Stereoselective Synthesis of (+)-Avarol, (+)-Avarone, and Some Nonracemic Analogues

Synthesis of the rearranged drimane sesquiterpenoids (+)-avarol and (+)-avarone from Wieland-Miescher ketone is described. This synthetic sequence provides convenient access to the natural enantiomers and, based on comparison of the optical rotation of synthetic avarol dimethyl ether with literature data, affords material of significantly higher optical rotation than a natural source. Similar synthetic strategies have been used to obtain several related compounds, including a decalin bearing an exocyclic olefin and a highly substituted cyclohexane, that can be viewed as hybrids of the trans-fused avarol and cis-fused arenarol skeletons.     

Infrared spectroscopic characterization of the buried interface and surfaces of bonded silicon wafers

Basic investigations have been carried out on the characterization of different processing steps in bond preparation using etched <111> faces of silicon wafers for the incidence of the infrared beam to a multiple internal reflection geometry. The method is very sensitive to the surface coverage and interface. Surface activation by RCA cleaning yields an increase of water coverage and a decrease of SiH and CH groups. The detection limit for an oxide layer between silicon wafers has been found to be about 3 nm. Received: 18 July 1997 / Revised: 15 August 1997 / Accepted: 19 August 1997     

Electrophilic aromatic prenylation via cascade cyclization

To gain access to prenylated hexahydroxanthenes, tandem cascade cyclization–electrophilic aromatic substitution reactions have been studied on substrates bearing allylic and propargylic substituents. Both BF₃·OEt₂ and TMSOTf can be used to initiate this reaction sequence, resulting in different ratios of the C-2 and C-6 substitution products. Even though allylic transposition is observed in some cases, the results of a crossover experiment are consistent with an intramolecular reaction sequence. Taken together, these studies now allow preparation of either the C-2 or C-6 prenylated hexahydroxanthene products.     

BF3 x Et2O-mediated cascade cyclizations: synthesis of schweinfurthins F and G

The total synthesis of the natural stilbene (+)-schweinfurthin G (8) has been accomplished through a sequence based on an efficient cationic cascade cyclization. This cascade process is initiated by Lewis acid promoted ring opening of an epoxide and terminated through a novel reaction with a phenolic oxygen "protected" as its MOM ether. Several Lewis acids have been examined for their ability to induce this new reaction, and BF3 x Et2O was found to be the most effective. The only major byproduct under these conditions was one where the expected secondary alcohol was found as its MOM ether derivative (e.g., 30). While this byproduct could be converted to the original target compound through hydrolysis, it also could be employed as a protected alcohol to allow preparation of a benzylic phosphonate (43) without dehydration of the secondary alcohol. The resulting phosphonate was employed in a Horner-Wadsworth-Emmons condensation with an aldehyde representing the right half of the target compounds, an approach complementary to previous studies based on condensation of a right-half phosphonate and a left-half aldehyde.     

Characterization of (E,E)-farnesol and its fatty acid esters from anal scent glands of nutria (Myocastor coypus) by gas chromatography-mass spectrometry and gas chromatography-infrared spectrometry

Several volatile compounds, including terpenoids, fatty alcohols, fatty acids and some of their esters, were identified from solvent extracts prepared from anal scent glands of nutria (a.k.a. coypu), a serious rodent pest ravaging wetlands in the USA. The major terpenoid constituents were identified as (E,E)-farnesol and its esters by a comparison of their gas chromatographic retention times, and electron-ionization (EI) and chemical-ionization (CI) mass spectra with those of authentic compounds. EI mass spectra of the four farnesol isomers are very similar, however, the ChemStation (Agilent) and GC-MS Solution (Shimadzu) software algorithms were able to identify the natural compound as the (E,E)-isomer, when a high-quality mass spectral library was compiled from reference samples and used for searching. Similarly, the esters were identified as those of (E,E)-farnesol. In contrast to EI spectra, the CI spectra of the (E,E)- and (E,Z)-isomers are distinctly different from those of the (Z,E)- and (Z,Z)-isomers. The intensities (I) of the peaks for the m/z 137 and 121 ions in the CI spectra offer a way of determining the configuration of the C-2 double bond of farnesols (for 2E isomers I(137)>I(121), whereas for 2Z isomers I(137)相似文献   

Temperature effects on stereocontrol in the Horner-Wadsworth-Emmons condensation of alpha-phosphono lactones

The Horner-Wadsworth-Emmons condensation of some alpha-phosphono lactones has been examined for conditions that impact product stereochemistry. The temperature employed to quench the reaction was found to be a major factor. For example, after the diethyl phosphonate derivative of gamma-butyrolactone was treated with potassium hexamethyldisilazane, 18-crown-6, and propionaldehyde at -78 degrees C in THF, an aliquot transferred to a flask at approximately 30 degrees C gave almost exclusively the Z-olefin product, while one allowed to warm to room temperature over several hours greatly favored the E-olefin.     

Direct conversion of benzylic and allylic alcohols to phosphonates

Benzyl phosphonate esters often serve as reagents in Horner-Wadsworth-Emmons reactions. In most cases, they can be prepared from benzylic alcohols via formation of the corresponding halide followed by an Arbuzov reaction. To identify a more direct synthesis of phosphonate esters, we have developed a one-flask procedure for conversion of benzylic and allylic alcohols to the corresponding phosphonates through treatment with triethyl phosphite and ZnI(2).     

Stereoselective synthesis of the 5'-hydroxy-5'-phosphonate derivatives of cytidine and cytosine arabinoside

Both the (R)- and (S)-5'-hydroxy 5'-phosphonate derivatives of cytidine and cytosine arabinoside (ara-C) have been prepared via phosphite addition or a Lewis acid mediated hydrophosphonylation of appropriately protected 5'-nucleoside aldehydes. Phosphite addition to a cytosine aldehyde protected as the 2',3'-acetonide gave predominately the 5'R isomer, while phosphite addition to the corresponding 2',3'-bis TBS derivative favored the 5'S stereochemistry. In contrast, phosphite addition to the 2',3'-bis TBS protected aldehyde derived from ara-C gave only the 5'R adduct. However, TiCl(4)-mediated hydrophosphonylation of the same ara-C aldehyde favored the 5'S stereoisomer by a 2:1 ratio. Once all four of the diastereomers were in hand, the stereochemistry of these compounds could be assigned based on their spectral data or that obtained from their O-methyl mandelate derivatives. After hydrolysis of the phosphonate esters and various protecting groups, the four alpha-hydroxy phosphonic acids were tested for their ability to serve as substrates for the enzyme nucleoside monophosphate kinase and for their toxicity to K562 cells.