Nature Chemistry 7, 171–177 (2015); published online 12 January 2015; corrected after print 25 January 2016.

In the version of this Article originally published, the absolute stereochemistry of compounds in Fig.2 and Table 3 was reversed. Additionally, the absolute stereochemistry was reversed in the Markush product structure shown in the reaction scheme at the top of Table 2. These structures have been corrected in the online versions of the Article.