Synthetic Studies toward GKK1032s, Novel Antibiotic Antitumor Agents: Enantioselective Synthesis of the Fully Elaborated Tricyclic Core via an Intramolecular Diels−Alder Cycloaddition

Enantioselective and Regiodivergent Synthesis of Dihydro-1,2-oxazines from Triene-Carbamates via Chiral Phosphoric Acid-Catalysis

Conjugated trienes are fascinating building blocks for the rapid construction of complex polycyclic compounds. However, limited success has been achieved due to the challenging regioselectivity control. Herein, we report an enantio- and diastereoselective process allowing to regioselectively control the functionalization of NH-triene-carbamates. Synthesis of chiral cis-3,6-dihydro-2H-1,2-oxazines is achieved by a chiral phosphoric acid catalyzed Nitroso-Diels-Alder cycloaddition involving [(1E,3E,5E)-hexa-1,3,5-trien-1-yl]carbamates. Moreover, modular access to three different regioisomers with excellent diastereoselectivities and high to excellent enantioselectivities is obtained by a careful choice of the reaction conditions. A computational study reveals that the regioselectivity is influenced by the steric demand of the substituents at the 6-position of the triene, as well as noncovalent interactions between the two cycloaddition partners. Utility of each regioisomeric cycloadduct is highlighted by a variety of synthetic transformations.

First Total Synthesis of Tanzawaic Acid B

The first total synthesis of (+)-tanzawaic acid B, a natural polyketide bearing a pentadienoic ester and octalin moiety, has been accomplished. The synthetic improvement from previous synthetic conditions facilitated our gram-scale synthesis of the chiral octalin that possesses seven stereogenic centers and that is the core skeleton of almost all of the tanzawaic acid family.

Tricarbocyclic core formation of tyrosine-decahydrofluorenes implies a three-enzyme cascade with XenF-mediated sigmatropic rearrangement as a prerequisite

Tyrosine-decahydrofluorene derivatives feature a fused [6.5.6] tricarbocyclic core and a 13-membered para-cyclophane ether. Herein, we identified new xenoacremones A, B, and C (1-3) from the fungal strain Xenoacremonium sinensis ML-31 and elucidated their biosynthetic pathway using gene deletion in the native strain and heterologous expression in Aspergillus nidulans. The hybrid polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) XenE together with enoyl reductase XenG were confirmed to be responsible for the formation of the tyrosine-nonaketide skeleton. This skeleton was subsequently dehydrated by XenA to afford a pyrrolidinone moiety. XenF catalyzed a novel sigmatropic rearrangement to yield a key cyclohexane intermediate as a prerequisite for the formation of the multi-ring system. Subsequent oxidation catalyzed by XenD supplied the substrate for XenC to link the para-cyclophane ether, which underwent subsequent spontaneous Diels-Alder reaction to give the end products. Thus, the results indicated that three novel enzymes XenF, XenD, and XenC coordinate to assemble the [6.5.6] tricarbocyclic ring and para-cyclophane ether during biosynthesis of complex tyrosine-decahydrofluorene derivatives.

Stereoselective Synthesis of the Core Structures of Pyrrocidines and Wortmannines through the Excited-State Nazarov Reactions

The reaction conditions and scope of the excited-state Nazarov reaction of dicyclicvinyl ketones were studied. The stereochemistry of this electrocyclization is consistent with the mechanism of the pericyclic reaction and Woodward-Hoffmann rule. UV-light-promoted excited-state Nazarov reactions gave hydrofluorenones bearing a syn-cis configuration via a disrotatory cyclization. The core tricyclic hydrofluorenones of pyrrocidines and wortmannines were constructed via the excited-state Nazarov reactions, which demonstrated their synthetic potential in complex natural product total synthesis.

Synthesis of Allene-Containing Apocarotenoids by Cross-Coupling Strategy

The stereocontrolled total synthesis of the allene and carbonyl conjugated apocarotenoids, paracentrone and 19-hexanoyloxyparacentrone 3-acetate, was achieved by sequential cross-coupling reactions using boronic acid ester and iodine- or tin-substituted C5 dienes, which were the building blocks for the elongation of the conjugated polyene systems at both terminals.

A mild preparation of alkynes from alkenyl triflates

We report herein a protocol for preparing alkynes from alkenyl triflates at ambient temperature with LiCl as a promoter.

