Abstract The regioselectivity of the nitroso-Diels–Alder reaction between unsymmetrical acyclic dienes and Boc-nitroso (Boc= tert-butoxycarbonyl) reagent or the Wightman chiral chloronitroso reagents has been studied. With the Boc-nitroso reagent, the selectivity is a consequence of steric effects at the C1-position in the diene and electronic effects at the C2- position in the diene. The combination of an unprotected hydroxyethyl side chain at C1 ...