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Intermolecular carbonylative cycloaddition reactions

Scheme 1.2 Ruthenium-catalyzed intermolecular carbonylative cycloaddition reaction. Scheme 1.2 Ruthenium-catalyzed intermolecular carbonylative cycloaddition reaction.
The ability to produce 1,3-dipoles, through the rhodium-catalyzed decomposition of diazo carbonyl compounds, provides unique opportunities for the accomplishment of a variety of cycloaddition reactions, in both an intra- and intermolecular sense. These transformations are often highly regio- and diastereoselective, making them extremely powerful tools for synthetic chemistry. This is exemplified in the number of applications of this chemistry to the construction of heterocyclic and natural-product ring systems. Future developments are likely to focus on the enantioselective and combinatorial variants of these reactions. [Pg.450]

The in situ formation of nitrones from oximes by 1,3-APT or 1,2-prototropy is clearly a powerful synthetic strategy but conventional nitrone generation, in particular hydroxylamine-carbonyl condensation, has been applied to numerous syntheses, in intra- and intermolecular mode (258). Accordingly, the ring systems similar to those synthesized using 1,3-APT/intramolecular nitrone-alkene cycloaddition (INAC) methodology by Heaney (313-315) (see Section 1.11.2) or Padwa and Norman (340) have been made using conventionally prepared nitrones (Scheme 1.67). As with the previous examples, once formed, the nitrones are suitably placed for a spontaneous intramolecular cycloaddition reaction with the... [Pg.55]

Hashimoto and co-workers (139) further looked at an intermolecular carbonyl ylide cycloaddition screening several different chiral rhodium catalysts. The Hashimoto group chose to study phthaloyl amino acid derivatives for enantiocon-trol of the cycloaddition reactions (Fig. 4.8). Using fluorinated or ethereal solvents with the phthaloyl catalysts gave ee ratios of 20-69%. [Pg.300]

Carbonyl ylides possess versatile reactivities, among which the 1,3-dipolar cycloaddition is the most common and important reaction. The reaction sequence of ylide formation and then 1,3-dipolar cycloaddition can occur in either inter- or intramolecular manner. When the reaction occurs intermolecularly, the overall reaction is a one-pot three-eomponent process leading to oxygen-containing five-membered cyclic compounds, as demonstrated by the example shown in Scheme 8. A mixture of diazo ester 64, benzaldehyde, and dimethyl maleate, upon heating to reflux in CH2CI2 in the presence of 1 mol% rhodium(ii) perfluorobutyrate [Rh2(pfb)4], yields tetrahedrofuran derivative 65 in 49% yield as single diastereomer. " ... [Pg.159]

The intermolecular Diels-Alder reaction between the dibromoenone (111) and dienes (112) provides access to bicyclo[5.4.0]undecane systems (113) that are common core structures of many natural products (Scheme 32).118 The alio-threonine-derived O-(/ -biphenyl carbonyl oxy)-/i-phenyloxazaborolidi none catalyses the enan-tioselective Diels-Alder reaction of acyclic enones with dienes.119 The reversal of facial selectivity in the Diels-Alder cycloaddition of a semicyclic diene with a bro-moenone was induced by the presence of the bromo substituent in the dienophile.120 Mixed Lewis acid catalyst (AlBr3/AIMe3) catalyses the Diels-Alder reaction of hindered silyloxydienes with substituted enones to produce highly substituted cyclohexenes.121 Chiral /V-enoyl sultams have been used as chiral auxiliaries in the asymmetric Diels-Alder reactions with cyclopentadiene.122... [Pg.370]

Electronically excited carbonyl compounds serve as versatile intermediates in countless reactions. They not only operate as reactive substrates for intra- and intermolecular hydrogen abstraction and cycloaddition reactions as well as C-C cleavage steps, but many of them also are useful sensitizers for the generation of triplet excited compounds. This variety of reaction possibilities makes carbonyl photochemistry sometimes very complex,... [Pg.11]

A review of the methods for the generation of cyclic carbonyl ylides from intramolecular carbene additions has recently appeared [64]. This intermediate was first exploited as the An component for cycloaddition reactions by Ibata [65]. ort/io-Disubstituted carboalkoxy aryl diazoketones such as 54 were decomposed by copper complexes, generating six-membered ring carbonyl ylides. These transient intermediates underwent subsequent intermolecular cycloadditions in the presence of ethylenic and acetylenic reagents to give predominantly exo products containing the oxabicyclo[3.2.1] nucleus, Eq. 38. [Pg.18]

