Last month, BOC agreed to a pounds 7.2bn takeover from Air Liquide of France and the US-based Air Products & Chemicals, a move that Mr Rosenkranz was widely believed to have opposed. He never denied his disapproval and described the takeover as "a deal tinged with sadness".
Mr Rosenkranz had always planned to leave the company once the transaction received regulatory approval later this year. But earlier this month, he said: "If I am required, I might give some support on integration."
Commenting on Mr Rosenkranz's departure, one analyst said: "It was a bit sudden. It suggests that some agreement regarding his future position in the company was not reached."
Mr Rosenkranz was expected to make pounds 7m out of BOC's takeover, including the sale of shares and options. According to BOC's annual report, published in September 1998, he owns 54,000 shares in the company, 622,000 share options, valued at pounds 6.62 each and 150,000 share incentive units. A spokesman for BOC said Mr Rosenkranz's right to exercise his share options would be unaffected by his resignation.
In a statement, Mr Rosenkranz said he had decided to leave after he "achieved most of what I set out to do," which, he said, was to reorganise the company to enhance shareholder value. He said: "The time is right for me to step aside and move on to other things."
A spokesman for BOC said this could include a number of non-executive chairmanships in other companies. One analyst speculated that Mr Rosenkranz may lead a buyout of the BOC's vacuum business, the future of which has not yet been decided.
Sir David John, BOC's chairman, said: "Danny Rosenkranz has done a terrific job in reorganising BOC and making it into a highly efficient operation." Tony Isaac, the firm's finance director, has been named acting chief executive.