8-Vinylflavan-3-ols are described in the literature as the intermediate compounds involved in the formation mechanism of pyranoanthocyanins in wines or in model-like solutions. 8-Vinyl-(+)-catechin has shown to be very unstable in solution and revealed a high reactivity with itself, undergoing an unusual acid-catalyzed dimerization in model wine solution involving the formation of a new dihydropyran ring. Two diasteroisomers, (9S,11R)- and (9R,11S)-vinylcatechin dimers, were obtained, and their structures were characterized by mass spectrometry (MS) and nuclear magnetic resonance (NMR). These compounds were also found to occur in reactions between (+)-catechin and acetaldehyde. The identification of different conformations of both isomers was achieved by computational methods. The enthalpy values calculated suggest that isomer (9S,11R) is more stable and its formation is thermodynamically favored when compared to isomer (9R,11S).