From Thomas Jefferson Encyclopedia
Despite claims to the contrary, researchers at Monticello have been unable to find any documentary evidence that Thomas Jefferson purchased or consumed Constantia wines or "vins du Constance." Jefferson does mention "Cape wine" once in passing, in 1788:
"Here [Cologne] the vines begin, and it is the most Northern spot on the earth on which wine is made. Their first grapes came from Orleans, since that from Alsace, Champagne &c. It is 32. years only since the first vines were sent from Cassel, near Mayence, to the Cape of good hope, of which the Cape wine is now made. Afterwards new supplies were sent from the same quarter. That I suppose is the most Southern spot on the globe where wine is made and it is singular that the same vine should have furnished two wines as much opposed to each other in quality, as in situation."
Jefferson noted planting "Cape of Good Hope" grapes and "Cape" grapes in his Garden Book; however, these are references to the Alexander grape, native to the U.S.
Claims that the signing of the U.S. Constitution was toasted with a Constantia wine seem also to be groundless.