Zinc oxide Knoevenagel reaction

Moison, H., Texier-Boullet, F. and Foucaud, A. Knoevenagel, Wittig and Wittig-Homer reactions in the presence of magnesium oxide or zinc oxide. Tetrahedron, 1987, 43, 537-542. 

In order to obtain thiophenes unsubstituted at C-4 and C-5, ethanal would be required as the aldehyde component, but there are only a few examples of this [63, 64], in one [64] of which zinc oxide was used instead of an organic base. In practice, l,4-dithiane-2,5-diol 24 is usually used (Scheme 35), and thus no sulfur needs to be introduced into the reaction mixture the reactant is a dimer of mercaptoacetaldehyde. Reaction is carried out in the presence of a base, which deprotonates an OH group and thus releases the aldehyde carbonyl from its hemithioacetal, ready for Knoevenagel condensation [56, 57]. 

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