Total synthesis of alkaloid 205B

Concise and highly stereocontrolled total syntheses of racemic and enantiopure frog alkaloid 205B (1) were accomplished in 11 steps from 4-methoxypyridines 6 and 7 in overall yields of 8 and 8%, respectively. The assembly of the core of the natural product relies on a stereoselective Tsuji-Trost allylic amination reaction and a ring-closing metathesis. The synthesis features the use of an N-acylpyridinium salt reaction to introduce the first stereocenter and an unprecedented trifluoroacetic anhydride-mediated addition of an allylstannane to a vinylogous amide with complete facial selectivity. Deoxygenation of the C4 ketone proved difficult but was accomplished via a modified Barton-McCombie reaction in the presence of a catalytic amount of diphenyl diselenide.

Our recent progress on the intramolecular Diels-Alder reaction approach in natural products synthesis: synthetic studies of the octahydronaphthalene substructure of versipelostatins and the A/B/C-tricyclic substructure of GKK1032s

During this decade, the enantio- and stereoselective synthesis of natural products has been actively explored in the author's laboratory. In this account, the author outlines practical syntheses of the polycyclic substructures of two novel structurally formidable antibiotics, namely, the trans-fused octahydronaphthalene moiety of versipelostatins and the A/B/C-tricyclic decahydrofluorene moiety of GKK1032s. Both syntheses have been achieved with remarkable regio- and stereoselectivity via intramolecular Diels-Alder reactions using well-designed enantiomeric substrates.

Hirsutellones and beyond: figuring out the biological and synthetic logics toward chemical complexity in fungal PKS-NRPS compounds

Covering: up to early 2013. Fungal polyketides and their hybrid non ribosomal peptide derivatives are characterized by often striking structural features and biological activities. Their diversity and their complexity arise from highly organized and programmable biosynthetic pathways and have been challenged by many synthetic chemists. This review will conceptually illustrate how complexity can be generated, starting from a general biosynthetic purpose (the fundaments of PKS-NRPS assembly lines) and finally showing how the particular class of hirsutellone compounds has emerged from such processes in relation to post-elongation and secondary tailoring events. Synthetic efforts to produce these natural products will be described with a special emphasis on complexity-generating strategies and steps. Thus, the biosynthetic and synthetic works will be analyzed in a continuous flow, focusing on both the logic of Nature and organic chemists.

Synthesis of a functionalized 7,6-bicyclic spiroimine ring fragment of the spirolides

The asymmetric synthesis of a functionalized 7,6-spiroimine related to the spirolides is described. Intermolecular Diels-Alder cycloaddition of a chiral trisubstituted dienophile and Danishefsky's diene enabled simultaneous installation of the C7 and C29 stereocenters. Further transformations and late-stage aza-Wittig cyclization afforded the spiroimine in good yield. During this study, an unprecedented 14-membered dialdimine was also obtained.

(1R,6R,13R,18R)-(Z,Z)-1,18-Bis[(4R)-2,2-dimethyl-1,3-dioxolan-4-yl]-3,16-dimethyl-ene-8,20-diaza-dispiro-[5.6.5.6]tetra-cosa-7,19-diene

The crystal structure of the title compound, C₃₄H₅₄N₂O₄, has been solved in order to prove the relative and absolute chirality of the newly-formed stereocentres which were established using an asymmetric Diels–Alder reaction at an earlier stage in the synthesis. This unprecedented stable dialdimine contains a 14-membered ring and was obtained as the minor diastereoisomer in the Diels–Alder reaction. The absolute stereochemistry of the stereocentres of the acetal functionality was known to be R based on the use of a chiral (R)-tris­ubstituted dienophile derived from enanti­opure (S)-glyceraldehyde. The assignment of the configuration in the dienophile and the title di-aldimine differs from (S)-glyceraldehyde due to a change in the priority order of the substituents. The crystal structure establishes the presence of six stereocentres all attributed to be R. The 14-membered ring contains two aldimine bonds [C—N = 1.258 (2) and 1.259 (2) Å]. It adopts a similar conformation to that proposed for trans–trans-cyclo­tetra­deca-1,8-dienes.

Synthesis of delta-tributylstannyl-alpha,beta,gamma,delta-unsaturated aldehydes from pyridines

Zincke aldehydes, which are readily available from the ring-opening reaction of pyridinium salts, are easily converted into delta-tributylstannyl-alpha,beta,gamma,delta-unsaturated aldehydes (stannyldienals) by the action of tributylstannyllithium. This reaction appears to proceed via 1,6-stannyllithium addition/elimination of lithium dialkylamide. Several stannyldienals of significant utility for the synthesis of polyene natural products have been made by this route, which proceeds in modest yields, but is successful on multigram scale using inexpensive reagents. Simple stannylenals and stannylenones are similarly available from the corresponding vinylogous amides.