Auction houses expect to sell as much as $2.3 billion of art in New York this month as billionaires from China to Brazil compete for trophy works by Claude Monet, Pablo Picasso and Jeff Koons in a surging market.

Two weeks of semiannual sales of Impressionist, modern, postwar and contemporary art at Christie’s, Sotheby’s (BID) and Phillips begin May 6, with online bidding as early as today. Their combined sales target represents a 77 percent increase from estimates for a similar round of auctions a year ago.