Novel ipso-Substitution of p-Sulfinylphenols through the Pummerer-Type Reaction: A Selective and Efficient Synthesis of p-Quinones and Protected p-Dihydroquinones

Nucleophilic functionalizations of indole derivatives using the aromatic Pummerer reaction

Because of the electron-rich property of indoles, direct functionalization strategies towards indoles generally involve electrophilic substitutions. In this paper, an efficient protocol for nucleophilic hydroxylation, halogenation and esterification of indoles via the aromatic Pummerer process was developed. With the advantages of readily accessible starting materials, simple operation and mild conditions, this protocol should be of interest to synthetic scientists.

Aromatic Pummerer reaction for the remote para- or ortho-benzyl nucleophilic functionalization

The classic Pummerer reaction involves an α-substituted sulfide via an elimination/addition of a thionium ion. In this paper, we reported a remote para- or ortho-benzyl nucleophilic functionalization using an aromatic Pummerer process. A plausible mechanism involving a quinone thionium intermediate was proposed to explain this reaction.

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.

Highly regioselective nucleophilic carbon-carbon bond formation on furans and thiophenes initiated by pummerer-type reaction

[reaction: see text] The reactions of (phenylsulfinyl)furans or -thiophenes with carbon nucleophiles in the presence of trifluoroacetic anhydride allowed the nucleophilic installation of carbon functional groups on the furan and thiophene nuclei with complete regioselectivity.

Asymmetric Total Synthesis of Fredericamycin A: An Intramolecular Cycloaddition Pathway

The asymmetric total synthesis of the potent antitumor antibiotic fredericamycin A ((S)-1) was achieved by the intramolecular [4+2] cycloaddition of the silylene-protected styrene derivative (S)-7 followed by the aromatic Pummerer-type reaction of the sulfoxide (S)-5. Although we had already succeeded in the total synthesis of racemic 1 by the same approach, synthesis of its asymmetric version was more complicated than we had expected due to the difficulties involved in constructing the quaternary carbon center and the tendency of this center to undergo facile racemization. Racemization of this center during the installation of the acetylene moiety on the dione (R)-8 was the most serious aspect. Systematic studies of its DE-ring analogue (R)-25 revealed that racemization of the quaternary carbon center proceeded by a retro-aldol-aldol reaction of the initial adduct, (1R)-39 a-Li, and that the degree of racemization was dependent on the reaction temperature. The racemization process could be completely depressed by keeping the reaction temperature at -78 degrees C. The construction of the stereogenic quaternary carbon center was achieved by the lipase-catalyzed desymmetrization of the prochiral 1,3-diol 9 a bearing the DEF-ring moiety. These studies enabled us to attain the asymmetric total synthesis of (S)-1 while completely retaining the chiral integrity created by the enzymatic reactions.

1-alkoxyvinyl esters: renaissance of half-century-old acyl donors with potential applicability

Among the various kinds of acyl donors, the 1-alkoxyvinyl esters have characteristic features, such as a high reactivity under nearly neutral conditions and the generation of neutral and volatile esters as single coproducts. Although their use in organic syntheses began in the middle of the 1950s, no significant progress has been seen. This is probably because the existing method of preparing alkoxyvinyl esters used toxic mercuric salts and was not totally applicable for those esters having functionalized acyl moieties. We have discovered that the use of a catalytic amount of the less toxic [RuCl2(p-cymene)]2 effectively accelerates the addition of carboxylic acids to ethoxyacetylene to give ethoxyvinyl esters bearing a variety of functionalized acyl groups in high yields. This discovery has opened a new avenue for developing new reactions and new synthetic methodologies based on the design and use of these acyl donors with suitable functional groups. Such examples include (i) the installation of hydrophilic acyl moieties on biologically active compounds, (ii) asymmetric Pummerer-type reactions, (iii) aromatic Pummerer-type reactions, (iv) the lipase-catalyzed desymmetrization of symmetrical 1,3-diols, and (v) lipase-catalyzed domino reactions. Future possibilities for these acyl donors are also discussed.

[Development of novel asymmetric reactions oriented to next-generation enzymatic organic syntheses]

Enzyme-catalyzed organic syntheses have enormous potential in the development of environmentally benign processes. In the last two decades, increased efforts have been devoted to making this a reality. However, the utility of the enzymes is generally limited. For example, although the lipases are extensively used for the kinetic resolution or the desymmetrization of alcohols and carboxylic acid derivatives, little is known about their ability to catalyze carbon-carbon bond-forming reactions. In the past several years, we have been engaged in exploiting a next-generation enzymatic synthesis by using the lipases for the construction of carbon skeletons. This review article describes the results. First, the development of a new type of acyl donor, 1-ethoxyvinyl esters (EVEs), that have significant advantages over common acyl donors (e.g., vinyl esters) for lipase-catalyzed esterification reactions. Second, the highly enantioselective desymmetrization of prochiral 1,3-propanediols and meso 1,2-diols using 1-ethoxyvinyl 2-furoate. The application of this technology for the asymmetric syntheses of fredericamycin A analogues and the oxindoles with a chiral, nonracemic quaternary carbon center has been demonstrated. Third, the first lipase-catalyzed domino reaction using EVEs possessing a suitably functionalized acyl moiety is described. The acyl moiety installed during the enzymatic kinetic resolution was used as a part of the constituent structure for the subsequent Diels--Alder reaction to product optically pure tricyclic compounds with five chiral carbon centers via a one-pot operation. The potential influence of the lipase on the Diets--Alder reaction and the domino process accompanied by the dynamic kinetic resolution are also described.

