Palladium-catalyzed asymmetric [4 + 3] cycloaddition of methylene-trimethylenemethane: access to seven-membered exocyclic axially chiral allenes

A palladium-catalyzed asymmetric [4 + 3] cycloaddition of the methylene-trimethylenemethane donor with an azadiene has been developed, affording benzofuro[3,2-b]azepine-derived exocyclic chiral allene with control of axial and point chirality. The target compounds were generated in good to excellent yields and with high diastereoselectivities and enantioselectivities (up to >20 : 1 dr, 99% ee). Furthermore, this cycloaddition reaction could be efficiently scaled-up and several synthetic transformations were accomplished for the construction of useful chiral allenol and chiral spirocyclic derivatives.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Nucleophilic vinylic substitution in bicyclic methyleneaziridines: SNVπ or SNVσ?

A stereoselective synthesis of the monodeuterated methyleneaziridine shown allowed the stereochemical course of formal S_NV-mode ring-opening with copper-based organometallics to be assigned.

Copper(I)-catalyzed diastereo- and enantio-selective construction of optically pure exocyclic allenes

Among about 150 identified allenic natural products, the exocyclic allenes constitute a major subclass. Substantial efforts are devoted to the construction of axially chiral allenes, however, the strategies to prepare chiral exocyclic allenes are still rare. Herein, we show an efficient strategy for the asymmetric synthesis of chiral exocyclic allenes with the simultaneous control of axial and central chirality through copper(I)-catalyzed asymmetric intramolecular reductive coupling of 1,3-enynes to cyclohexadienones. This tandem reaction exhibits good functional group compatibility and the corresponding optically pure exocyclic allenes bearing cis-hydrobenzofuran, cis-hydroindole, and cis-hydroindene frameworks, are obtained with high yields (up to 99% yield), excellent diastereoselectivities (generally >20:1 dr) and enantioselectivities (mostly >99% ee). Furthermore, a gram-scale experiment and several synthetic transformations of the chiral exocyclic allenes are also presented.

Brown Algae (Phaeophyceae) from the Coast of Madagascar: preliminary Bioactivity Studies and Isolation of Natural Products

Eight species of brown algae (Phaeophyceae) from the coast of Madagascar have been investigated for their chemical constituents. Fucosterol (3) was obtained as the most abundant compound. The brown alga Sargassum ilicifolium was the source for the first isolation of the terpenoid C27-alcohol 1,1',2-trinorsqualenol (1) from marine sources. From S. incisifolium we isolated the highly unsaturated glycolipid 1-O-palmitoyl-2-O-stearidonoyl-3-O-β-D-galactopyranosylglycerol (4) and we report the first full assignment of its (1)H and (13)C NMR data. Apo-9'-fucoxanthinone (8) along with 24-ketocholesterol (5), (22E)-3β-hydroxycholesta-5,22-dien-24-one (6), and saringosterol (7) were obtained from Turbinaria ornata. The crude extracts of all eight species of brown algae exhibited a pronounced antimicrobial activity against the Gram-positive bacteria Bacillus cereus, Staphylococcus aureus, and Streptococcus pneumoniae.

Allenyl and alkynyl phenyl ethers from the endolichenic fungus Neurospora terricola

Terricollenes A-C (1-3), new allenyl phenyl ethers, and terricolynes (4-6), new alkynyl phenyl ethers, have been isolated from the crude extract of the endolichenic fungus Neurospora terricola. The structures of these compounds were elucidated primarily by NMR experiments. The absolute configurations of 1 and 2 were determined by reversed-phase HPLC analysis on the o-tolylthiocarbamates of the liberated sugars upon acid hydrolysis, whereas that of 4 was assigned using the modified Mosher method. Compound 1 showed modest cytotoxicity against the human tumor cell lines HeLa and MCF-7.

