To some observers, the absence of two of the UK's biggest law firms from a place around the deal table at one of the standout takeovers of the year so far will have raised eyebrows.

Despite winning mandates on previous transactions involving the main players in the deal, neither Clifford Chance nor Slaughter and May were involved when US drugs giant Walgreens revealed it was to complete its takeover of UK pharmacy chain Alliance Boots earlier this month.

Instead, Simpson Thacher & Bartlett cemented its position as go to adviser for Alliance Boots' owner, private equity firm Kohlberg Kravis Roberts (KKR).

Simpsons' relationship with KKR, which has a market cap of around $1.84bn (£1.1bn), goes back almost 30 years, to KKR's $25.1bn (£14.95bn) acquisition of RJR Nabisco in 1988 – at the time the largest private equity purchase in history.

Since then, the ties between the two have only strengthened, with Simpsons acting on the US private equity giant's original £9.7bn takeover of Alliance Boots in 2007. The Wall Street stalwart has since become counsel for Alliance Boots with respect to financing matters following the acquisition.

Given its history of acting for UK blue chips on 'bet the company deals', it was no surprise that Slaughters advised Alliance Boots on the 2007 takeover. The firm had previously acted for Boots on the £7bn merger with Alliance UniChem that created the company in 2005.

However, with the company subsequently private equity owned and, now, headquartered in Switzerland, Slaughters has not been involved since KKR took control and was not mandated when Walgreens first took a 45% stake in Alliance Boots in 2012.

But if Slaughters' ties to Boots now appear purely historical, CC may have been expected to have a role for KKR, given its involvement in recent deals.

The firm was instructed, alongside Simpson Thacher, on Walgreens' 2012 minority stake acquisition and KKR's original 2007 buy out. The pair had also previously hooked up for KKR's acquisition of Philips' semiconductors business in 2006,an enterprise then valued at €8.3bn (£5.7bn).

Sources close to the deal, however, stress that the involvement, or lack thereof, of CC is not as meaningful as it may first seem.

For a start, to complete the merger, Walgreens only had to exercise an option acquired in 2012 to buy the 55% of Boots it did not already own, a feat that, sources say, simply did not require a plethora of legal advisors to be involved. According to the market, the role in 2012 was the prize mandate.

And a lack of involvement does not mean the end of CC's work with KKR. It advised the buyout house as recently as June on an investment in African rose grower and distributor Afriflora, and it could potentially become involved in later stage of the Alliance Boots takeover, with the deal not expected to complete until the first quarter of 2015.

"When the client (KKR) sees it's appropriate they will engage them," one source familiar with the situation explains. "To say there's something in the lack of roles is making a mountain out of a molehill."

Indeed Simpson Thacher is KKR's go-to adviser, with other firms involved as and when they have particular expertise or have a unique involvement in the relevant jurisdiction anyway. That certainly seems to be the case for CC.

On the other side of the negotiating table, Walgreens turned to another New York powerhouse in the shape of Wachtell Lipton Rosen & Katz. There was some cheer for the UK contingent though, with Allen & Overy also acting for the US client.

A&O fielded an Amsterdam-led team for the deal, with that office being the one that boasted ties with senior figures at Walgreens.

Meanwhile, French M&A specialists Darrois Villey Maillot Brochier won the mandate to advise Stefano Pessina, executive chairman of Alliance Boots, and his AB Acquisitions investment vehicle, echoing roles the firm had on the 2007 and 2012 deals.

Darrois also represented Alliance Boots last year when it and Walgreens entered a strategic partnership with AmerisourceBergen, one of the largest pharmaceutical services companies in North America.