Synthesis of medium-ring lactones via tandem methylenation/Claisen rearrangement of cyclic carbonates

Products Generated by Amine-Catalyzed Strand Cleavage at Apurinic/Apyrimidinic Sites in DNA: New Insights from a Biomimetic Nucleoside Model System

Abasic sites are common in cellular and synthetic DNA. As a result, it is important to characterize the chemical fate of these lesions. Amine-catalyzed strand cleavage at abasic sites in DNA is an important process in which conversion of small amounts of the ring-opened abasic aldehyde residue to an iminium ion facilitates β-elimination of the 3'-phosphoryl group. This reaction generates a trans-α,β-unsaturated iminium ion on the 3'-terminus of the strand break as an obligate intermediate. The canonical product expected from amine-catalyzed cleavage at an AP site is the corresponding trans-α,β-unsaturated aldehyde sugar remnant resulting from hydrolysis of this iminium ion. Interestingly, a handful of studies have reported noncanonical 3'-sugar remnants generated by amine-catalyzed strand cleavage, but the formation and properties of these products are not well-understood. To address this knowledge gap, a nucleoside system was developed that enabled chemical characterization of the sugar remnants generated by amine-catalyzed β-elimination in the 2-deoxyribose system. The results predict that amine-catalyzed strand cleavage at an AP site under physiological conditions has the potential to reversibly generate noncanonical cleavage products including cis-alkenal, 3-thio-2,3-dideoxyribose, and 2-deoxyribose groups alongside the canonical trans-alkenal residue on the 3'-terminus of the strand break. Thus, the model reactions provide evidence that the products generated by amine-catalyzed strand cleavage at abasic sites in cellular DNA may be more complex that commonly thought, with trans-α,β-unsaturated iminium ion intermediates residing at the hub of interconverting product mixtures. The results expand the list of possible 3'-sugar remnants arising from amine-catalyzed cleavage of abasic sites in DNA that must be chemically or enzymatically removed for the completion of base excision repair and single-strand break repair in cells.

Highly Reactive Cyclic Carbonates with a Fused Ring toward Functionalizable and Recyclable Polycarbonates

Monomer design plays an important role in the development of polymers with desired thermal properties and chemical recyclability. Here we prepared a class of seven-membered ring carbonates containing trans-cyclohexyl fused rings. These monomers showed excellent activity for ring-opening polymerization (ROP) with turnover frequency (TOF) up to 6 × 10⁵ h^-1 and catalyst loading down to 50 ppm, which yielded high-molecular-weight polycarbonates (M_n up to 673 kg/mol) with great thermostability (T_d > 300 °C). Ultimately, the resulting polycarbonates can completely depolymerize into their corresponding cyclic dimers that can repolymerize to synthesize the starting polymers in moderate yields, demonstrating a potential route to achieve chemical recycling. Postfunctionalization of the unsaturated polycarbonate was conducted through cross-linking reaction and "click" reaction under UV irradiation.

Development of the Claisen Rearrangement/Organocatalytic Diels-Alder Approach for the Synthesis of Eunicellins

The intramolecular Diels-Alder approach to synthesizing eunicellins has proved to be a powerful method for the synthesis of this class of natural products. The key to the success of this strategy is control over the endo/exo selectivity of the cycloaddition reaction, which we have addressed through an organocatalytic reaction employing the MacMillan imidazolidinone catalyst. This approach has been further developed to address the issue of functionality at the C8 position, and a novel scalable method for the extension of the medium-ring lactone has been developed.

Triphosgene-amine base promoted chlorination of unactivated aliphatic alcohols

Unactivated α-branched primary and secondary aliphatic alcohols have been successfully transformed into their corresponding alkyl chlorides in high yields upon treatment with a mixture of triphosgene and pyridine in dichloromethane at reflux. These mild chlorination conditions are high yielding, stereospecific, and well tolerated by numerous sensitive functionalities. Furthermore, no nuisance waste products are generated in the course of the reactions.

Asymmetric Synthesis of a Hydroxylated Nine-membered Lactone from Tartaric Acid using the Claisen Rearrangement

The synthesis of a hydroxylated vinyl-appended lactone, in five synthetic steps from tartaric acid, is reported. The C2-symmetrical bis-vinyl diol 12 was converted into the ketene acetal 14 via methylenation of the cyclic carbonate 13 or thermal elimination of benzeneselenenic acid from the selenoxide 17. In both cases, the in situ generated ketene acetal 14 underwent spontaneous Claisen rearrangement to give the nine-membered lactone (+)-15. Lactones of this type are potentially advanced precursors to simplified eleutherobin analogues or other medium-ring lactone natural products.

Total Synthesis of Solandelactones E and F, Homoeicosanoids from the Hydroid Solanderia secunda

Asymmetric total syntheses of solandelactones E and F confirmed that hydroxyl configuration at C11 in these oxylipins had been misassigned and that the stereochemistry at this center should be reversed. Key steps in the synthesis involved a Nagao asymmetric acetate aldol reaction, a directed Simmons-Smith cyclopropanation, a Holmes-Claisen rearrangement to establish the unsaturated octalactone, and a Nozaki-Hiyama-Kishi coupling to connect two major fragments at C11-C12.

An organocatalytic approach to the core of eunicellin

A stereocontrolled synthesis of the core of eunicellin is described featuring a Claisen rearrangement and a diastereoselective organocatalytic Diels-Alder reaction as the key steps.