Gas‐Phase Structure Determination of Dihydroxycarbene,One of the Smallest Stable Singlet Carbenes

Carbenes are reactive molecules of the form R¹ C̈ R² that play a role in topics ranging from organic synthesis to gas‐phase oxidation chemistry. We report the first experimental structure determination of dihydroxycarbene (HO C̈ OH), one of the smallest stable singlet carbenes, using a combination of microwave rotational spectroscopy and high‐level coupled‐cluster calculations. The semi‐experimental equilibrium structure derived from five isotopic variants of HO C̈ OH contains two very short CO single bonds (ca. 1.32 Å). Detection of HO C̈ OH in the gas phase firmly establishes that it is stable to isomerization, yet it has been underrepresented in discussions of the CH₂O₂ chemical system and its atmospherically relevant isomers: formic acid and the Criegee intermediate CH₂OO.     

A general equation describing frequency discrimination as a function of frequency and sensation level

Frequency-discrimination thresholds, for a wide range of stimulus frequencies and stimulus levels, were obtained from three normal-hearing listeners. Linear regression analyses of the present data, and of data from two previous studies, indicate that an SL-1 transformation of stimulus level and a square root F transformation of stimulus frequency yield linear dimensions that allow accurate predictions of frequency-discrimination thresholds from normal-hearing listeners over a wide range of stimulus frequencies (125-8000 Hz) and stimulus levels (5-8 dB SL) with a single prediction equation.     

Asymmetric total synthesis of (-)-laulimalide: exploiting the asymmetric glycolate alkylation reaction

A concise total synthesis of the potent antitumor macrolide (-)-laulimalide is described. The observation that homoallylic (or latent homoallylic) C-O bonds are present at C5, C9, C15, C19, and C23 led to the strategic decision to rely heavily on the asymmetric glycolate alkylation to construct both the C1-C14 fragment 3 and the C15-C27 subunit 4. A diastereoselective addition of a C1-C14 allylstannane to a C15-C27 alpha,beta-epoxyaldehyde served to join the two advanced fragments. A Mitsunobu macrolactonization of hydroxy acid 2 avoided isomerization of the sensitive 2,3-Z-enoate, which has been observed in base-catalyzed macrolactonizations. Removal of two TBS protecting groups to reveal the C15 and C20 hydroxyls occurred without rearrangement to isolaulimalide.     

Correlations between reported and measured ultraviolet radiation exposure of mothers and young children

Cumulative and intermittent sun exposure are risk factors for skin cancer, highlighting the need to monitor exposure during childhood. There is currently very little available information concerning the accuracy of self-reported levels of sun exposure, particularly for very young children. In this study, UV radiation (UVR) exposure measured by polysulfone dosimeters worn on the wrist was compared with a measure of estimated exposure using a diary based on recall at the end of the 4 day study period and ambient dosimetric measures. Results of the study indicate that the relative UVR exposures expressed as a fraction of daily total ambient received during the 4 day period by young children and mothers are similar. A high level of association was obtained for the estimated levels of exposure between young children and their mothers. Moderate correlations were evident for dosimeter readings of mother and child on weekends with no significant association on weekdays. The association between estimated exposure and dosimeter readings was poor and needs improvement. This may be achieved by greater consideration of structural and environmental factors that influence the levels of UVR exposure received by individuals and by increasing the level of specificity in the measurement instruments. Methodological issues such as recall of exposure after several days, quantity of sun exposure and more precise measurement of grades of exposure when outside may have more impact than previously expected, and further information is being sought with a larger sample for exposure during the summer months.