Intramolecular 1,4-oxyacetoxylation

In MeOH, l,4-dimethoxy-2-cyclohexene (379) is obtainejl from 1,3-cydo-hexadiene[315]. Acetoxylation and the intramolecular alkoxylation took place in the synthesis of the naturally occurring tetrahydrofuran derivative 380 and is another example of the selective introduction of different nucleo-philes[316]. In intramolecular 1,4-oxyacetoxylation to form the fused tetrahy-drofurans and tetrahydropyrans 381, cis addition takes place in the presence of a catalytic amount of LiCI, whereas the trans product is obtained in its absence[317]. The stereocontrolled oxaspirocyclization proceeds to afford the Irons product 382 in the presence of Li2C03 and the cis product in the presence of LiCl[ 318,319]. 

Palladium-catalyzed reaction of dienol 80 in acetone in the presence of acetic acid and benzoquinone resulted in an intramolecular 1,4-oxyacetoxylation (Scheme 8-28) [107], The stereochemistry of the reaction can be controlled via a slight variation of the ligand environment. Thus, under chloride-ion-free conditions, a rrawj-oxyacetoxylation occurs. Usually this reaction is highly stereoselective (>98% tram addition), except m = n = 2 in Scheme 8-28, where the tramlcis ratio is 75 25. When the reaction is run in the presence of a catalytic amount of chloride, the stereochemistry is reversed and now a 1,4-cts-oxyacetoxylation takes place. The effect of the chloride is the same as discussed above, i.e., it blocks the coordination of acetate so that cis migration by acetate cannot occur. 

Synthetic Applications In several syntheses toward naturally occurring furanoid terpenes, the intramolecular oxyacetoxylation was applied as a key step [125]. For example, in the synthesis of marmelo oxides A and B,... 

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