Reaction of diacetyl with acid
AKM: In your response to MM the self-condensation of diacetyl 1 occurs stepwise and not with the double enol of diacetyl 1 (biacetyl; butan-2,3-dione). Although your question relates to the acid-catalyzed condensation of diacetyl, historical condensations of diacetyl were conducted under alkaline conditions. The products formed from the base or acid-catalyzed are numerous and dependent on reaction conditions.
Quinone 2 was prepared under basic conditions by von Pechmann.1 Diels and coworkers2 isolated from an alkaline reaction a dimer, C8H12O4, which was assigned lactonic structure 3 with no spectroscopic evidence.3 The structures are color-coded to display the four carbon moiety of diacetyl. Clearly, lactone 3 shows a branched red carbon framework. An undefined "benzylic acid rearrangement" was suggested without details. The structure was later revised to diketone 4 based upon spectroscopic evidence and a more reasonable normal carbon chain.4  
Trimer 5, C12H18O6, has also been isolated under alkaline conditions and the stereochemistry confirmed by NMR and IR.5 
An acid-catalyzed condensation of diacetyl in dilute aqueous acetic acid (70oC, 13 days) leads to C10 structure 10.6 The structure was confirmed spectroscopically and by periodate cleavage with loss of acetic acid with concommitant formation of diketone 11. Structure 10 does not form a hemiketal as does structure 5 owing to the resonance stabilizing effect of the &alpha;, &beta;-unsaturated ketone. The mechanism proposed is described in the sequence of steps 4 --> 10. 
As can be seen from the examples provided --- and that's not all of them --- the self-condensation of diacetyl is not straightforward.



1) Pechmann, H. v., Berichte, 1888, 21, 1417; Otte, R. and Pechmann, H. v., 1889, 22, 2115.
2) Diels, O., Blanchard, W. M., Heyden, H. v. d., Berichte, 1914, 47, 2355.
3) Machell, G., J. Chem. Soc., 1960, 683.
4) Birch, A. J. and Moye, C. J., J. Chem. Soc., 1957, 412.
5) Cresswell, R. M., Smith, W. R. D. and Wood, H. C. S., J. Chem. Soc., 1961, 4882.
6) Shapiro, R., Hachman, J., Wahl, R., J. Org. Chem., 1966, 31, 2710.