Reactions of dipivaloylketene dimer with nucleophiles : new access to the 2,6,9-trioxabicyclo[3.3.1]nona-3,7-diene ring system (bridged bis-dioxines)

Novel and convenient methods to prepare several functionalized 2,6,9-trioxabicyclo[3.3.1]nonadienes (''bridged bis-dioxines'') 6, 10, 11, and 12 are described. In general, these are obtained by addition of nucleophiles to alpha-oxoketene 5 in a multistep reaction sequence, where the specific reaction pathway depends strongly on the type of nucleophile employed: Aromatic amines with electron-donating substituents afford the trioxabicyclononadienes 6, while with aliphatic amines or thiolates, 2 equiv of the corresponding dipivaloylacetamides 13 or thioesters 14, respectively, are formed. In the case of OH nucleophiles, the primary 1:1 adducts 8 are isolable, characterized by an X-ray analysis of 8a. Under acidic conditions, 8 can be cyclized to the bifunctionalized bis-dioxines 10. The bis-carboxylic acid 10c serves as a valuable intermediate to synthesize other derivatives, e.g. the bis-acid chloride 11 and the bis-ester 12. Mechanistic pathways leading to the various reaction products are discussed in detail and supported by semiempirical molecular orbital calculations using the AM1 method. The spontaneous formation of the bis-dioxine skeleton in the reaction 5 --> 6 is likely to proceed via an unusual [4 + 4] type tandem cyclization reaction.