1, 3-Dipropyl-8-(3-pentyl) xanthine (14), a open-chain analogue of 4, had about 63-fold higher affinity to the A, receptor than a parent compound (3) and moderate A, selectivity. But 14 was a less potent A, antagonist than 4. A further increase or decrease in the size of alkyl substitutent (R'or R2) caused a loss of affinity at the Al receptor (compare 9, 10, and 17 with 14). An amino or a phenoxy group reduced affinity to Al and A2 receptors (1 1 and 16). ...