Microsoft has trumped a private equity shop with a $486 million bid to acquire Greenfield Online, the parent of European price comparison engine Ciao.com, the company said Friday.

In June, Steve Rattner’s Quadrangle Group cut a deal to buy Greenfield, based in Wilton, Connecticut, for $15.50 per share, or about $426 million. A “go shop” provision in the agreement let Greenfield seek alternative buyers and earlier this month, the company announced that a new “strategic” bidder had emerged.

In a research note, UBS analyst Heather Bellini said that the deal is designed to pump life into Microsoft’s online businesses in Europe.

In a statement, Microsoft said the acquisition for $17.50 per Greenfield share is aimed at bolstering Microsoft Live Search. Google has dominated European search much as it has in the U.S. market. It had 79.2 percent share, according to comScore’s March rankings for Europe, versus 2 percent for Yahoo and 1.9 percent for Microsoft.

In late morning trading, shares of Greenfield rose $.08, or .5 percent, to $17.33. Microsoft, meanwhile, whose valuations are near historic lows, according to Ms. Bellini, slid $.52, or 1.9 percent, to $27.42.

Greenfield management noted Microsoft’s cash offer represents a premium of about 13 percent over the Quadrangle proposal and about 32 percent over the company stock’s $13.28 closing price on June 13, the last trading day before the Quadrangle agreement was announced.

Quadrangle has stakes in a broad array of companies, including Cablevision Systems, a cable and telecom provider, and Alpha Media, the parent of “laddie” magazine Maxim. As part of its deal with Greenfield, Quadrangle has received a $5 million breakup fee.

Microsoft, whose interest is centered on Ciao.com, has lined up an unnamed “financial buyer” for Greenfield’s marketing survey business, which accounts for more than half of revenue.  Both deals are expected to close in the fourth quarter pending shareholder and regulatory approval.

Greenfield posted net income of $12.9 million on revenue of $129 million in 2007.