A de novo synthesis of pentoses is described starting from (Z)-2-buten-1,4-diol (1). The key step is the enzyme catalysed enantioselective HCN-addition to O-protected 4-hydroxybut-2-enal using the hydroxynitrilelyase from Hevea brasiliensis, followed by an asymmetric dihydroxylation. For the cyanohydrin reaction the influence of the configuration of the double bond and of the protecting group was