Allylation, oxygen, esterification

Alkenyloxirans 16 are reactive allylating agents used under neutral conditions. The epoxy ring is opened by Pd(0) to form 7r-allylpalladium with generation of an alkoxide anion, which abstracts a proton from a pronucleophile to produce the a-hydroxy- 7r-allylpalladium 17. Reductive elimination of 17 affords either 1,4-adducts 18 or 1,2-adducts 19 [8,9]. The 1,4-adducts are mainly obtained under usual conditions due to the electronic effect of the epoxide oxygen. The 1,4-adducts 18 are allylic alcohols, and can be used again for the allylation after esterification to yield 19a. 

Much effort, using mainly Fusarium spp., has been devoted to the identification of the oxygenation, cyclisation and esterification steps which take place after the formation of trichodiene. Fermentations carried out on a large scale, or in the presence of enzyme inhibitors, such as ancymidol (225) or xanthotoxin (219), or with mutant strains (223), have yielded metabolic products [(47), (52), (53), (59) Table 8] which are proven intermediates. They are derived from trichodiene by plausible pathways involving allylic hydroxylation at positions 2a and 11a, fol-... 

This one-step transformation of an alkene to an allylic acetate compares well with other methods of preparation such as hydride reduction of a, 8-unsaturated carbonyl compounds followed by esterification. The scope and limitations of the reaction have been investigated. The allylic acetoxylation proceeds via a TT-allylpalladium intermediate, and as a result, substituted and linear alkenes generally give several isomeric allylic acetates. With oxygen nucleophiles the reaction is quite general, and reactants and products are stable towards the reaction conditions. This is normally not yet the case with nitrogen nucleophiles, although one intramolecular palladium-catalyzed allylic amination mechanistically related to allylic acetoxylation has been reported. ... 

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