Spiro hepta-4,6-diene cycloaddition reactions

The Diels-Alder reaction of methylenecyclopropane with cyclohexa-1,3-diene at 120°C gave the spirocyclopropanebicyclooctene in 50% yield.When cyclopentadiene and spiro[2.4]hepta-4,6-diene were used as dienes the respective spirocyclopropanenorbornenes were obtained. At 190°C, (chloromethylene)cyclopropane underwent [2 + 4] cycloaddition reactions with cyclopentadiene, furan and cyclohexa-1,3-diene to give the respective Diels-Alder adducts. The reaction of buta-1,3-diene and cyclohexa-1,3-diene with bicyclopropylidene as dienophile gave predominantly the [2 + 2] cycloadduct in addition to a small quantity of the Diels-Alder product. Cyclopentadiene, however, formed exclusively the dispirocyclo-propanenorbomene as result of a formal [2 + 4] cycloaddition. 

Among the known [4 + 3] cycloaddition protocols examined, only the Schmid2 and Fohlisch4-10 procedures were found to afford the [4 + 3] cycloadducts in synthetically useful yields. Schmid s rarely-used "aminoallyi" reaction appears to be most effective, particularly when sterically demanding 1,3-dienes [e.g., spiro[2.4]hepta-4,6-diene] or otherwise recalcitrant N-acylpyrroles are required for the cycloadditions. On the other hand, with sterically unencumbered 1,3-dienes and also with furans, the Fohlisch reaction has emerged as the method of choice in view of the practical aspects of simple and convenient execution. 

The method described here is the direct adaptation of the original procedure developed by the late Professor Hans Schmid- As summarized in Scheme 1, the Schmid reaction has been successfully applied to [4 + 3] cycloadditions with cyclopentadiene, spiro[2.4]hepta-4,6-diene, and N-Boc-pyrrole. Additional examples can be found in references 8 and 9. Use of functionalized six-membered oxyallyls and synthetic applications of the [4 + 3] cycloadducts have also been described.8... 

Lanthanide /3-diketonates have been used as catalysts in Diels-Alder reactions. The first example of a lanthanide-catalyzed cycloaddition was the dimerization of spiro[2.4]hepta-4,6-diene by [Eu(tfn)3] (Morrill et al., 1975) (scheme 2). In the absence of the europiitm(lll) complex no dimerization took place. Because of the mild experimental conditions, this catalyst has potential in Diels-Alder reactions where acid labile components are combined. An example is the cycloaddition of cyclopentadiene with acrolein (Danishefsky and Bednarski, 1985). 

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