The novel and efficient direct synthesis of N,O-acetal compounds using a hypervalent iodine(III) reagent: an improved synthetic method for a key intermediate of discorhabdins

Unified Divergent Total Synthesis of Discorhabdin B, H, K, and Aleutianamine via the Late-Stage Oxidative N,S-Acetal Formation

This study achieved the total syntheses of (+)-discorhabdin B, (-)-discorhabdin H, (+)-discorhabdin K, and (-)-aleutianamine. A phenethylamine fragment bearing a o-pivaloylthio group, corresponding to the D/E/G ring moiety, was prepared from benzothiophen-2-carboxylic acid methyl ester and condensed with a known pyrroloiminoquinone derivative. The adduct was subjected to [bis(trifluoroacetoxy)iodo]benzene (PIFA)-promoted oxidative spirocyclization to furnish the A/B/C/D/E spirocyclohexadienone fused with pyrroloiminoquinone. The total synthesis of (±)-discorhabdin B was completed via the key construction of the highly strained G ring with the N,S-acetal moiety featuring a newly developed CuBr2-mediated oxidative cascade cyclization. The stereocontrolled total synthesis of (+)-discorhabdin B was accomplished by a diastereoselective PIFA-promoted oxidative spirocyclization using a chiral thioester. (-)-Disocrhabdin H and (+)-discorhabdin K were synthesized by the site- and face-selective thia-Michael addition of l-ovothiol A to (+)-N-Ts-discorhabdin B with the concomitant formation of the F ring by forming the C2-N18 bond. The total synthesis of (-)-aleutianamine was achieved via a skeletal rearrangement initiated by the Luche reduction of the dienone moiety of (+)-N-Ts-discorhabdin B.

The role of proton shuttling mechanisms in solvent-free and catalyst-free acetalization reactions of imines

Catalyst-free and solvent-free reactions of the type NuH + E → Nu–EH are NuH-catalyzed processes in which Grotthuss-like proton shuttling pays a key role.

XtalFluor-E® mediated proto-functionalization of N-vinyl amides: access to N-acetyl N,O-acetals

XtalFluor-E® has been extensively used in a broad range of reactions in the past few years. Here we report its use with protic nucleophiles in a catalytic manner for the in situ generation of protons that lead to the proto-functionalization of activated olefins. Utilizing the latter protocol, proto etherification of enamides gives rise to N,O-acetals in nearly quantitative yields.

Redox-neutral α-oxygenation of amines: reaction development and elucidation of the mechanism

Cyclic secondary amines and 2-hydroxybenzaldehydes or related ketones react to furnish benzo[e][1,3]oxazine structures in generally good yields. This overall redox-neutral amine α-C-H functionalization features a combined reductive N-alkylation/oxidative α-functionalization and is catalyzed by acetic acid. In contrast to previous reports, no external oxidants or metal catalysts are required. Reactions performed under modified conditions lead to an apparent reductive amination and the formation of o-hydroxybenzylamines in a process that involves the oxidation of a second equivalent of amine. A detailed computational study employing density functional theory compares different mechanistic pathways and is used to explain the observed experimental findings. Furthermore, these results also reveal the origin of the catalytic efficiency of acetic acid in these transformations.

Marine pyrroloiminoquinone alkaloids

Recent reports on the synthetic studies of marine pyrroloiminoquinone alakloids and their analogs are reviewed.

Hypervalent iodine(III)-induced methylene acetoxylation of 3-oxo-N-substituted butanamides

1-Carbamoyl-2-oxopropyl acetate derivatives were synthesized through an acetoxylation process to methylene with the aid of (diacetoxyiodo)benzene (DIB) as the oxidant. Not only mild reaction conditions, but also excellent yields and good substrate scope make the present protocol potentially useful in organic synthesis.

