Chemsiddiqui Chemistry Blog

Two organic chemists claim to have settled a major dispute over the first synthesis of quinine by revisiting an experiment performed over 90 years ago.

Aaron Smith and Robert Williams from Colorado State University, US, have reproduced a three-step conversion of d-quinotoxine into quinine [1] first performed by chemists Paul Rabe and Karl Kindler in 1918 [2] that was referenced but - crucially - not carried out in the first published total synthesis of the compound.

The controversy dates back to 1944, when two rising stars at Harvard University - Robert Burns Woodward and William von Eggers Doering - claimed to have come up with the first total synthesis of quinine [3], an alkaloid with impressive antimalarial properties that occurs naturally in the bark of the Cinchona tree.

The report of quinine’s total synthesis caused a sensation, not least because many of the natural sources of quinine were in the Dutch East Indies (modern day Indonesia), which were then in enemy hands. The hope - never realised - was that Woodward and Doering’s quinine synthesis could be scaled up to help fight malaria on the battlefield.

But, despite press reports at the time, Woodward and Doering had not actually produced quinine. Instead, their 17 step synthesis ended with d-quinotoxine and they referred to Rabe and Kindler’s 1918 work to demonstrate that total synthesis of the compound was possible.

That led to doubts over the validity of their pathway and controversy over Woodward and Doering’s claim has continued ever since.

Competing claims

Perhaps their most vocal critic has been Gilbert Stork, based at the University of Wisconsin and then Harvard, who had also begun work on his own total synthesis of the compound in the 1940s.

In 2001, when Stork and his colleagues published their own synthesis of quinine via a completely different route [4], he was quoted as saying that had Woodward and Doering attempted the Rabe-Kindler conversion, it ‘would not have worked’ [5]. The implication was that their synthesis was no synthesis at all.

Now, after 90 years, Smith and Williams have published their attempts to replicate the Rabe and Kindler conversion and say that this should, at last, remove any blemish on the reputations of Rabe, Kindler, Woodward and Doering. ‘Stork has done a major injustice to their work,’ Williams said. ‘We’ve proven that he’s dead wrong.’ The secret, they say, lies in the last step of the conversion.

In Rabe and Kindler’s 1918 paper, they first oxidised d-quinotoxine using sodium hypobromite to give N-bromoquinotoxine. The second step was base-mediated cyclization to produce quininone. Finally, quinone was reduced to quinine with aluminium powder.

‘The first two steps worked perfectly,’ says Williams. However, the final reduction produced barely detectable quantities of quinine.

Purity problem

Smith and Williams hypothesized that the problem lay in the purity of their aluminium powder. ‘It would have been kept in a stoppered apothecary jar open to the air,’ says Williams. Replicating these conditions by exposing their elemental aluminium to oxygen, the final step of the conversion went ahead smoothly. ‘We’re 100 per cent sure that had Woodward and Doering been motivated to convert their d-quinotoxine to quinine, they would have succeeded,’ says Williams.

“This has closed the door on any doubts”
- Jeffrey I. Seeman
‘This has closed the door on any doubts,’ says Jeffrey I. Seeman, an expert on alkaloids at the University of Richmond, Virginia and a historian of chemistry. Seeman recently reconstructed the ins and outs of this controversy in an article published last year [6]. ‘My historical research provides much evidence that Rabe and Kindler did, in fact, convert d-quinotoxine to quinine,’ he says.

Woodward and Doering set as their target the preparation of d-quinotoxine from 7-hydroxyisoquinoline, says Seeman, relying on the Rabe-Kindler conversion to do the rest. ‘Smith and Williams’ experimental evidence unambiguously and conclusively established the validity of Rabe and Kindler’s 1918 work and, in consequence, the validity of the Woodward-Doering/Rabe Kindler total synthesis of quinine,’ he says.

“The problem was that Woodward and Doering were immediately caught in a whirlwind of hype because of the enormous relief felt that we now had a solution to the critical quinine shortage during the US World War II operations in Asia”
- Gilbert Stork
But why did Woodward and Doering not go on to complete the synthesis of quinone themselves? Stork, now emeritus professor at Columbia University, suggests the pair knew that the attempt was likely to end badly - either failing because of a lack of detail in the original Rabe and Kindler method, or by producing an unimpressively low yield of quinine. ‘Why should they sully their superb synthesis of “homomeroquinene” by advertising that fact?’ Stork told Chemistry World.

