Telescoping a Prenyltransferase and a Diterpene Synthase to Transform Unnatural FPP Derivatives to Diterpenoids

Abstract

New diterpenoids are accessible from non-natural FPP derivatives as substrates for an enzymatic elongation cyclization cascade using the geranylgeranyl pyrophosphate synthase (GGPPS) from Streptomyces cyaneofuscatus and the spata-13,17-diene synthase (SpS) from Streptomyces xinghaiensis. This approach led to four new biotransformation products including three new cyclododecane cores and a macrocyclic ether. For the first time, a 1,12-terpene cyclization was observed when shifting the central olefinic double bond toward the geminial methyl groups creating a nonconjugated 1,4-diene.

Scheme 1. (A) Overview on the Biosynthesis of Spata-13,17-diene (4) from C₅ Precursors DMAPP (1) and IPP (2) via GGPP (3) and Structure of Spatol (5) and (B) Elongation of FPP Derivatives 6–8 and Cyclization of Intermediate GGPP Derivatives 9a–9c Reported in This Work

Scheme 2. Synthesis of FPP Derivative 7

Reagents and conditions: (a) H₂SO₄, MeOH, Δ (90%); (b) DIBAL-H, THF, −78 °C (96%); (c) TBDPSCl, imidazole, N,N-DMF, rt (67%); (d) Dess-Martin periodinane, THF, 0 °C to rt; (e) i-PrPPh₃I, n-BuLi, THF, 0 °C to rt to 40 °C; (f) TBAF, THF, 0 °C to rt (50% over three steps); (g) CBr₄, PPh₃, CH₂Cl₂, 0 °C to rt (quant.); (h) NaSO₂Ph, N,N-DMF, 50 °C to rt (60%); (i) n-BuLi, THF, bromide 14, −78 °C to rt (80%); (j) Na₂HPO₄, Na–Hg, THF, MeOH, −14 °C to rt (95%); (k) TBAF, THF, 0 °C to rt (91%); (l) Et₃N, MsCl, LiCl, THF, 0 °C, then ((n-Bu)₄N)₃P₂O₇H, MeCN, rt (91% over two steps).

Scheme 3. Overview on Biotransformations (Ring Sizes Are Labelled)

Scheme 4. Details on the Structure Elucidation of Macrocycles 16–18 by NMR Analysis

1D NOE correlation between methyl groups (δ= 1.44 ppm and δ= 1.59 ppm); the alternate conformation with both methyl groups pointing upward not shown.

Scheme 5. Proposed Mechanisms That Lead to Macrocycles 17 and 18a with Shifted Double Bond