Syntheses of allene-modified derivatives of peridinin toward elucidation of the effective role of the allene function in high energy transfer efficiencies in photosynthesis

Peridinin is known as the main light-harvesting pigment in photosynthesis in the sea and exhibits exceptionally high energy transfer efficiencies to chlorophyll a. This energy transfer efficiency is thought to be related to the intricate structure of peridinin, which possesses allene and ylidenbutenolide functions in the polyene backbone. There are, however, no studies on the relationship between the structural features of peridinin and its super ability for energy transfer. We then focused on the subjects of why peridinin possesses a unique allene group and how the allene function plays a role in the exceptionally high energy transfer. Toward elucidation of the exact role of the allene function, we now describe the syntheses of three relatively unstable allene-modified derivatives of peridinin along with the results of the Stark spectroscopy of peridinin and the synthesized peridinin derivatives.

Sesquiterpene-like inhibitors of a 9-cis-epoxycarotenoid dioxygenase regulating abscisic acid biosynthesis in higher plants

Abscisic acid (ABA) is a carotenoid-derived plant hormone known to regulate critical functions in growth, development and responses to environmental stress. The key enzyme which carries out the first committed step in ABA biosynthesis is the carotenoid cleavage 9-cis-epoxycarotenoid dioxygenase (NCED). We have developed a series of sulfur and nitrogen-containing compounds as potential ABA biosynthesis inhibitors of the NCED, based on modification of the sesquiterpenoid segment of the 9-cis-xanthophyll substrates and product. In in vitro assays, three sesquiterpene-like carotenoid cleavage dioxygenase (SLCCD) inhibitor compounds 13, 17 and 18 were found to act as inhibitors of Arabidopsis thaliana NCED 3 (AtNCED3) with K(i)'s of 93, 57 and 87 microM, respectively. Computational docking to a model of AtNCED3 supports a mechanism of inhibition through coordination of the heteroatom with the non-heme iron in the enzyme active site. In pilot studies, pretreatment of osmotically stressed Arabidopsis plants with compound 13 resulted lower levels of ABA and catabolite accumulation compared to levels in mannitol-stressed plant controls. This same inhibitor moderated known ABA-induced gene regulation effects and was only weakly active in inhibition of seed germination. Interestingly, all three inhibitors led to moderation of the stress-induced transcription of AtNCED3 itself, which could further contribute to lowering ABA biosynthesis in planta. Overall, these sesquiterpenoid-like inhibitors present new tools for controlling and investigating ABA biosynthesis and regulation.

Total synthesis of paracentrone, C31-allenic apo-carotenoid

The stereocontrolled total synthesis of a C31-allenic apo-carotenoid, paracentrone, was achieved by the convergent C20 + C11 = C31 strategy. The key elements of our synthesis were the Pd-catalyzed cross-coupling to stereoselectively construct the conjugated polyene backbone skeleton and the designed geometrical isomerization at the central double bond of the conjugated polyene chain. In addition, the terminal oxygenated cyclohexane ring having the allenic moiety was prepared by the highly diastereoselective Sharpless epoxidation under our own reaction conditions.

Stereocontrolled Total Synthesis of a Polyfunctional Carotenoid, Peridinin

Peridinin, which was isolated from the planktonic algae dinoflagellates causing red tides, is a highly oxidized carotenoid containing an allene and a characteristic (Z)-gamma-ylidenebutenolide function in the main conjugated polyene chain in addition to functionalized cyclohexane rings at both ends of the molecule. We achieved a stereocontrolled total synthesis of peridinin by featuring the Sharpless asymmetric epoxidation under precise reaction conditions, Wittig reaction with silylfuranmethylide followed by photosensitized oxygenation, stereocontrolled Pd-catalyzed one-pot (Z)-gamma-ylidenebutenolide synthesis, and modified Julia-Kocienski olefination. This synthesis is the first example of controlling the stereochemistry of polyfunctional allenic carotenoids.

Isolation and phytotoxicity of apocarotenoids from Chenopodium album

Two new compounds (1, 2) and 16 apocarotenoids (3-18) were isolated from the weed Chenopodium album. The structures of new apocarotenoids were determined to be (3R,6R,7E,9E,11E)-3-hydroxy-13-apo-alpha-caroten-13-one (1) and (6S,7E,9E,11E)-3-oxo-13-apo-alpha-caroten-13-one (2) by spectroscopic, NMR, and MS analysis. Five of the known compounds (5, 6, 13, 15, and 17) were previously reported only as synthetic compounds. Effects of these compounds on germination and growth of Lactuca sativa (lettuce) were studied in the 10(-4)-10(-7) M concentration range.