Pd-catalyzed β-carbon elimination of 3-hydroxy-4-pentenoic acid derivatives promoted by triethylborane proceeded to form conjugated dienes via a decarboxylation process. The formed conjugated dienes underwent the Prins reaction with aldehydes in situ to afford conjugated homoallylic alcohols. These sequential transformations enabled conversion of a diastereomeric mixture of 3-hydroxy-4-pentenoic acids, which were readily prepared from the simple crossed aldol reaction of esters and α,β-unsaturated aldehydes, to 3,5-hexadienyl alcohols with high regio- and stereoselectivities in a single manipulation.