β-Lactones are a privileged structural motif as enzyme inhibitors and chemical probes, particularly for the inhibition of enzymes from the serine hydrolase class. Herein, we demonstrate that cross-metathesis (CM) of α-methylene-β-lactones offers rapid access to structurally diverse, previously unexplored β-lactones. Combining this approach with competitive activity-based protein profiling (ABPP) identified lead β-lactone inhibitors/probes for several serine hydrolases, including disease-associated enzymes and enzymes of uncharacterized function. The structural diversity afforded by the α-methylene-β-lactone scaffold thus expands the landscape of serine hydrolases that can be targeted by small-molecule inhibitors and should further the functional characterization of enzymes from this class through the optimization of target-selective probes.

A novel Pt-catalyzed rearrangement of oxaspirohexanes to 3-methylenetetrahydrofurans is reported. Mechanistic studies by 13C-labeling experiments confirm oxidative addition of Pt(II) regioselectively to the least substituted carbon–carbon bond of the cyclopropane to form a platinacyclobutane intermediate. To our knowledge, this is the first alkoxy-substituted platinacyclobutane that has been observed spectroscopically. The scope and a proposed mechanism of this new Pt-catalyzed transformation are described.

Oxetanes are valuable intermediates in organic synthesis, and strategic manipulations of this strained heterocycle continue to emerge. In this Synopsis, recent, distinct approaches to construct heterocyclic systems using oxetanes are described. These include ring expansion, ring opening, and C-2 functionalization.

The first rearrangement of 2-methyleneoxetanes to α,β-unsaturated methylketones is reported. It is proposed that when these substrates are heated, the corresponding oxetenes are formed and subsequently undergo electrocyclic ring-opening to methyl vinylketones. In particular, α-silyl-α,β-unsaturated methylketones were isolated in moderate to high yields and with high stereoselectivities. Based on the proposed mechanism, density functional theory explains the differential kinetics and stereoselectivities among substrates.

Laureatin, a metabolite of the red algae Laurencia nipponica, has shown potent activity as a mosquito larvicide. The two previously published syntheses of laureatin involved an initial preparation of the 8-membered cyclic ether, followed by formation of the oxetane ring. Our strategy was the reverse, i.e., to utilize an oxetane as the framework to construct the larger ring. During this work, attempted N-bromosuccinimide (NBS)-mediated cyclization of oxetane alcohol 17, prepared from readily accessible 2-methyleneoxetane 12, yielded epoxytetrahydrofuran 19 rather than the expected laureatin core. Further derivatization of 19 yielded trans fused bis-tetrahydrofuran 32. The synthesis of 19 and 32, as well as structural and stereochemical elucidation studies, are described.

Natural killer T (NKT) cells recognize glycolipids presented by CD1d. The first antigen described, α-galactosyl ceramide (αGalCer), is a potential anticancer agent whose activity depends upon IFN-γ secretion. We report two analogs of αGalCer based on a naturally occurring glycosphingolipid, plakoside A. These compounds induce enhanced IFN-γ that correlates with detergent-resistant binding to CD1d and an increased stability of the lipid-CD1d complexes on antigen-presenting cells. Structural analysis on one of the analogs indicates that it is more deeply bound inside the CD1d groove, suggesting tighter lipid-CD1d interactions. To our knowledge, this is the first example in which structural information provides an explanation for the increased lipid-CD1d stability, likely responsible for the Th1 bias. We provide insights into the mechanism of IFN-γ-inducing compounds, and because our compounds activate human NKT cells, they could have therapeutic utility.Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (385 K)Download as PowerPoint slideHighlights► Plakoside A analogs are antigens for human and mouse invariant natural killer T cell ► These analogs induce a potent Th1-biased immune response ► Th1-skewing correlates with an increased biological stability ► Structural studies may provide insights to account for the Th1-biased response

The first psico-oxetanocin analogue of the powerful antiviral natural product, oxetanocin A, has been readily synthesized from cis-2-butene-1,4-diol. Key 2-methyleneoxetane precursors were derived from β-lactones prepared by the carbonylation of epoxides. F+-mediated nucleobase incorporation provided the corresponding nucleosides in good yield but with low diastereoselectivity. Surprisingly, attempted exploitation of anchimeric assistance to increase the selectivity was not fruitful. A range of 2-methyleneoxetane and related 2-methylenetetrahydrofuran substrates was prepared to explore the basis for this. With one exception, these substrates also showed little stereoselectivity in nucleobase incorporation. Computational studies were undertaken to examine if neighboring group participation involving fused [4.2.0] or [4.3.0] intermediates is favorable.

An unanticipated cleavage of 2-azido-2-(hydroxymethyl)oxetanes is reported. In attempts to oxidize the title oxetanyl alcohols to the corresponding carboxylic acids with RuO4, cleaved nitriles were formed as the sole isolable products, while a closely related tetrahydrofuran gave solely the expected carboxylic acid. Quantum chemical calculations suggest that the divergent outcomes are governed by conformational differences in the azidoalcohols.

Four 3″- and 4″-deoxy and -fluorogalactosyl ceramides were synthesized, and their ability to stimulate iNKT cells, based on levels of IL-2 production, was assessed in three NKT cell receptor hybridomas. In two of the hybridomas, 1.2 and 2H4, all of the analogs were immunostimulatory, while in the 1.4 hybridoma only the 4″-fluoro analog led to the production of significant levels of IL-2.

An α-galactosyl ceramide (α-GalCer) 2 was synthesized and evaluated for its ability to stimulate iNKT-cell proliferation and elicit T-helper cytokines, IL-4 and IFNγ. Compound 2 combines the acyl chain of the potent, Th2 biasing α-GalCer 1 with a sphingoid base of the same length as that found in OCH, which also exhibits Th2 skewing, Such complementation may enhance cytokine bias, which is thought to be important for therapeutic applications of iNKT cell stimulation. Two related α-GalCers, 3 and 4, with saturated acyl chains were prepared for comparison.

The synthesis and evaluation for iNKT stimulation of α-S-galactosylceramide is reported. Prepared by alkylation of a galactosylthiol, this analog of the potent immunostimulatory agent, KRN7000, did not stimulate iNKT cells either in vitro or in vivo.

2-Methyleneoxetanes were converted in excellent yields to 4-oxaspiro[2.3]hexanes under modified Simmons–Smith conditions. Treatment of the oxaspirohexanes with BF3·Et2O provided cyclopentanones, cyclobutanones or 4-methylenetetrahydrofurans, depending on the substituents.