at the two apical positions. One of the 3,3 -phenyl groups would effectively shield one face of an imine, and consequently, a diene attacks from the opposite side. Judging from this model, similar selectivities were expected in the Mannich-type reactions of imines with silyl eno-lates. Actually, when ligand 10 was used in the reaction of imine la with S-ethyl-thio-l-trimethylsiloxyethene, the corresponding / -amino thioester was obtained in 84% ee (Scheme 5.6). As expected, the sense of the chiral induction in this case was the reverse of that observed when using catalyst 6 [12, 25]. 

In general, 2,3-dialkyl-p-benzoquinones exhibit site selectivity in that they tend to give predominantly Diels-Alder adducts resulting from diene attack on the external rather than the internal double bond. This external site selectivity is, however, dramatically reversed when a (substituted) cyclobutane ring is fused to p-benzoquinone. Paddon-Row and coworkers103 studied the reactions of p-benzoquinones such as 129 with several... 

The regiochemistry 81 is fine the nucleophilic end of the diene attacks the electrophilic end of 80. But stereochemistry is all important. Undoubtedly the trans isomers -79 and -80 will be easiest to make so we should explore the result of the Diels-Alder with these two. We expect an endo transition state 82 and this gives 83. Unfortunately this stereochemistry is wrong for all the lycoranes. 

The next step is something quite new. No new C-Cbond is formed instead, the diene attacks the hydroperoxide to give an epoxide and a fully conjugated triene. The new double bond is cis this time, which is what we should expect from the conformation we have been using. This is LTA4 and all the other leukotrienes are made from this compound,... 

These woikers have also examined asymmetric induction in the process in some detail. For instance, cycloaddition of sugar aldehyde (167) occurred to afford only adduct (169) at high pressure (equation 80). It was suggested that the Diels-Alder reaction proceeds via diene attack on the aldehyde conformation shown in (168) from the least congested face. Other chiral aldehydes have been investigated by this group, as has the e ect of lanthanide catalysts upon the extent of asymmetric induction. Summaries of this work have recently been published. ... 

This is a straightforward Diels-Alder reaction. The product is the racemic acid 3. The faces of the dienophile 2 are enantiotopic - if the diene attacks from one face 4a we get one enantiomer (R) -3 and the other (S)-3 if it attacks from the other 4b. However, it doesn t matter at all which face of the diene 1 reacts. Its faces are homotopic. 

Fig. 26) in which the exocyclic carbonyl group is bound in the apical position and the dienophile reacts out of the s-cis conformation. Steric shielding of the Si face by the ligand phenyl group would favor diene attack on the exposed Re face. As with Mg(II)bis(oxazoline) complexes (Sect. 3.1.3), the presence of ligands other than the dienophile appear to be important in the creation of a stereode-fined environment about the metal center. 

The two faces of the double bond of the dienophile are now different because of the chiral centre they are diastereotopic, and the diene can distinguish between them. If we now do the Diels-Alder reaction in the presence of a Lewis acid, Et2AlCl, the aluminium chelates the oxygen atoms of the dienophile to form the rigid and reactive structure shown below. The isopropyl group is held in such a way that its steric bulk prevents the diene attacking that face of the prochiral alkene. The diene has no choice but to attack from above, and only one of the possible diastereoisomeric products is formed. 

The observed high diastereofacial selectivity is attributable to conformational restriction at the allylic center in dienophile 20. The steric bulk of the sugar chain and the acetyl group effectively limit the C-3—C-4 rotamers to a single conformer, as shown in the following scheme. The diene attacks almost exclusively from the same side as the allylic oxygen atom fsi-face for the D-enonate 20), where the steric hindrance is lowest. 

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