Formal syntheses of tetracyclic terpenoids frondosin B and liphagal are described. Both synthetic routes rely on the use of platinum-catalyzed α,β-unsaturated carbene formation for the key C–C bond forming transformations. The successful route toward frondosin B utilizes a formal (4 + 3) cycloaddition, while the liphagal synthesis features the vinylogous addition of an enol nucleophile as a key step. Both synthetic routes are discussed, revealing insights into structural requirements in the catalytic α,β-unsaturated carbene reaction manifold.

Materials and Methods. All reactions were performed under an argon atmosphere unless otherwise noted. Tetrahydrofuran, N,N-dimethylformamide, dichloromethane, hexanes, and toluene were purified by passing through activated alumina columns. Diisopropylamine was distilled over CaH 2. 2-Fluoropyridine was freshly distilled before use. All other reagents were used as received unless otherwise noted. Commercially available chemicals were purchased from Alfa Aesar (Ward Hill, MA), Sigma-Aldrich (St. Louis, MO), Gelest (Morrisville, PA), Oakwood Products (West Columbia, SC), Strem (Newburport, MA), Mallinckrodt Chemicals (Phillipsburg, NJ), Spectrum (Gardena, CA) Fischer Scientific (Fair Lawn) and TCI America (Portland, OR). Qualitative TLC analysis was performed on 250 mm thick, 60 Å, glass backed, F254 silica (Silicycle, Quebec City, Canada). Visualization was accomplished with UV light and exposure to either p-anisaldehyde or KMnO 4 solution followed by heating. Flash chromatography was performed using Silicycle silica gel (230-400 mesh). 1 H NMR spectra were acquired on either a Varian Mercury 300 (at 300 MHz), a Varian Inova 400 (at 400 MHz), or a Varian 400 MR (at 400 MHz) and are reported relative to SiMe 4 (δ 0.00). 13 C NMR spectra were acquired on either a Varian Inova 400 (at 100 MHz), a Varian Mercury 300 (at 75 MHz), or a Varian 400 MR (at 100 MHz) and are reported relative to SiMe 4 (δ 0.0). All IR spectra were obtained on NaCl plates (film) with either a Nicolet Magna FTIR 760, a Nicolet 380 FTIR, or a Bruker Tensor 27. High resolution mass spectrometry data were acquired by the Colorado State University Central Instrument Facility on an Agilent 6210 TOF LC/MS. Optical rotations were obtained with an Autopol-III automatic polarimeter.

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The Ireland-Claisen rearrangement of boron ketene acetals is described. The boron ketene acetal intermediates are formed through a soft enolization that obviates the use of strong bases and the intermediacy of alkali metal enolates. Yields and diastereoselectivities of these rearrangements are very sensitive to the choice of boron reagent, even among those that have been shown to effect quantitative formation of boron ketene acetals from esters. The rearrangement occurs at room temperature for all substrates with generally high levels of stereoselectivity. In contrast to previous reports using boron triflates, the use of a commercially available boron iodide reagent allows for a wider substrate scope that extends to propionates and arylacetates, as well as the previously described α-oxygenated esters. This work also provides insight into the dynamic nature of boron ketene acetals and the ramifications of this behavior for reactions in which they are intermediates.

2-exo-Vinyl-7-alkylidenenorbornanes, readily prepared from fulvene Diels–Alder adducts, undergo smooth Cope rearrangement at elevated temperatures to produce hexahydroazulene derivatives.