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Acknowledgements This work was supported by an NSF predoctoral fellowship
(B.R.R.), National Institute of General Medical Sciences grant GM-097444,
Asymchem and Bristol–Myers Squibb. We thank D.-H. Huang and L. Pasternack
for assistance with NMR spectroscopy; A. L. Rheingold, C. E. Moore and M. A.
Galella for X-ray crystallographic analysis; and D. G. Blackmond, O. Luca, T.
Paschkewitz, Y. Ishihara and T. Razler for discussions.
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Author Contributions E.J.H., B.R.R. and P.S.B. conceived this work; E.J.H., B.R.R.,
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the experiments and analysed the data; Y.C. and J.T. performed the large-scale
experiments; E.J.H., B.R.R., K.C., M.D.E. and P.S.B. wrote the manuscript.
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