The absolute rate constants for the C 6 H 5 + C 6 H 6 and C 6 D 6 reactions have been measured by cavity ringdown spectrometry at temperatures between 298 and 495 K at a constant 40 Torr Ar pressure. The new results, which reveal no detectable kinetic isotope effect, can be represented by the Arrhenius equation, k 1 ) 10 (11.91(0.13) exp [-(2102 ( 106)/T] cm 3 /(mol s). Our low-temperature data for the addition/stabilization process, C 6 H 5 + C 6 H 6 f C 12 H 11 , can be correlated with those obtained in a low-pressure, high-temperature Knudsen cell study for the addition/displacement reaction, C 6 H 5 + C 6 H 6 f C 12 H 10 + H, by the RRKM theory using the molecular and transition-state parameters computed at the B3LYP/6-311G(d,p) level of theory. Combination of these two sets of data gives k 1 ) 10 (11.98(0.03) exp [-(2168 ( 34)/T] cm 3 /(mol s) covering the temperature range 298-1330 K. The RRKM theory also correlates satisfactorily the forward reaction data with the hightemperature shock-tube result for the reverse H-for-C 6 H 5 substitution process with 2.7 and 4.7 kcal/mol barriers for the entrance (C 6 H 5 + C 6 H 6 ) and reverse (H + C 12 H 10 ) reactions, respectively. For modeling applications, we have calculated the forward reaction rate constants for the formation of the two competing products, H + C 12 H 10 and C 12 H 11 , at several pressures covering 300 K < T < 2500 K.

The total synthesis of the natural product RK-397 is based on a new synthetic strategy for assembling polyacetate structures, by efficient cross-coupling of nucleophilic terminal alkyne modules with electrophilic epoxides bearing another alkyne at the opposite terminus. The natural product is constructed from four principal modules: a polyene precursor for carbons 3-9, and three alkyne-terminated modules for carbons 10-16, 17-22, and 23-33. Each module is prepared with control of all stereochemical elements, and the alkynyl alcohols obtained from alkyne-epoxide couplings are converted into 1,3-diols by a sequence of hydroxyl-directed hydrosilylation, C-Si bond oxidation, and stereoselective ketone reduction with induction from the beta-hydroxyl group. The highly convergent nature of our synthetic pathway and the flexibility of the modular synthesis strategy for virtually any stereoisomer can provide access to other members of the polyene-polyol macrolides, including stereoisomers of RK-397.

Cycloisomerization of homopropargylic thiols to dihydrothiophenes is promoted by group VI metal carbonyls. Related thiacyclization transformations under basic and radical conditions are also described, including regioselective formation of benzothiophenes from aryl methyl sulfides and alkynes.

Our new synthetic strategy for assembling polyacetate structures features efficient cross-couplings of six-carbon modules derived from any stereoisomer of the epoxyalkynol derivative (1). Hydration of the internal alkyne in the coupled products and stereoselective reduction of the resulting ketone intermediate provides a general approach to a library of stereoisomeric 1, 3, 5, ... alternating polyols (2). The strategy is demonstrated in a stereoselective synthesis of the C11-C28 polyol substructure of the natural product RK-397.