Allyl compounds Subject

Palladium-mediated substitutions of silylated allylic compounds are not subject to steric hindrances. The silicon atom exerts control over the reaction site snch thaty-substitution results [64]. While the electrophiles are unavoidably disjoint with respect to the acceptor silicon, one of the two limiting forms, i.e., the a-silylcarbenium ion, is much more unfavorable (a-a arrangement) and therefore its population is expectedly low. 

When an allylic compound such as 23 is subjected to nucleophilic attack as shown in Scheme 7.5, the product is that of the SN2 reaction. If the allylic compound now has the structure shown in 24, there is significant steric hindrance to approach of the nucleophile to the chlorine-bearing carbon. There is, however, an alternative site, available to the nucleophile, and this is shown in Scheme 7.6. 

In the studies of syrrngm, an active component in traditional Chinese medicine, it was shown that 4-hydroxy-3,5-drmethoxybenzoic acid 167 reacted with allyl bromide under basic conditions to produce a mixture of O- and C-allylated compounds 168, 169. After the mixture was subjected to heating at about 200 °C, a para-Claisen rearrangement took place to form the main product 169 in 71% yield (equation 76) . 

Very recently, Compain proposed an approach to spirocyclic imino sugars (Scheme 41). The synthetic route started from known cyclobutanol 208, which was transformed into carbamate 209. Rhodium-catalyzed intramolecular C-H amination performed on that compound led to oxazolidinone 211. Subsequently, this compound was Al-allylated and subjected to the ring-closing metathesis with the Grubbs II cat. As a... 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

The outcomes of intramolecular cyclizations of hydroxy vinylepoxides in more complicated systems can be difficult to predict. In a study of the synthesis of the JKLM ring fragment of dguatoxin, epoxide 44 was prepared and subjected to acid-mediated cydization conditions (Scheme 9.24) [114]. Somewhat surprisingly, the expected oxepane 45 was not formed, but instead a mixture of tetrahydropyran 46 and tetrahydrofuran 47 was obtained, both compounds products of attack of the C6 and C5 benzyl ether oxygens, respectively, on the allylic oxirane position (C3). Repetition of the reaction with dimsylpotassium gave a low yield of the desired 45 along with considerable amounts of tetrahydropyran 48. 

The synthesis in Scheme 13.21 starts with a lactone that is available in enantiomer-ically pure form. It was first subjected to an enolate alkylation that was stereocontrolled by the convex shape of the cis ring junction (Step A). A stereospecific Pd-mediated allylic substitution followed by LiAlH4 reduction generated the first key intermediate (Step B). This compound was oxidized with NaI04, converted to the methyl ester, and subjected to a base-catalyzed conjugation. After oxidation of the primary alcohol to an aldehyde, a Wittig-Horner olefination completed the side chain. 

Mechanistic explanations are still subject to debate. It is well known that n-allylpalladium compounds may show distortions such that one end of the allyl group assumes a G-character, while the other part assumes a Ti-character. The differentiation between C-l and C-3 is due to the different steric interaction... 

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