Nature Chemistry 5, 428–432 (2013); published online 7 April 2013; corrected after print 26 April 2013.

In the version of this Article originally published, the reaction conditions given in Fig. 3 for conversion of compound 2 to compound 4 were incorrect, and should have been stated as 'Acetic acid/H2O, 100 °C, 3 h'; this has now been corrected in the HTML and PDF versions of this Article.