How do we know that Dre (Taye Diggs), a suave New York music executive, misses the glory days of ’80s hip-hop, when break-dancers twirled in the boogie-down Bronx? We know it because Dre never, ever stops talking about it; he reveres the lost innocence of the pre-gangsta era the way that some baby boomers never got over Woodstock Nation. As a love-jones soap opera, Brown Sugar feeds right into Dre’s nostalgic crankiness. He and Sidney (Sanaa Lathan), a prominent hip-hop journalist, have been best friends ever since they bonded as kids in that age of rappers’ delight, but now, despite a persistent if barely acknowledged attraction, both have paired off with different lovers. Can they find their way back to each other — and to the ”true” spirit of hip-hop? The answer is at once predictable and cloying, though ”Brown Sugar” is so earnest in its yearning for the days before rap went nihilistic that it summons more spirit and bite than your average formulaic romantic quadrangle.