[Structure: see text] A mild and efficient [3+2] nitrile oxide/olefin cycloaddition allows coupling of the highly functionalized naphthalene and isocoumarin hemispheres of purpuromycin. A rationale of the inability of advanced keto alcohols to spirocyclize is presented based upon a systematic examination of the electronic factors present in these systems and suggests that the biosynthesis of purpuromycin does not proceed through open-chain intermediates.

Protoplasts from 8-to 9-day-old wheat (Triticum aestivum L.) leaves were used to isolate organelles which were examined for their contents of peptide hydrolase enzymes and, in the case of vacuoles, other acid hydrolases. High yields of intact chloroplasts were obtained using both equilibrium density gradient centrifugation and velocity sedimentation centrifugation on sucrose-sorbitol gradients. Aminopeptidase activity was found to be distributed, in approximately equal proportions, between the chloroplasts and cytoplasm. Leucyltyrosine dipeptidase was mainly found in the cytoplasm, although about 27% was associated with the chloroplasts. Vacuoles shown to be free from Ceilulysin contamination contained all of the protoplast carboxypeptidase and hemoglobin-degrading activities. The acid hydrolases, phosphodiesterase, acid phosphatase, a-mannosidase, and fP-N-acetylgucosamidase were found in the vacuole to varying degrees, but no f8-glucosidase was localized in the vacuole.Studies of protein degradation in plant tissues have demonstrated the existence of a diverse group of peptide hydrolase enzymes. Protein degradation has been envisaged as a process initiated by endopeptidase action and continued through the action of exopeptidases, including aminopeptidases, carboxypeptidase, and di-and tri-peptidases to eventually release amino acids for storage and/or transport (24). Characterization of these enzymes (5,15,22,29) has shown that, despite their common role in protein breakdown, they have few similarities with respect to the conditions required for optimal in vitro activity. These differences, particularly differences in optimum assay pH, have led to suggestions of enzyme compartmentalization as a means of regulating protein degradation, since the plant cell contains a number of localized pH zones by virtue of its various organelles.Although acid proteinases are known to be localized in leaf vacuoles (3,11), no other peptide hydrolases have been reported there. Carboxypeptidase activity has been located in protein bodies from the cotyledons of germinating mung beans (7,27) and in vacuoles prepared from the endosperm tissue of 4-d-old castor bean seedlings (18) but has not been reported in vacuoles isolated from leaf tissue. In a previous publication (30), we examined the patterns of activity of a number of enzymes during senescence of the wheat plant. The present study was undertaken to examine the spatial relationship between these enzymes, which include acid proteinase, amino-and carboxypeptidases, and an alkaline dipep- MATERIALS AND METHODS Plant Material. Wheat seeds (Triticum aestivum cv. Egret) were imbibed for 24 h prior to being transferred to 34-x 29-x 6-cm trays containing a compost soil mixture. Plants were grown in a glasshouse under natural daylength and light-intensity conditions. Plants were watered daily without additional nutrients.Protoplast Preparation. Primary leaves from 8-to 9-d-old wheat seedlings were used to isolate protoplasts. The lower epidermis was removed, and the leaves wer...

The total synthesis of (-)-scabronine G has been achieved in a concise manner from the (-)-Wieland-Miescher ketone. Scabronine G and its more potent methyl ester (also prepared) display activity as nonpeptidyl inducers of nerve growth factor production.

No abstract

A convergent, stereocontrolled route to either antipode of the cell adhesion inhibitor, peribysin E, has been achieved from carvone. Highlights of the synthesis include a Diels−Alder reaction to generate a cis-decalin framework, followed by semipinacol-type ring contraction to secure the stereochemistry of the C7 quaternary center. Potential mechanistic pathways for the critical ring contraction were studied through deuterium incorporation studies. In addition, an optimized olefin isomerization/Saegusa oxidation protocol is described for the conversion of [4+2] cycloadducts of 2-(trialkylsilyloxy)-1,3-dienes to 1,6(2H,7H)-naphthalenediones, having stereochemical arrangements not accessible via conventional Robinson annulation protocols. Finally, the ability to independently prepare either enantiomer of peribysin E from the corresponding antipode of carvone led to a reassignment of the absolute configuration of peribysin E.

