In summary, we have found that the peptidic linkage joining the two cysteine residues in Leu-Arg-Arg-Cys-Cys-Leu-Gly exists predominantly in the cis isomeric form. Since the Cys-Cys dyad is easy to prepare, such a molecular constraint should prove to be of general utility in conferring a configurational bias on otherwise conformationally flexible peptides. In addition, recent efforts in de novo protein design have served to generate interest in the construction of secondary structural elements, both artificial and peptidic. The Cys-Cys dyad offers the potential of incorporating bends or turns into natural and unnatural proteins via insertion of the unoxidized precursor by the standard protocols of either solid-phase peptide synthesis or the recombinant DNA methodology; While the efrapeptins have been exploited for their interesting biological properties, their structures have not been elucidated. As part of our work on fungal insect toxins, we reisolated the efrapeptins and in this communication report the complete stereostructures of efrapeptins D and F as 1 and 2, respectively. The amino acid sequences of the two major efrapeptins, D and F, were established by amino acid analysis, FABMS fragmentation pattern, and GC-MS analysis of volatile derivatives of partial hydrolysates. All of the chiral amino acids were shown to have the S (L) absolute configuration by chiral GC and other analyses. Efrapeptin D (1) differed from efrapeptin F (2) by one Gly/Ala replacement. The nature of the C-terminal blocking group was elucidated by a single-crystal X-ray diffraction analysis of 4, the trifluoroacetate salt of the trifluoroacetyl derivative of hydrolytic fragment 3. Efrapeptins D (1) and F (2) were toxic to Leptinotarsa decemlineata and inhibited the mitochondrial ATPase from fungi and insects. The activity of the efrapeptins primarily resides in the C-terminus since one of the hydrolytic fragments showed a dose response inhibition of M. domestica ATPase albeit with a lower specific activity than that of the full peptide; One of the unsolved puzzles of organic photochemistry has been the efficient formation of benzocyclobutenols from 2,6-dialkylphenyl ketones but not from simple o-alkylphenyl ketones. We report that a variety of o-alkyl ketones do in fact form cyclobutenols efficiently, quantitatively, and often stereoselectively and that the mechanism involves thermal electrocyclic closure of the initial dienol photoproducts. Scheme I depicts several acetophenones that we have found to yield benzocyclobutenols as the major or only photoproducts upon irradiation of dilute ketone with wavelengths >290 nm. In all cases a single diastereomer was produced. Heating the alcohols at 80 OC or higher converts them quantitatively to the starting ketones. Since the cyclobutenols are stable to acid while the dienols are rapidly converted to ketone by acid, we conclude that all cyclobutenol formation must occur from the first-formed dienols