1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.

An accurate gas-phase structure determination of cis-hex-3-ene-1,5-diyne (1), the parent molecule for the Bergman cyclization reaction, has been achieved through a combination of Fourier transform microwave spectroscopy and computational quantum chemistry. The microwave spectra of seven isotopes of enediyne 1 have been observed using a pulsed-nozzle Fourier transform spectrometer. The ground-state rotational constants were corrected for vibrational effects and the equilibrium structure (R e ) was deduced. The equilibrium structure displays excellent agreement with that obtained from high-level ab initio calculations (CCSD(T)/cc-pVTZ). Both approaches confirm that the molecule has C 2V symmetry with slight deviations of the alkyne units (C-CtC-H) from linearity. These groups distort away from each other in contra-distinction to their distortion during the Bergman cyclization reaction. The C1-C6 distance (4.32 Å), which has been viewed as a critical parameter governing reactivity in the Bergman cyclization, is notably longer than an earlier, widely publicized value (4.12 Å). An important role of the macrocyclic ring in enediyne antibiotics is to overcome an intrinsic distortion of the enediyne moiety in the direction opposite that of the reaction coordinate for Bergman cyclization.

Recent synthetic and biological studies of the viridin class of steroidal furans have revealed multiple opportunities for fundamental discoveries as well as advanced drug design. Wortmannin is a potent enzyme inhibitor that binds to the ATP site of important regulatory kinases such as PI-3 kinase and Polo-like kinase. The natural product shares a unique mechanism-based biological activation pathway with other viridins. Furthermore, while there have been several encouraging approaches toward the total synthesis of these compounds, there is still ample room for improvements in synthetic strategies and tactics, and the development of structurally simplified analogs that exert more specific biological effects and are devoid of toxicity issues that have thwarted the clinical development of the parent compounds.