Allyl external stereoselection

The stereochemistry of the dialkoxylation arises from two external attacks by the alcohol, one on the r-diene complex and the second on the intermediate 7r-allyl complex. This is in accordance with the other palladium-catalyzed 1,4-syn additions discussed above. Also, the 1,4-dialkoxylation of acyclic 1,3-dienes was stereoselective. For example, the reaction of ( , )-2,4-hexadiene gave the d,l products 38 by a 1,4-syn addition. The double bond was exclusively of E configuration (equation 18),... 

In general, allyl ethers are less reactive than esters and therefore rarely used as substrates in the palladium-assisted alkylation with soft nucleophiles. Phenyl a- or )5-D-eor//jro-hex-2-enopyra-nosides react with a variety of soft carbanions under neutral conditions to give the a- or t -C -g]ycopyranosides, respectively in good to excellent yields both stereoselectively and with complete regioselectivity88. Since the liberated phenoxide ion deprotonates the active methylene compounds, no external base has to be added. 

The influence of an external, oxygen-substituted allylic stereocenter in C-4-(Z)-alkenyl nitrones has shown that the extent of stereoselection depends on ... 

Since the AAA in aqueous solution has been most studied with Pd(diphosphine)-bearing catalysts, we screened the library of biotinylated diphosphine ligands that were initially used in the hydrogenation study (Sect. 2.1). Our initial experiments focused on symmetric 1,3-diphenylallyl acetate and dimethytmalonate as soft nucleophiles (Scheme 3) [50]. It is widely accepted that the enantiodiscrimmation event occurs through the external attack of a soft nucleophile on a palladium allyl intermediate. The AAA thus bears resemblance to enzymatic reactions, in which a reactant need not necessarily bind to the active site of the enzyme for the reaction to proceed with high stereoselectivity [51]. The second coordination sphere may thus play an important role in ensuring selectivity. 

Microencapsulated palladium (MC[Pd(PPh3)j) was found to catalyze the allylic alkylation of 1,3-diphenylpropenyl ester with good stereoselectivity (up to 83%) in the presence of an external chiral ligand 107 (Scheme 36) [122]. 

If the oxidation is performed in the presence of an external dienophile, the respective products of [4+2] cycloaddition are formed [351-356]. Typical examples are illustrated by a one-pot synthesis of several silyl bicyclic alkenes 283 by intermolecular Diels-Alder reactions of 4-trimethylsilyl substituted masked o-benzoquinones 282 generated by oxidation of the corresponding 2-methoxyphenols 281 [351] and by the hypervalent iodine-mediated oxidative dearomatization/Diels-Alder cascade reaction of phenols 284 with allyl alcohol affording polycyclic acetals 285 (Scheme 3.118) [352]. This hypervalent iodine-promoted tandem phenolic oxidation/Diels-Alder reaction has been utilized in the stereoselective synthesis of the bacchopetiolone carbocyclic core [353]. 

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