Frank K. Brown

Evidence for the concerted mechanism of the Diels-Alder reaction of butadiene with ethylene.

Diels-Alder reactions of substituted dienes and dienophiles have been found to cccur by both concerted and stepwise mechanisms.’.* The nature of the mechanism of the prototype Diels-Alder reaction of butadiene with ethylene to form cyclohexene is not only of fundamental interest but has generated fervid fulminations for favorite We wish to report new experimental evidence that is compatible only with the concerted mechanism, as well as further theoretical evidence for the synchronicity of this concerted mechanism.6 The reactions of 1,1,4,4-tetradeuterio-1,3-butadiene with cisor trans-1,2-dideuterioethylene were carried out at 185 “ C for 36 h at a pressure of 1800 psi in a stainless steel b~mb.~-’O Infrared spectra of the unreacted dideuterioethylenes indicate that they do not isomerize under the conditions of the reaction. Each cyclohexene product was separated from butadiene dimers by preparative GLC” and was then epoxidized with m-chloroperbenzoic acid (Figure 1). The N M R spectrum of cyclohexene oxide (Figure 2A) has resonances for the protons at C, and C, at 6 1.19 and 1.38, corresponding to protons that are cis and trans to the epoxide oxygen.12 The proton NMR spectra of the epoxides obtained from the reactions of cis and trans-dideuterioethylenes are clearly different. The DH couplings were partially eliminated by irradiation of the C3 and C6 deuteriums to give the spectra shown in Figure 2. The cis-dideuterio compound has a singlet resonance at d 1.19, due to the product with C4 and Cs hydrogens cis to the epoxide, and a second singlet a t 6 1.38, due to the

The influence of substituent induced asynchronicity on the stereochemistries of intramolecular diels-alder reactions

Abstract Trends in stereoselectivities of intramolecular Diels-Aider reactions upon substitution are rationalized by a “twist-asynchronous” model, which is supported by MM2 model calculations.

The STO-3G transition structure of the diels-alder reaction

A transition structure with CS symmetry for the Diels-Alder reaction of butadiene with ethylene has been obtained with the STO-3G basis set. Stereochemical consequences are discussed.

The inside alkoxy effect in intramolecular nitrile oxide cycloadditions : ab initio and force field modeling

Abstract MM2 parameters based on ab initotransition structures for nitrile oxide cycloaddition on allyl ethers were developed for INOC reactions.

A force field model for the transition structures of intramolecular nitrile oxide cycloadditions

Abstract A force field model for nitrile oxide cycloadditions accounts for observed diastereoselectivities in inter- and intramolecular reactions controlled by steric factors.

Studies on the Structure and Function of Ubiquitin

Ubiquitin is a small (76 amino acid) protein found in all eukaryotes either free or covalently attached to proteins in the nucleus, cytosol, or plasma membrane (Wilkinson, 1986; Busch, 1984; Hershko, 1988). It is the most highly conserved protein yet discovered in eukaryotes, with only three conservative amino acid substitutions separating yeast ubiquitin from the human (Fig. 1). The protein is water soluble, extremely stable, and may be boiled without loss of activity. Ubiquitin is stable to proteases and has not been shown to be glycosylated (Ozkaynak et al., 1984; Vierstra et al., 1986; Schlesinger and Goldstein, 1975). It contains no cysteine and only one histidine at position 68. The crystal structure of human ubiquitin has been solved at 1.8 A resolution and demonstrates that ubiquitin is a compact molecule with a hydrophobic core containing three and one-half turns of α helix and five strands of β sheet. At the carboxy terminus, four amino acids protrude from the core of the molecule and have considerable freedom of motion (Vijay-Kumar et al., 1987).