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Construction of a Heterocyclic Ring

While application of the transition-metal-catalyzed [2 - - 2 - - 2] cycloaddition reaction and its variants for the construction of a benzene unit led to a plethora of natural products with different molecular structures and architectures, its use for the construction of a pyridine moiety within a natural product synthesis is less well developed. The reason for this is uncertain and should not account for the pyridine formation per se the co-cyclization of two alkynes with a nitrile unit to give a pyridine core can be catalyzed efficiently by cobalt, ruthenium, and cationic rhodium complexes, as shown in many methodology-oriented studies [35]. [Pg.226]

Further manipulations of 123 afforded vitamin Be (124). Unfortunately, the key cyclotrimerization was based on a high load of CpCo(CO)2 catalyst and therefore was less efficient economically. However, a recent improvement in this sequence involved the light-promoted cyclization (300 to 800 nm) of a related 3,3 bis-silyl-2-dipropynyl ether and acetonitrile in the presence of a truly catalytic amount (1 mol %) of CpCo(cod) acting as the catalyst [36c], Advantages of this modified protocol [Pg.226]

The CpCo(CO)2 (10 mol %) catalyzed reaction of diyne 125 and cyanomesylate 126 under microwave heating (120 °C) delivered dehydrotylophorine (127) directly in 78% yield. Notably, during the course of this reaction cascade, the nonnucleophilic N-center of a nitrile is converted into a nucleophilic pyridine moiety that subsequently undergoes an intramolecular nucleophilic reaction with the tethered sulfonate leaving group. This facile tandem reaction has also been used in a synthetic approach to the spiroindolinone framework of the marine alkaloids citranin A and B [38]. [Pg.227]

Notably, use of the diyne 139 having a terminal ynamide and an internal alkyne moiety led in a related ruthenium-catalyzed [2 + 2 + 2] cycloaddition with [Pg.228]


The construction of a heterocyclic ring from two reagents, one of which contains a nitro group, is widely used in the synthesis of the nitro derivatives of pyrazole, isoxazole, and 1,2,3-triazole. Thus, for example, the reaction of sodionitromalonal-dehyde with substituted hydrazines leads to the corresponding derivatives of 4-nitropyrazole [33, 61, 471 173] (Scheme 63). [Pg.40]

Bis(tributylstannyl)oxide ° andtrimethylstannyldimethylamide react similarly. A recent example of a more elaborate tinalkoxide, 407 is used in the construction of a heterocyclic ring 408. ... [Pg.80]

The Pauson-Khand reaction provides another new approach to the metal-catalyzed synthesis of heterocycles. This reaction involves the interaction of the multiple bonds of an alkyne with an alkene and carbon monoxide in the presence of dicobalt octacarbonyl (Co2(CO)g), or with just this reagent as a source of CO. The overall process has been described as a [2 -h 2 -h 1] cycloaddition. Only a few applications to heterocyclic synthesis have been reported so far. A 2008 paper that is illustrative of the process describes the use of this reaction for the construction of a heterocyclic ring that is part of an azabicy-clo[3.3.1]nonane derivative. This ring system is present in the alkaloid (-)-alstonerine (4.37), which prompted this study. [Pg.81]

The only report concerning the construction of the heterocyclic ring of diazafluorenes was described in CHEC-11(1996) <1996CHEC-II(7)921> using a variant of the Hantzsch pyridine synthesis. Somewhat surprisingly, no further reports using this method have been found. [Pg.1247]

Under Lewis acid catalysis several imines react with the sterically hindered diene shown in Eq. (73) to form cyclic products. This acts as a useful means of construction of fused heterocyclic ring systems bearing an angular methyl group as a characteristic feature [111]. [Pg.424]

This chapter shows how just a few general principles allow one to understand the methods which are used in the construction of the heterocyclic ring of an aromatic heterocyclic compound from precursors that do not have that ring. It discusses the principles, and analyses the types of reaction frequently used in constructing an aromatic heterocycle, and also the way in which appropriate functional groups are placed in the reactants, in order to achieve the desired ring synthesis. [Pg.107]

The construction of two heterocyclic rings in one S)mthetic step has been developed for the preparation of coumarin derivatives. In this process, the thiazole ring (31 - 40) is accomplished by Flantzsch reaction monitored by fabrication of pyrazole by reacting a 3-(2-bromoacetyl) coumarin with thiosemicarbazide and acetylacetone at room temperature [87]. [Pg.13]

