Late-Stage Chemoenzymatic Installation of Hydroxy-Bearing Allyl Moiety on the Indole Ring of Tryptophan-Containing Peptides

Abstract

The late-stage functionalization of indole- and tryptophan-containing compounds with reactive moieties facilitates downstream diversification and leads to changes in their biological properties. Here, the synthesis of two hydroxy-bearing allyl pyrophosphates is described. A chemoenzymatic method is demonstrated which uses a promiscuous indole prenyltransferase enzyme to install a dual reactive hydroxy-bearing allyl moiety directly on the indole ring of tryptophan-containing peptides. This is the first report of late-stage indole modifications with this reactive group.

Keywords: C−H activation; allylation; biocatalysis; functionalization; indole prenyltransferase.

Figure 1

Previous and current introduction of allyl and/or hydroxy groups on indole‐derived compounds. a) previously reported late‐stage allylic alkylation (i and ii)[ ¹⁶ , ¹⁷ , ¹⁸ ] and hydroxylation (iii) catalyzed by transition metals, indole prenyltransferases (IPTs) or other methods. b) Aim of current study. Hydroxy‐bearing allyl (HBA) group shown in red.

Scheme 1

Synthesis of 4  a and 4  b HBA pyrophosphates. Dimethylallyl fragment of native acceptor DMAPP is highlighted. Purified yields after column chromatography are stated.

Figure 2

Tryptophan‐containing peptides used in this study. Structures of a) cyclic dipeptides 7–9; b) daptomycin 10.

Figure 3

Chemoenzymatic method used and products obtained. a) CdpNPT‐catalyzed reaction between 4  b and Trp‐containing dipeptides 7–9. Reaction conditions: 7–9 (0.4 mM), 4  b (1.6 mM), 33.6 μM CdpNPT, in 50 mM TrisCl/ 10 mM CaCl₂ at 37 °C for 16 h; b) structures of HBA‐modified products of cyclo‐(l‐Trp₁‐l‐Trp₂), 11  a–11  d confirmed by HR‐MS and 1‐ and 2D NMR spectroscopy. 11  d is probably an artifact that arose from HPLC conditions.

Figure 4

Derivatization of daptomycin 10 with HBA group. a) HPLC analysis of in vitro assays containing 10 (0.8 mM) with 4  a or 4  b (1.6 mM) in Tris 50 mm/CaCl₂ 10 mm (pH 8.0) incubated with CdpNPT (32 μM) for 16 h at 37 ^οC. b) ¹H‐¹H COSY, ¹H−¹³C HMBC and ¹H‐¹H NOESY correlations of the tryptophan of 12 (full NMR correlations are demonstrated in Figure S13). c) Structure of HBA‐modified daptomycin 12. Long‐range ¹H−¹H COSY correlations are shown between H‐2′′/H‐4′′, H‐5′′.