Cyclopentadienes cyclocondensations

Grieco PA, Bahsas A (1988) Role reversal in the cyclocondensation of cyclopentadiene with heterodienophiles derived from aryl amines and aldehydes synthesis of novel tetrahydroqui-nolines. Tetrahedron Lett 29 5855-5858... 

The original synthesis of indenes alkylated on the benzene ring required multistep sequences, often with low regioselectivity and low overall yield. More efficiently, 4,7-alkyl-substituted indenes can be prepared in a one-step synthesis by cyclocondensation of alkanediones with cyclopentadienes (eq. (1)) [13, 14]. 

For example, the cyclocondensation of cyclopentadiene with 2,5-hexanedione in the presence of sodium affords 4,7-dimethylindene in 65 % yield. Subsequent standard transformations yield the desired bridged-metallocene structures. 

An interesting role reversal is observed in the cyclocondensation of cyclopentadiene with iminium ions derived from aryl amines and aldehydes [5]. The use of aniline as the amine component with formaldehyde and cyclopentadiene in aqueous acetonitrile fails to produce any of the corresponding iV-phenyl-2-azanorbornene 11. Instead, the novel tetrahydroquino-line-based pentacyclic products 12 and 13 are isolated in a near-quantitative yield as a 3.7/1 isomeric ratio ... 

Grieco et al. have also reported the use of C-acyl iminium ions as heterodienophiles in the aqueous Diels-Alder reaction with cyclopentadiene [8]. This work details the cyclocondensation of C-acyl A-alkyl iminium ions 18 and C-acyl A-unsubstituted iminium ions 19 with cyclopentadiene in water ... 

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