Study of Direct N7 Regioselective tert-Alkylation of 6-Substituted Purines and Their Modification at Position C6 through O, S, N, and C Substituents

A new N7 direct regioselective method allowing the introduction of tert-alkyl groups into appropriate 6-substituted purine derivatives is developed. This method is based on a reaction of N-trimethylsilylated purines with a tert-alkyl halide using SnCl4 as a catalyst. In this work, we study the structure and optimal reaction conditions leading to the N7 isomer and in some cases also to the N9 isomer. The main goal is devoted to preparing 7-(tert-butyl)-6-chloropurine as a suitable compound for other purine transformations. The stability of the tert-butyl group at the N7 position is tested for classic model reactions, leading to the preparation of new 6,7-disubstituted purine derivatives, which is also interesting from the point of view of possible biological activity.

Rapid profiling of cytokinins using supercritical fluid chromatography coupled with tandem mass spectrometry

BACKGROUND

The determination of plant hormones is still a very challenging analytical discipline, mainly due to their low concentration in complex plant matrices. Therefore, the involvement of very sensitive high-throughput techniques is required. Cytokinins (CKs) are semi-polar basic plant hormones regulating plant growth and development. Modern methods for CK determination are currently based on ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS), which enables the separation of CK isomeric forms occurring endogenously in plants. Here, ultra-high performance supercritical fluid chromatography coupled with tandem mass spectrometry (UHPSFC-MS/MS) was used for the simultaneous determination of 37 CK metabolites.

RESULTS

The chromatographic conditions were tested on three different columns with various retention mechanisms. Hybrid silica modified with 2-picolylamine was selected as the stationary phase. Several parameters such as column temperature, back pressure regulation, mobile phase composition and make-up solvent were investigated to achieve efficient separation of CK isomers and reasonable sensitivity. Compared to UHPLC-MS/MS, a 9-min chromatographic analysis using a mobile phase of supercritical CO2 and 5 mM ammonia in methanol represents a three-fold acceleration of total run time. The quantification limit of UHPSFC-MS/MS method was in the range of 0.03-0.19 fmol per injection and the method validation showed high accuracy and precision (below 15 % for most analytes). The method was finally applied to the complex plant matrix of the model plant Arabidopsis thaliana and the obtained profiles of CK metabolites were compared with the results from the conventional UHPLC-MS/MS method.

SIGNIFICANCE

The presented work offers a novel approach for quantification of endogenous CKs in plants. Compared to the conventional UHPLC-MS/MS, the total run time is shorter and the matrix effect lower for the key CK metabolites. This approach opens the opportunity to utilize UHPSFC-MS/MS instrumentation for targeted plant hormonomics including other plant hormone families.

Study of the N7 Regioselective Glycosylation of 6-Chloropurine and 2,6-Dichloropurine with Tin and Titanium Tetrachloride

6-Chloropurine and 2,6-dichloropurine were regioselectively glycosylated at position 7 to give the corresponding peracetylated N⁷-nucleosides, which can be suitable for other purine transformations. In this work, we study the distribution of N⁷/N⁹-isomers produced via the Vorbrüggen method under different conditions, using an N-trimethylsilylated purine derivative and SnCl₄ or TiCl₄ as a catalyst. The main effort is devoted to reversing the disadvantageous predominant selectivity of most glycosylation reactions at the N⁹ position and thus to determining conditions that maximize the regioselectivity of glycosylation toward the desired N⁷-isomer.

Synthesis of [15 N4 ] purine labeled cytokinin glycosides derived from zeatins and topolins with 9-β-d, 7-β-d-glucopyranosyl, or 9-β-d-ribofuranosyl group

Synthesis of [¹⁵ N₄ ] purine labeled cytokinine glycosides derived from zeatins and topolins containing a 9-β-d, 7-β-d-glucopyranosyl, or 9-β-d-ribofuranosyl group is described. These N⁶ -substituted adenine derivatives are intended as internal analytic standards for phytohormone analysis. All labeled compounds were prepared from 6-chloro[¹⁵ N₄ ]purine (1). The equilibrium reaction of 1 with acetobromo-α-d-glucose gave isomeric 7-β-d (3) and 9-β-d (4) chloro glucosyl precursors, which were treated with the corresponding amines to get desired labeled cytokinin 7-β-d (6) and 9-β-d (5) glucopyranosides. Cytokinins containing 9-β-d-ribofuranosyl group (8) were obtained by direct enzymatic transglycosylation reaction of cytokinins (7) prepared from 6-chloro[¹⁵ N₄ ] purine (1).