Rhodium (I)‐katalysierte Reaktionen der [2+4]‐Cycloaddukte aus 3,4‐Dimethoxyfuran und Acetylendicarbonsäure‐estern

[2+4]‐Cycloaddition Products of 3,4‐Dimethoxyfuran with Acetylenedicarboxylates and Their Transformations under the Influence of Rhodium(I) Catalysts 3,4‐Dimethoxyfuran ( 1 ) readily reacts with acetylenedicarboxylates ( 2 ) at room temperature in a [2+4]‐cycloaddition to give a mono‐( 3 ) and several di‐addition products. 90% of the latter consists of the endo‐exo compound 4 . Under the influence of catalytic amounts of [Rh(CO) 2 Cl] 2 the mixture of mono‐ and di‐adducts in methanolic solution is smoothly transformed into endo ‐5,5,6‐trimethoxy‐7‐oxabicyclo [2.2.1]hept‐2‐ene‐2,3‐dicarboxylate Alle Verbindungen sind racemisch. Die Formeln stellen jeweils nur ein Enantiomeres dar. (5) , 3‐hydroxy‐4,5‐dimethoxyphthalate ( 6 ) and (I R *, 2 S *, 4 R *, 5 R *, 7 R *, 11 R *, 12 R *) ‐5,8,8,9,12‐pentamethoxy‐3,6‐dioxatetracyclo [5.3.1.1 2,5 . 0 4,11 ]dodec‐9‐ene‐1,11‐dicarboxylate ( 7 ).