In a tall glass filled with ice stir together rum, juice, and syrup until combined well.
Garnish drink with pineapple and mint.
Chop mint. In a saucepan bring sugar, water and mint to a boil, stirring until sugar is dissolved. Simmer syrup, undisturbed, 2 minutes. Pour syrup through a fine sieve, pressing hard on solids, and cool. Syrup keeps, covered and chilled, 2 weeks. Makes about 1 1/2 cups.
