Chemoenzymatic synthesis of spinosyn A

Abstract

Following the biosynthesis of polyketide backbones by polyketide synthases (PKSs), post-PKS modifications result in a significantly elevated level of structural complexity that renders the chemical synthesis of these natural products challenging. We report herein a total synthesis of the widely used polyketide insecticide spinosyn A by exploiting the prowess of both chemical and enzymatic methods. As more polyketide biosynthetic pathways are characterized, this chemoenzymatic approach is expected to become readily adaptable to streamlining the synthesis of other complex polyketides with more elaborate post-PKS modifications.

Keywords: biosynthesis; polyketide macrolides; post-polyketide synthase modifications; spinosyns; total synthesis.

Figure 1

(A) The organization of putative operons encoding enzymes responsible for the post-PKS modifications in the spinosyn biosynthetic gene cluster. (B) HPLC analysis of the enzymatic tandem reactions to convert 11 to 17; 11 alone (a), enzyme reaction mixture (b), and 17

Scheme 1

Established biosynthetic pathway for spinosyn A (1).

Scheme 2

Retrosynthetic analysis.

Scheme 3

Synthesis of (A) fragment A (19) and (B) fragment B (20). Reagents and conditions: a) (−)-DBNP, Et₂Zn, hexanes, 0 °C, 66%, 92% ee; b) TESOTf, 2,6-lutidine, CH₂Cl₂, −78 °C; c) i-Bu₂AlH, toluene, −78 °C, 2 steps 78%; d) TiCl₄, (−)-sparteine, CH₂Cl₂, 0 °C, 84%; e) TBSOTf, 2,6-lutidine, CH₂Cl₂, −78 °C; f) LiBH₄, Et₂O, CH₃OH, 0 °C, 2 steps 75%; g) TPAP, NMO, 4Å MS, CH₂Cl₂, rt; h) (+)-Ipc₂B(allyl), Et₂O, −78 °C, 2 steps 60%; i) TBSOTf, 2,6-lutidine, CH₂Cl₂, −78 °C, 87%; j) OsO₄, NMO, THF, acetone, pH 7 buffer, rt; k) NaIO₄, THF, pH 7 buffer, rt; l) NaBH₄, EtOH, rt, 3 steps 67%; m) PTSH, PPh₃, DIAD, THF, rt, 86%; n) (NH₄)₆Mo₇O₂₄, H₂O₂, EtOH, H₂O, 4 °C; o) TESOTf, 2,6-lutidine, CH₂Cl₂, −78 °C, 2 steps 91%; p) 1,3-dithiane, n-BuLi, THF, 0 °C, 94%; q) TBSOTf, 2,6-lutidine, CH₂Cl₂, −78 °C, 95%; r) MeI, CaCO₃, CH₃CN, H₂O, reflux, quantitative; s) (+)-Ipc₂B(allyl), THF, −78 °C; t) TBSOTf, 2,6-lutidine, CH₂Cl₂, −78 °C, 2 steps 67%; u) OsO₄, NaIO₄, 2,6-lutidine, dioxane, H₂O, rt, 83%; v) CrCl₂, HCI₃, dioxane, THF, rt, 87%; w) DDQ, aq CH₂Cl₂, rt; x) Dess-Martin periodinane, CH₂Cl₂, rt, 2 steps 83%.

Scheme 4

(A) Synthesis of fragment C (21) and (B) completion of preparation of 11. Reagents and conditions: a) Bu₃SnH, AIBN, benzene, reflux, 50%; b) MnO₂, acetone, rt, 85%; c) EtO₂CCH₂P(O)(OEt)₂, NaH, THF, 0 °C to rt, 74%; d) KHMDS, THF, −78 °C, 82%; e) 21, Pd₂(dba)₃, Ph₃As, DMF, rt, 70%; f) PPTS, EtOH, 0 °C; g) LiOH, THF, CH₃OH, H₂O, reflux, 2 steps 62%; h) 2,4,6-Cl₃C₆H₃COCl, Et₃N, THF; DMAP, toluene, rt, 75%; o) HF•pyr, EtOH, 4 °C, 4 days, 64%.