[themagicbean]

In performing a basic Diels-Alder between furan and maleimide, our product was yellow (solid).   Solvent was ethyl acetate, products were heated at 90C for three hours (trying to get the exo product):

Desired NMR spectra 11.1; 1   + 6.5;2 + 5.1; 2 + 2.8; 2 (exo)
According to http://nmrshiftdb.nmr.uni-koeln.de/ the Hs are HN, CH=CH, H next to ether bridge, and H on the carbons connecting the rings. 

Result:  11.1; 1 + 10.9;1 (appeared almost to be a doublet but given the remaining spectra probably not) 6.8; 2 + 6.5; 2 + 5.1; 2 + 2.8; 2

Suggestion was that there was some sort of animal-based contaminant (skin oil or flora?) ?

Yield was awful in terms of mass and our vial also seemed to lose liquid compared to the others in the lab (smaller volume of liquid when we took it out of the bath).

Any ideas?  What are likely contaminants or reactions?  Possible structures?  At first it seems like just 2H on a double bond next to an ether are added and another H on an N, but I am having trouble adding those without destroying the apparently preserved structures (HN, HC b/t carbonyl and ether bicyclic bridge, HC=CH) 

I browsed the list of sites but couldn't find one that would predict structure from NMR.  Any of those resources are welcome too if I overlooked them.   Thanks.

[discodermolide]

Looks like your yellow product is a mixture of exo and endo products.

[themagicbean]

Derp!  Thank you!  Simple answer is a good one. 

[Dan]

I'm not convinced - the aliphatic CH region does not add up to a mixture of isomeric products.

To me it looks like a 4:5 mixture of DA adduct:starting maleimide.

[phth]

I know this may sound simple, but did you take IR of it?  Should tell you if you got starting material in the mix.