Intramolecular Hydroamination of Aminoalkenes using Rhodium(I) and Iridium(I) Complexes with N,N- and P,N-Donor Ligands

Cationic rhodium(i) and iridium(i) complexes containing the bidentate heterocyclic N,N-donor ligand bis(1-pyrazolyl)methane (bpm) and the counterion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF–) were found to be efficient catalysts for the cyclization of activated and unactivated aminoalkenes. The rates of cyclization were found to be highly dependent upon the size and steric bulk of the substituents at the γ-position of the aminoalkene, with large steric bulk leading to faster rates of cyclization. In comparison, analogous rhodium(i) and iridium(i) complexes with the mixed P,N-donor ligand 1-[2-(diphenylphosphino)ethyl]pyrazole (PyP) were found to be less effective as catalysts for this reaction.

Synthesis of the marine pyrroloiminoquinone alkaloids, discorhabdins

Many natural products with biologically interesting structures have been isolated from marine animals and plants such as sponges, corals, worms, etc. Some of them are discorhabdin alkaloids. The discorhabdin alkaloids (discorhabdin A-X), isolated from marine sponges, have a unique structure with azacarbocyclic spirocyclohexanone and pyrroloiminoquinone units. Due to their prominent potent antitumor activity, discorhabdins have attracted considerable attention. Many studies have been reported toward the synthesis of discorhabdins. We have accomplished the first total synthesis of discorhabdin A (1), having the strongest activity in vitro among discorhabdins in 2003. In 2009, we have also accomplished the first total synthesis of prianosin B (2), having the 16,17-dehydropyrroloiminoquinone moiety, by a novel dehydrogenation reaction with a catalytic amount of NaN(3). These synthetic studies, as well as syntheses of the discorhabdins by various chemists to-date, are reviewed here.

Synthesis of antitumor marine alkaloid discorhabdin A oxa analogues

Discorhabdin A (1) exhibits a strong cytotoxic activity in vitro, but it is difficult to synthesize and handle due to the instability of its highly strained N,S-acetal structure. We then designed the analogues of discorhabdin A which also have strong cytotoxic activity and stability. The synthesis and examination of the biological activity of various types of stable discorhabdin A oxa analogues (2) were achieved.

Facile and efficient synthesis of lactols by a domino reaction of 2,3-epoxy alcohols with a hypervalent iodine(III) reagent and its application to the synthesis of lactones and the asymmetric synthesis of (+)-tanikolide

The domino reaction of 2,3-epoxy-1-alcohol derivatives, namely tetrasubstituted 2,3-epoxy-1-alcohols and 2- or 3-alkyl trisubstituted 2,3-epoxy-1-alcohols, with PhI(OCOCF(3))(2) in the presence of H(2)O is described in detail. In this reaction, several types of lactol derivatives can be directly obtained from the 2,3-epoxy-1-alcohol derivatives in a single operation. The obtained lactols were successively converted into the corresponding lactones. This reaction is applicable to the construction of optically active lactone compounds. The asymmetric total synthesis of (+)-tanikolide, an antifungal marine natural product, has been effectively achieved by using this reaction.

The efficient direct synthesis of N,O-acetal compounds as key intermediates of discorhabdin A: oxidative fragmentation reaction of alpha-amino acids or beta-amino alcohols by using hypervalent iodine(III) reagents

Hypervalent iodine(III) reagents are readily available, easy to handle, and have a low toxicity and similar reactivities to those of heavy metal reagents, and hence they are used for various oxidative reactions. The oxidative cleavage of alkynes or carbonyl compounds by using bis(trifluoroacetoxy)iodo(III) pentafluorobenzene (C(6)F(5)I(OCOCF(3))(2)) has been reported. Herein, the efficient direct synthesis of N,O-acetal compounds as key intermediates of discorhabdin A, by the oxidative fragmentation reaction of alpha-amino acids or beta-amino alcohols by using C(6)F(5)I(OCOCF(3))(2), is described.

Enantioselective constructions of quaternary carbons and their application to the asymmetric total syntheses of fredericamycin A and discorhabdin A

An efficient enantioselective construction of quaternary carbons including spiro carbons is an area of intense interest due to the importance of these units as components of biologically active natural products. Prominent methods are presented for the synthesis of chiral, nonracemic quaternary carbon centers by (i) stereospecific rearrangement of optically active epoxides, (ii) enzyme-catalyzed resolution, and (iii) hypervalent iodine reagent-induced ipso-substitution of para-substituted phenol derivatives. These methods were applied to the total syntheses of fredericamycin A and discorhabdin A.