yl of 1 to 6 carbon atoms; or R1 and R2 together are polymethylene; R3 is hydrogen, hydroxy, alkoxy of 1 to 6 carbon atoms or acyloxy of 1 to 7 carbon atoms; R4 is hydrogen; or R3 and R4 together are a bond; R5 is hydrogen; and R7 and R8 together with the nitrogen atom to which they are attached form a 6-membered unsubstituted or substituted, unsaturated heterocyclic ring, the ring being a single