Vinyl ethers oxypalladation

Oxypalladation of vinyl ether, followed by alkene insertion, is an interesting synthetic route to functionalized cyclic ethers. In prostaglandin synthesis, the oxypalladation of ethyl vinyl ether (40) with the protected cyclopentenediol 39 generates 41 and its intramolecular alkene insertion generates 42. The intermolecular insertion of the alkene 43, and /1-elimination of 44 occurred as one-pot reaction at room temperature, giving the final product 45 in 72% yield [46], The stereochemistry of the product shows that the alkene insertion (carbopalladation of 41) is syn. It should be noted that the elimination of /1-hydrogen from the intermediate 42 is not possible, because there is no /1-hydrogen syn coplanar to the Pd and, instead, the insertion of alkene 43 occurs. 

The use of other oxygen nncleophiles such as acetic acid or alcohols also results in oxypalladation. Subsequent 8-Pd-H elimination produces vinyl acetates or vinyl ethers. However, these are not necessarily the final products. When acetic acid is used as the nucleophile, ally lie acetates often become the major product. In the case of alcohols, another alcohol reacts with the resulting vinyl ether to give an acetal (Scheme 2). Focusing on such product compositions, the oxypalladation of alkenes with carboxylic acids and alcohols followed by dehydropalladation is described here. 

As shown in Scheme 32, when allylilc alcohols are allowed to react with vinyl ethers, the resulting a--bond in oxypalladation intermediate adds intramolecularly to the C=C bond of allylic moiety.f Subsequent (3-Pd—elimination leads to cyclic acetals. With this method, albeit stoichiometric in palladium(II), a unique approach to the synthesis of prostaglandins was attained by Larock and Lee as shown in Scheme 33.1 ... 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

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