Mapping the sirodesmin PL biosynthetic pathway — A remarkable intrinsic steric deuterium isotope effect on a 1 H NMR chemical shift determines β-proton exchange in tyrosine

Sirodesmin PL is both an antibiotic and a phytotoxin produced by a fungal plant pathogen ( Leptosphaeria maculans, asexual stage Phoma lingam ) that causes blackleg disease on crucifers. To determine potential biosynthetic precursors of sirodesmin PL, deuterated compounds were synthesized and incubated with cultures of L. maculans. Incorporations of deuterium into sirodesmin PL (7) and its precursor phomamide (4) were determined using 1 H and 13 C NMR spectroscopy, LC-HRMS-ESI and HRMS-EI spectrometry. Spectroscopic analyses established that [3,3- 2 H 2 ]l-tyrosine (1a), [3,3- 2 H 2 ]O-prenyl-l-tyrosine (9a), [3,3,5′,5′,5′- 2 H 5 ]O-prenyl-l-tyrosine (9b), and [5,5- 2 H 2 ]phomamide (4a) were incorporated efficiently into sirodesmin PL (7). Interestingly, an unexpected “twist” revealed that one of the β-deuteria (pro-R) of [3,3- 2 H 2 ]l-tyrosine (1a) was exchanged stereospecifically before incorporation into sirodesmin PL (7). As well, our studies revealed that O-prenyl-l-tyrosine is likely to be the first committed precursor en route to sirodesmin PL (7).