Pillsbury Winthrop Shaw Pittman has celebrated its merger by landing the lead role on Chevron-Texaco’s $18bn (£9.57bn) purchase of Unocal – the largest M&A deal the firm has handled for years.

The deal sees the California-based ChevronTexaco, a long-term Pillsbury client, acquire the independent US oil company Unocal in a cash-and-stock transaction worth $18bn, including net debt.