Abstract2-Iodoxybenzoic acid (IBX) is a convenient reagent for the dehydrogenation of tetrahydro-β-carbolines to their aromatic forms under mild conditions. The utility of the method was demonstrated in a total synthesis of the marine indole alkaloid eudistomin U.The aromatic β-carboline moiety is found in numerous natural products and synthetic congeners.1 Compounds bearing this ring system display a diverse range of biological properties including antimalarial,2 antitumor,3 and anti-HIV activities.4 Others show potent binding affinities toward benzodiazepine receptors in the central nervous system, thereby acting as diazepam antagonists.5 In light of these pharmacological properties, mild synthetic methods for the construction of the β-carboline unit are desirable. One strategy for its preparation centers on the formal dehydrogenation of a suitable tetrahydro-β-carboline precursor. Transformations of this type have been previously conducted by heating the substrate with palladium on carbon6 or sulfur7 in refluxing cumene or xylenes over extended periods of time. Other oxidizing agents such SeO 2 8 and MnO 2 9 also require high temperatures and must often be used in excess. Organic-based reagents capable of effecting the dehydrogenation are limited to quinone-derived reagents such as chloranil10 and DDQ11 and yields are often unsatisfactory. Trichloroisocyanuric acid (TCCA) has recently been identified as an additional oxidant.12We were prompted to seek a mild set of conditions for the aromatization of tetrahydro-β-carbolines during our studies in alkaloid synthesis. Aiming to improve upon existing methodologies, we desired a process that would employ an inexpensive oxidant and proceed smoothly at ambient temperature. In this Letter, we describe a new method to achieve this transformation and demonstrate its utility in a total synthesis of the marine indole alkaloid eudistomin U.Drawn to examples by Nicolaou and co-workers of iodine(V)-mediated syntheses of pyridines from N-heterocyclic precursors,13 our attention turned to hypervalent iodine reagents such as the Dess-Martin periodinane and 2-iodoxybenzoic acid (IBX).14 A survey

The isolation and characterization of the API form of aminopeptidase (EC 3.4.11.) previously identified (Waters, Dalling 1979 Aust J Plant Physiol 6: 595-606) in the primary leaves of wheat (Triticum aesti'um L. cv Egret) seedlings is reported. The enzyme shows a high preference for a substrate which contains an aromatic side chain, whether this be either a synthetic j-naphthylamide or a peptide substrate. Maximum activity with both types of substrates occurred around pH 7.6. The stability of API was reduced by exposure to high pH and by incubation at temperatures above 20C in the absence ofsubstrate. API was inhibited by the metal chelators bathocuproine and bathophenanthroline and the sulfhydryl group inhibitorsp-chloromercuribenzoate and N-ethylmaleimide. The molecular weight, estimated by gel filtration, was 57,000. The K., value for activity against the synthetic substrate Phe-a-NA (0.20 millimolar) was slightly lower than that for Phe-Phe (0.50 millimolar) although the enzyme activity against peptide substrates was considerably greater than with Phe-,8-NA.Aminopeptidases have been purified and characterized in a number of economically important plant species (6,8,14). With only a few exceptions, the plant aminopeptidases have a number ofproperties in common: low mol wt (<100,000) compared with mammalian aminopeptidases; neutral assay-pH optima; the ability to hydrolyze aminoacyl-,8-naphthylamides, di-and tripeptides, and amino acid and dipeptide amides but an inability to hydrolyze Z-dipeptides3, synthetic ester substrates, and proteins; all contain an essential sulfhydryl group, as evidenced by their sensitivity to p-CMB and heavy metal ions, and the stabilizing effects of sulfhydryl reagents such as DTT and ME; they are insensitive to the carboxypeptidase inhibitor, DFP; and they are

A concise, enantioselective total synthesis of the Lycopodium alkaloid (−)-lyconadin C was achieved in 12 steps and high overall yield. Key features include construction of a luciduline congener through Mannich-type cyclization and a one-pot, tandem Curtius rearrangement/6π-electrocyclization to fashion the 2-pyridone system of lyconadin C.