Intermediate compounds, including 2-(t-butyldimethylsilyloxymethyl)-3,4-[(dimethylmethylene)dioxy]-5-hydroxy -tricyclo[5.2.1.0.sup.2,6 ]dec-8-ene, which are useful for the synthesis of neplanocin A having strong antitumor activity. Improved processes for the preparation of neplanocin A, starting from optically active 2-hydroxymethyl-5-hydroxy-tricyclo[5.2.1.0.sup.2,6 ]deca-3,8-diene and via a key step comprising a retro-Diels-Alder reaction of the above intermediate.