Nature 501, 195–199 (2013); doi:10.1038/nature12472

In Fig. 3 of this Letter and on pages SI-24 to SI-28 and SI-84 to SI-93 of the Supplementary Information, the Me and n-Pr labels should be reversed in compounds 17 to 21. Corresponding errors also affect the precursor alcohols 42, 44, 46, 47 and 48 on pages SI-24 to SI-28. The conclusions of the paper are not affected by these errors, which have been corrected in the online HTML and PDF versions of this Letter.