‘The problem was that Woodward and Doering were immediately caught in a whirlwind of hype by the media (creating the myth of feasible large scale synthesis) because of the enormous relief felt that we now had a solution to the critical quinine shortage during the US World War II operations in Asia,’ he added.

Henry Nicholls

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Researchers in the US have developed a new way to detect DNA synthesis in living cells by using click chemistry - the concept of reacting together two ‘high energy’ molecules that ‘click’ together efficiently under mild conditions. Experts believe the technique could lead to new ways of identifying cancer cells.

To see how fast cells are proliferating, biologists measure the rate of DNA synthesis by feeding them analogues of the natural building blocks of the molecule. These synthetic compounds become incorporated into the growing DNA chain and can be detected after the cells are killed.

The two most widely used analogues are [3H]thymidine, whose radioactivity can be measured in the fixed cells, or 5-bromo-2′-deoxyuridine (BrdU), which can be detected by specific antibodies raised against BrdU.

However, both have drawbacks. Tritiated thymidine produces only a weak signal and the technique is time consuming. The disadvantage of the BrdU method, on the other hand, is that the bulky antibody may not diffuse through large tissue samples to reach the cells of interest.

Adrian Salic of Harvard Medical School in Boston, Massachusetts, and his colleague Timothy Mitchison have overcome these hurdles by using click chemistry.

‘We made a very simple chemical analogue of thymidine which contains a terminal alkyne in the form of an ethynyl group,’ Salic told Chemistry World. The triple bond of the ethynyl can be ‘clicked’ with a fluorescent azide, a compound containing three double-bonded nitrogens in a row. This is a classic ‘3+2′ cycloaddition - a rapid, efficient click-reaction catalysed by copper.

Cells fed the alkyne-containing analogue incorporate it into their DNA. The team then add the fluorescent azide, which quickly diffuses into cells and reacts with the ethynl group. ‘You obtain a strong signal that is detectable under standard fluorescence microscopy at low magnification,’ said Salic.

“The method is foolproof”
- Adrian Salic
The technique works in cultured cells and in whole animals. ‘The method is foolproof,’ Salic added.

‘The authors have used elegant, cutting edge chemistry to develop a novel approach to label replicating DNA in cells,’ John Diffley, deputy head of Cancer Research UK’s London Research Institute told Chemistry World. ‘This work may lead to new diagnostic approaches for the early identification of cancer cells.’

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A biodegradable polymer made with green solvents can mend broken thigh bones in mice, UK researchers have shown.

The sponge-like polylactic acid scaffold was coated with vascular endothelial growth factor protein and seeded with human bone marrow cells to mimic the biological factors that stimulate natural bone regrowth. After slowly delivering its contents, the polymer is metabolised by the body leaving no trace, said Steve Howdle, the green chemist at Nottingham University who led the team.

The breakthrough means the innovative polymer is a step closer to being used in hospitals. According to Richard Oreffo of Southampton University, who works on the team, human trials could be less than 5 years away.

Many researchers are working on similar systems because they promise to avoid the problems of harvesting bone grafts from patients and donors. ‘But a key difficulty is how to make a polymer that’s holey and sponge-like, but which also encapsulates a highly expensive and delicate protein or drug inside,’ Howdle explained.

Supercritical solution

The solution Howdle’s team has been developing over the last decade is to use supercritical carbon dioxide - a solvent with properties midway between those of a gas and a liquid. Formed under pressure, the supercritical fluid melts the polymer and allows any delicate protein to be mixed into it just above body temperature. When the pressure is released, carbon dioxide gas foams up this mixture, automatically forming the porous scaffold ready-coated with active drug or protein.

Biomedical engineer Steve Curran, who researches bone repair for medical supplier Smith & Nephew, said the material processing was very important. ‘Conventional manufacturing methods for polymers with the appropriate mechanical integrity to support bone regeneration require processing conditions (such as high temperatures and pressures, and cytotoxic solvents) that do not support the stability or long-term functionality of bioactives,’ he explained.

‘The retention of biological activity of . growth factors and the ability to release proteins is a tremendous boost in the reparative process,’ said Oreffo. He said that his group was looking at releasing more than one growth factor to better mimic the body’s bone regeneration environment.

Trials in rabbits and sheep are likely next, said Howdle. ‘Bone is a good testing ground, but other areas could be to repair cartilage, muscles, or neurons - you just need the right porosity of scaffold and the right growth factors,’ he added.

Howdle also hopes to use the scaffolds to slowly release hormones, such as human growth hormone or alpha-interferon. At the moment, patients with hormone deficiencies face daily injections.

Richard Van Noorden

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