[Development of novel synthetic organic reactions: synthesis of antitumor natural products and leading compounds for new pharmaceuticals]

Biologically active natural products with unique, highly complex molecular skeletons have been used as leading compounds for raw materials of new drugs. Due to the limitations of natural supply, highly efficient, large-scale syntheses and molecular design have been sought in drug discovery. For this purpose, we have focused on a synthetic strategy effective in developing novel reactions and reagents and found several useful regio- and stereospecific reactions, contributing to the synthesis of otherwise unattainable target molecules. The application of these reactions for the total synthesis of three types of potent cytotoxic natural products for the first time is described in this paper. The basic concept is first described. Then the total synthesis of anthracyclines, fredericamycin A, and discorhabdins is reported. Novel reactions using hypervalent iodine reagents under environmentally benign conditions are also described. The future prospects for this method are discussed.

Enantiodivergent synthesis of either enantiomer of ABCDE-ring analogue of antitumor antibiotic fredericamycin A via intramolecular [4 + 2] cycloaddition approach

[reaction: see text] An intramolecular enantiodivergent synthesis of both enantiomers of the ABCDE-ring analogue 22 of fredericamycin A is reported. Key steps involve an intramolecular [4 + 2] cycloaddition of 17 and an aromatic Pummerer-type reaction of 19. A lipase-catalyzed enantioselective desymmetrization of prochiral diol 2 using 1-ethoxyvinyl 2-furoate 3 led to the pivotal intermediate (R)-4.

Regioselective nucleophilic addition of methoxybenzene derivatives to the beta-carbon of p-benzoquinone mono O,S-acetal

Regioselective nucleophilic addition of electron rich aromatics to the beta-position of acetal carbon of p-benzoquinone mono O,S-acetal was achieved by modifying the acetal moiety.

Facile and efficient sulfenylation method using quinone mono-O,S-acetals under mild conditions

A novel sulfenylation method induced by aromatization of quinone mono-O,S-acetals is described. These sulfenylation reagents readily react with silyl enolethers or electron rich aromatic compounds to give sulfenylation products under mild conditions. In particular, O,S-acetal 2j, which possesses a pentafluorophenylthio function, is the most effective reagent from the standpoint of the adaptability for various substrates.

Ambident effect of a p-sulfinyl group for the introduction of two carbon substituents to phenol rings: a convergent synthesis of diverse benzofuran neolignans

A convergent synthesis of diversely substituted benzofuran neolignans (8) is described employing a single p-sulfinyl group on the phenols (3) as an ambident functional group for two types of carbon-carbon bond-forming reactions: (i) the direct synthesis of the dihydrobenzofuran skeletons through an aromatic Pummerer-type reaction and (ii) the ipso-substitution of the sulfur functional group by carbon substituents through a ligand exchange reaction.

Remote Pummerer Reaction via Intermolecular Through-Space Interaction between Sulfonium and Sulfenyl Sulfur Atoms

The remote Pummerer reaction of the mono-sulfoxide of p-bis(methylthio)-aromatic 1 is described. The reaction of 1 with (CF(3)CO)(2)O in CH(2)Cl(2) gave a mixture of the corresponding mono-Pummerer product 2, the bis-Pummerer product 3, and the bis-sulfide 4 in an n:1:1 ratio (n >/= 2). The 1:1 formation of 3 and 4 indicates an intermolecular interaction between sulfur atoms. The reaction with the deuterium-labeled 1-d(3)() showed that the formation of 2 occurs at the sulfenyl as well as sulfinyl groups in 1a-d(3)()-1d-d(3)(), in which the product ratio of 2-d(3)():2-d(2)() is in the range of 3.5-10. On the other hand, the Pummerer reaction of 1e-d(3)() occurred preferably at the sulfinyl group in a usual manner. The reaction of a 1:1 mixture of the bis-sulfide 4 and the bis-sulfoxide 5 with (CF(3)CO)(2)O also gave 2, 3, and 4 in a ratio similar to that for the reaction of 1. The mechanism in the present remote Pummerer reactions is discussed in light of an intermolecular through-space interaction between the sulfonium and sulfenyl sulfur atoms, and a dithia dication dimer B and/or a bis(dithia dication) cyclic dimer C are proposed as intermediates.