An efficient synthesis of the precursor of AI-2, the signalling molecule for inter-species quorum sensing
Graphical abstract
Introduction
(S)-4,5-Dihydroxypentane-2,3-dione (DPD) 1 is the uncyclized precursor of AI-2, a signalling molecule for bacterial inter-species communication.1, 2 DPD is a reactive 1,2-diketone which in aqueous medium forms an equilibrium mixture of the linear form and two anomeric cyclic forms 2 and 3, their hydrated versions 4 and 5 (Scheme 1).3 The hydrated isomer 4 can exist also as its 2,3-borate diester 6. It has been shown that distinct bacteria can detect different forms of this molecule and thus all these forms are known collectively as AI-2. Specifically, many Vibrio have the LuxP-type of AI-2 receptor which recognizes 6,4 whereas members belonging to phylogenetically distinct families such as the human pathogens Salmonella typhimurium and Bacillus anthracis as well as the plant symbiont Sinorhizobium meliloti contain the LsrB-type of receptors and recognize the non-borated diastereoisomer 5.3, 5
Bacterial populations use cell–cell communication in order to coordinate their behaviour and function in such a way that they can adapt to changing environments. Chemical communication among bacteria is called ‘quorum sensing’. Examples of quorum sensing behaviours are biofilm formation, virulence-factor expression, antibiotic production and bioluminescence.6 AI-2 is unique in that it is produced and detected by a wide variety of bacteria.2, 7 Thus, using AI-2 bacteria are able to detect the presence of other bacterial species in their vicinity and regulate gene expression according to the species composition in the environment.8 Ultimately, the understanding of the molecular mechanisms that bacteria use to regulate their behaviours can lead to the development of new therapies to control bacterial infections, and also to develop biotechnological applications for the control of industrial scale production of beneficial bacterial products, such as antibiotics or recombinant proteins.
Although a small molecule, DPD is highly functionalised, optically active and highly reactive. This presents a significant synthetic challenge and the production of reasonable quantities presents many problems. The absence of an accessible synthesis of DPD has been a major drawback in studies aimed at the understanding of AI-2 quorum sensing.7 In this work we describe an efficient and economic method to synthesise DPD on a reasonable scale. We have also developed a simple procedure to remove the last protective group thus furnishing uncontaminated DPD solutions.
Section snippets
Results and discussion
Currently, there are five published9, 10, 11, 12, 13 syntheses of DPD but all have problems and hence were considered unsuitable for the routine production of larger quantities required for biological studies. These routes generally use optically active starting materials and in most cases glyceraldehyde, derived from sugars. Each presented chemical or practical problems and we reasoned that a route in which the asymmetric centre was produced by the asymmetric reduction of a ketone could
Conclusion
In conclusion, (R)- and (S)-DPD were obtained in 8 steps with a 33% overall yield and >98% ee using a common strategy. Synthetic (S)-DPD proved to have the same biological activity as that of the enzymatically produced DPD. All intermediates were easy to handle and presented low volatility and excellent stability, the reactions were very reproducible and afforded high yields. Liberation of DPD from acetal 15 with Dowex resin and washing with chloroform allowed us to obtain a final DPD
General
1H NMR spectra were obtained at 400 MHz in CDCl3 or D2O with chemical shift values (δ) in ppm downfield from tetramethylsilane in the case of CDCl3, and 13C NMR spectra were obtained at 100.61 MHz in CDCl3 or D2O. Assignments are supported by 2D correlation NMR studies. Flash column chromatography: silica gel Merck 60, 0.040–0.063 mm (230–400 mesh ASTM). Analytical TLC: Aluminium-backed silica gel Merck 60 F254. Specific rotations () were measured using an automatic polarimeter. Reagents and
Acknowledgements
We acknowledge the generous financial support provided by Fundação para a Ciência e Tecnologia (PPCDT/DG/BIA/82010/2006 Portugal). We thank CERMAX for the use of the NMR spectrometers, which were purchased within the framework of the National Programme for Scientific Re-equipment, contract REDE/1517/RMN/2005, with funds from POCI 2010 (FEDER) and Fundação para a Ciência e a Tecnologia (FCT). We thank Pedro Lamosa for help in quantifying the DPD samples by NMR and Paula Chicau for help with HPLC
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