Synthesis of legonmycins A and B, C(7a)-hydroxylated bacterial pyrrolizidines

• Wilfred J. M. Lewis,
• David M. Shaw and
• Jeremy Robertson

Beilstein J. Org. Chem. 2021, 17, 334–342, doi:10.3762/bjoc.17.31

• pyrrolizidines; cyanoketones; legonmycin; vinylogous ureas; Introduction At least 40 members of the large class of pyrrolizidine alkaloids [1][2][3][4] have so far been characterized from bacterial cultures. Of these ‘bacterial pyrrolizidines’, those of the vinylogous urea type are particularly intriguing from

• , irrespective of the site of activation, the subsequent ejection of the activating reagent would generate an extended iminium ion whose capture by adventitious water would produce legonmycin A directly. For context, the treatment of enol esters with oxidizing agents to give α-hydroxyketones, or halogens to give

• pyrrole 18, and the appearance of new resonances at 3.21 (ddd, J = 11.5, 9.0, 2.5 Hz, 1H) and 3.64 (dt, J = 11.5, 8.5 Hz, 1H) ppm that correspond visually with those reported for the diastereotopic CH2N protons in legonmycin A. HRMS (ESI+) analysis of material isolated from this NMR experiment showed m/z