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Poly(ADP-ribosyl)ation is a common post-translational modification that mediates a wide variety of cellular processes including DNA damage repair, chromatin regulation, transcription, and apoptosis. The difficulty associated with accessing poly(ADP-ribose) (PAR) in a homogeneous form has been an impediment to understanding the interactions of PAR with poly(ADP-ribose) glycohydrolase (PARG) and other binding proteins. Here we describe the chemical synthesis of the ADP-ribose dimer, and we use this compound to obtain the first human PARG substrate-enzyme co-crystal structure. Chemical synthesis of PAR is an attractive alternative to traditional enzymatic synthesis and fractionation, allowing access to products such as dimeric ADP-ribose, which has been detected but never isolated from natural sources. Additionally, we describe the synthesis of an alkynylated dimer and demonstrate that this compound can be used to synthesize PAR probes including biotin and fluorophore-labeled compounds. The fluorescently labeled ADP-ribose dimer was then utilized in a general fluorescence polarization-based PAR-protein binding assay. Finally, we use intermediates of our synthesis to access various PAR fragments and evaluation of these compounds as substrates for PARG reveals the minimal features for substrate recognition and enzymatic cleavage. Homogeneous PAR oligomers and unnatural variants produced from chemical synthesis will allow for further detailed structural and biochemical studies on the interaction of PAR with its many protein binding partners.

This communication details the copper catalyzed ring expansion of vinyl aziridines to 3-pyrrolines. Broad substrate scope (24 examples) using tosyl and phthalimide protected vinyl aziridine substrates is observed. Cu(hfacac) 2 was determined to be superior to all other catalysts tested.3-Pyrrolines (2,5-dihydropyrroles), while important in their own right, are synthetically versatile heterocycles which allow ready access to pyrrolidines and pyrroles. 1 Collectively these ring systems occur in countless natural products and pharmaceuticals. 2 Not surprisingly, a number of different synthetic approaches toward pyrrolines have been developed but the most popular are Birch reduction of pyrroles, 3 [3+2] cyclization, 4 and ring closing metathesis. 5 Despite these and other creative approaches, a requisite exists for a simple and selective method which allows access to structurally complex 3-pyrrolines. Herein, we demonstrate that vinyl aziridines can be efficiently converted to 3-pyrrolines using commercially available copper(II) catalysts.Analogous to the vinyl cyclopropane rearrangement, 6 the thermolysis of vinyl aziridines to 3-pyrrolines was first observed in the late 1960's. Vinyl aziridines are most easily synthesized by one of three methods: the addition of a nitrene to a diene, the addition of an allylic ylide to an imine or the cyclization of unsaturated amino alcohols. 11 Each method has limitations often directly related to the nitrogen protecting group. This paper focuses on the p-toluenesulfonyl (Ts) and phthalimido (Phth) protecting groups. Tosyl aziridines are easily accessed by employing an aziridination protocol developed by Sharpless 12 or by catalytic decomposition of N-phenyliodinanes. 13 Although phthalimide substituted aziridines are less commonly used, they offer an attractive option based on their ease of synthesis utilizing a stabilized singlet nitrene and broader substrate scope. 14 Vinyl aziridine 1 was chosen as a model substrate due to the growing interest in dehydroprolines. 15 Based on previous work by our group on the catalytic ring expansion of vinyl oxiranes and vinyl thiiranes, 16 copper catalysts were tested ( Table 1). The more electrophilic copper(II) hexafluoroacetylacetonate (entries 14-18) proved far superior to other commercially available and synthetic catalysts. Interestingly, the reaction proceeds faster and with a higher yield when the Cu(hfacac) 2 hydrate is dried prior to use. Optimized reaction conditions were found to be 150 °C, 0.1 M [substrate], and 5 mol % catalyst loading. Table 2 displays the results of applying these optimized conditions to a variety of phthalimide and tosyl protected vinyl aziridines. These products can be readily deprotected to give the N-H pyrrolines. 17 Simple substrates (Entries 1-11) demonstrate that all substitution patterns are well tolerated resulting in excellent yields. Entries 4, 5, and 8 display an important synthetic aspect of this rearrangement, as each 3-pyrroline can originate from two regioisomeric vinyl aziridine prec...

In this report, it is demonstrated that chiral vinyl aziridines can be stereospecifically ring expanded. This synthetic approach allows controlled access to chiral 2,5-cis or 2,5-trans-3-pyrroline products from starting materials with the appropriate aziridine geometry. Twenty three ring expansion examples, most of which feature a stereospecific cyclization, are presented.A cursory review of the structural motifs of the top 200 top selling drugs 1 reveals that around 90% contain at least one nitrogen atom and approximately 65% are decorated with a heterocycle. Not surprisingly, the majority of these heterocycles are nitrogenous, with pyrrolidines a commonly occurring heterocyclic scaffold. Given the success of chiral pyrrolidines as important pharmaceutical building blocks it follows that a range of practical synthetic methods are needed 2 to provide access to any targeted structural and stereochemical pyrrolidine pattern.We have chosen to tackle the challenge of developing useful pyrrolidine forming methods by revisiting the ring expansion of vinyl aziridines, first reported by Atkinson. 3 Surprisinigly, despite the potential usefulness of converting a vinylaziridine into a 3-pyrroline, there had only been a single study focused on using metal catalysts to aid the rearrangement prior to our contribution to this field. 4 Oshima and coworkers found that tosylated dieneaziridines could be ring expanded in the presence of a palladium catalyst to the 3-pyrrolines. Both the N-tosyl group and the diene moiety were reported to be essential for the success of this rearrangement. Simple non-dienic vinyl aziridines did not ring expand, furnishing instead a complex mixture of products. In our recent report, we demonstrated that this significant substrate limitation could be solved using Cu(hfacac) 2 as a catalyst. 5 The substrate scope of this new transformation, which we demonstrated for a range of tosyl (Ts) and N-phthalimide (NPhth) protected vinyl aziridines, was shown to be quite broad. In this report we expand these investigations further and focus our attention on stereospecific vinyl aziridine ring expansions and application of this new methods towards accessing chiral pyrroline products.njardars@email.arizona.edu. Supporting Information Available Full experimental details for all new compounds reported in this article including X-ray data for compounds 15d, 15j, 15l, 21d and 21f are available free of charge via the Internet at http://pubs.acs.org. In order to maximize the synthetic potential of our method for preparing chiral pyrroline products, it is essential that reliable, asymmetric, convergent, and scalable routes be available to access the requisite starting materials (chiral vinyl aziridines). 6 The union of an imine and suitably activated nucleophile quickly emerged as the optimal approach (Scheme 1). The imine based retrosynthetic analysis is highlighted for chiral pyrroline 3, which we envisioned would originate from the copper catalyzed ring expansion of a trans-or cis-vinyl aziridine (4 and ...