In 27 years of corporate dealmaking, Alan Klein has had more than his share of all-nighters. But when Microsoft Corporation asked Klein and his colleagues at Simpson Thacher & Bartlett to negotiate its largest deal to date—an $8.5 billion acquisition of Skype Global S.àr.l. from a disparate group of private equity firms, individuals, and eBay Inc.—in just three weeks, “it felt like we were trying to climb Mount Everest as part of a speed race,” Klein says.

The rush wasn’t Microsoft’s idea. When the software giant first expressed interest in Skype in late March of 2011, the consortium of investors who owned the Luxembourg-based Internet telephone service were intrigued but wary. With more than $50 billion in cash on its balance sheet, Microsoft could make an all-cash offer for Skype, an attractive option for such investors as Silver Lake, venture capital firm Andreessen Horowitz, and the Canada Pension Plan Investment Board.