| 摘要 |
<p>There are described compounds of the formula <IMAGE> (I) in which R1 is hydrogen or C1-6 alkyl, R2 is hydrogen, C1-6 alkyl or C3-6 alkenyl, R3 is hydrogen or C1-6 alkyl, R4 is hydrogen or an N-protecting group, n is 2, 3, 4 or 5, and X is oxygen, sulphur or -CH2-, provided that when X is -CH2- n is 0; and salts thereof. The compounds in which R4 is hydrogen have pharmaceutical activity.</p> |
