A small group of high-value works from the collection of entrepreneur and philanthropist Jennifer Gilbert will go to auction at Sotheby’s this spring, with proceeds directed toward Lumana, a Detroit-based arts nonprofit she is developing. The works will be split across Sotheby’s May contemporary sales and its June design auction. The top lots include Joan Mitchell’s Loom II (1976), estimated at $5 million to $7 million, and Kenneth Noland’s Circle (1958), carrying a $4 million to $6 million estimate—an ambitious figure that, if achieved, would set a new high-water mark for the artist. Other works by George Rickey and Harry Bertoia round out the group, which leans heavily on midcentury abstraction and design—material that remains dependable, if not especially speculative, in the current market.  The sale arrives at a moment when collectors and institutions alike are leaning on established names , even as broader demand has softened outside the top tier.…