Wafer Ash

Ptelea trifoliata

Wafer Ash or Hop Tree

Small flowers with "the powerful scent of honeysuckle, ..probably the most fragrant of any hardy tree" (Hillier)

- Forest Farm (link here)

AKA Stinking Ash

http://davesgarden.com/guides/pf/go/1710/

This might work as an understory tree in the garden.

Zone 4 and shade tolerant.

There seems to be some disagreement on the scent of the flowers.

"Carrion flies pollinate the flowers. "

- Missouri Botanical Garden (link here)