Pyrimidines used to be prepared via the construction of the heterocyclic ring as shown in Fig. 35. A 4-substituted-cyano-benzene is subjected to treatment with hydrogen chloride gas in an ethanolic solution, after the reaction is complete ammonia is bubbled into the solution to yield the ami-no-imine, which in turn is reacted with a suitably substituted phenyl diethyl malo-nate to give the dihydroxypyrimidine. The hydroxyl groups are removed by chlorination with phosphorus oxychloride followed by hydrogenation over palladium. The first part of this synthetic pathway is relatively straightforward, but the latter stages can result in poor yields if not performed correctly. [Pg.1411]

Intramolecular cyclizations of ort/io-functionaUzed 0, 0-difluorostyrenes provide a general access to ring-lluorinated heterocycles. In this methodology, both the construction of a heterocyclic nucleus and the introduction of a fluorine substituent are simultaneously effected. [Pg.194]

The axially chiral catalyst 26 was prepared by construction of the heterocyclic ring via a cobalt-catalyzed [2+2-I-2] cycloaddition of diyne 52 and the enantiopure. [Pg.391]

This survey has shown the usefulness of Pd-catalyzed oxycarbonylation in numerous natural product syntheses. In many cases, the chemo-, regio-, and stereoselectivities of Pd-mediated reactions have succeeded in building a complex structure. Construction of oxa-heterocyclic rings has been realized under mild reaction conditions and in the presence of several functional groups. [Pg.437]

This ring system can be constructed by building one of the two heterocyclic rings on the other preformed ring the construction of the thiazeto ring on a quinoline moiety or generating a pyridine ring on a... [Pg.77]

Many versatile approaches to the construction of fused heterocyclic systems (6 5 6) with ring junction heteroatoms have been reported. More general reactions which can be used for synthesis of derivatives of several tricyclic systems, and transformations which have potential for use in the preparation of a series of substituted compounds, are discussed in this section. Formation of the five-membered ring is presented first because it is a conceptually simple approach. It should be noted, however, that the addition of a fused six-membered ring to a bicyclic component offers much more versatility in the construction of a (6 5 6) system. Each subsection below starts with intramolecular cyclization of an isolated intermediate product. Reactions which follow are one-pot intermolecular cyclizations. [Pg.990]

Another approach to the construction of the central ring is an intramolecular cyclization of a substrate composed of two six-membered heterocycles and bridged by a one-atom unit. This method has been applied to the synthesis of derivatives of 3, 6, 12, and 13 C1995BML163, 1996JHC1147, 2001JHC1173, 2002T5733>. [Pg.1017]

The construction of the central ring can also be accomplished by a radical cyclization of a non-fused heterobiaryl system composed of two six-membered heterocycles and linked by a two-atom bridge. An example is the synthesis of 106 from the bromo derivative 105 in the presence of AIBN and Bu3SnH (Equation 8) <1999TL4073>. [Pg.1020]

N-Methyl-2-phenyl-A2-tetraJiydropyridine and similar compounds have previously been prepared by the hydrolysis and decarboxylation of a-benzoyl-N-methyl-2-piperidone3 and by the addition of phenyl Grignard reagents to N-methyl-2-piperidone followed by dehydration.4 Both of these methods require that a heterocyclic ring already be present in the system. In contrast, this procedure offers a new flexible route to the construction of five- or six-membered heterocyclic rings which may easily be incorporated into a larger polycyclic product. Several examples of this process that can be cited to demonstrate this utility are... [Pg.48]

A powerful method to convert non-conjugated keto olefins into cyclic alcohols [46,47] utilizes the cathodic reduction of a ketone to afford a ketyl, which subsequently undergoes cyclization onto the pendant alkene. As illustrated in Eqs. (27H30), the process provides access to mono-, bi-, and heterocyclic systems from very simple starting materials. It is especially well-suited to the construction of five-membered rings, less so for six-, and is not effective in producing seven-membered rings. [Pg.18]

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. [Pg.43]


See other pages where Construction of a Heterocyclic Ring is mentioned: [Pg.226]    [Pg.227]    [Pg.229]    [Pg.226]    [Pg.227]    [Pg.229]    [Pg.382]    [Pg.73]    [Pg.1]    [Pg.888]    [Pg.287]    [Pg.98]    [Pg.259]    [Pg.321]    [Pg.133]    [Pg.347]    [Pg.321]    [Pg.74]    [Pg.133]    [Pg.394]    [Pg.1]    [Pg.241]    [Pg.2]    [Pg.14]    [Pg.168]    [Pg.164]    [Pg.109]    [Pg.733]    [Pg.2]    [Pg.225]    [Pg.23]    [Pg.429]   


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