Perhaps the most successful approach for promoting the 47r participation of 1-oxabutadiene systems in intermolecular Diels-Alder reactions employs a,/8-unsaturated carbonyl compounds substituted with an additional C-3 (a) electron-withdrawing group. The addition of the C-3 electron-withdrawing substituent increases the electron-deficient character of the oxabutadiene system, decreases the LUM0oxabutadiene, and as expected, enhances the observed [4 + 2] cycloaddition rate and regioselec-... [Pg.275]

Recently, Dong and Sun disclosed an unprecedented intermolecular asymmetric a-aldol reaction of vinylogous NHC-enolates, a type of versatile but less explored species relative to simple NHC-enolates. In contrast to the known C—C bond formation at the y-position of vinylogous NHC-enolates, this reaction exhibits complete a-selectivity. Unlike most cycloaddition reactions of NHC-enolates with external carbonyl electrophiles, this reaction does not involve a cycloaddition step. Notably, two challengingstereocenters, one quaternary and the other labile tertiary (both allylic and a-carbonyl), are also established in an acyclic product with excellent stereocontrol. A range of highly enantioenriched p,y-unsaturated P -fluoroalkylated esters have been synthesized with high efficiency under mild conditions. These products can be easily transformed into other useful molecules, such as densely functionalized tetrahydrofurans (Scheme 7.90). [Pg.334]

It is generally accepted that a typical carbonyl yhde reaction proceeds as shown in Fig. 2. Interaction of diazo compound 1 with the metal forms diazonium complex 2, which then extrudes nitrogen forming carbenoid intermediate 3. Reaction of 3 with the carbonyl group present in the substrate forms intramolecular carbonyl yhde 4 (or an intermolecular carbonyl yhde) in which the metal catalyst may or may not remain associated with the ylide [13]. Finally, the [3+2]-cycloaddition and regeneration of the active cat-... [Pg.159]

The synthesis of biologically important trifluoromethyl-substituted diox-olanes has been accomplished via 1,3-dipolar cycloaddition reaction of an intermolecularly generated carbonyl yhde with an aryl aldehyde. For example, the reaction of methyl diazo(trifluoromethyl)acetate (25) with two equivalents of aryl aldehydes afforded dioxolanes 27,28 [71]. The diastereoselectivity of these reactions depends on the substituent present on the aryl aldehyde (Scheme 8). [Pg.163]

It is also worthwhile comparing the intramolecular photochemical cycloaddition reactions of ethylenic aldehydes and ketones with free radical intramolecular additions. For instance, irradiation of 5-hexen-2-one (470) (Scheme 161) in the gas phase gives the oxetane 471 as only cyclized product, as expected from the known photochemical intermolecular reaction between olefins and ketones. If the irradiation is conducted in solution 470 gives 471 (26%) and 472 (18%). With other y,< -unsaturated ketones, the bicyclic compound analogous to 472 may become the major product. With 2-allylcyclanones such as 473 (Scheme 161) bicyclic compounds are obtained (80% yield) as a mixture of 474 and 475, with 475 being the major product, but such compounds are difficult to isolate. " In the same manner, selective irradiation of the carbonyl group of 2-acyl-2,3-dihydro-4/f-pyrans (476) leads exclusively (23% yield) to exo-brevicomin (477) (a sex attractant), neither oxetane formation nor Norrish type II reaction being observed. The formation of the compounds 472, 475, and 477 which was considered as unexpected... [Pg.265]

Zhang and co vorkers reported a number of cycloaddition reactions, somewhat related to enyne cycloisomerization, in which a gold-mediated cascade was initiated by the addition of a carbonyl moiety onto an alkyne. Furans and thiophenes were also found to react with alkynes in an intra- or intermolecular manner in the presence of an NHC-gold catalyst to generate complex structures. [Pg.463]

As was mentioned in Section 13.2, the [27t + 27i] photocycloaddition of alkenes is an allowed reaction according to orbital symmetry considerations. Among the most useful reactions in this categoty, from a synthetic point of view, are intramolecular [27t + 2ti] cycloadditions of dienes and intermolecular [2ti + 2ti] cycloadditions of alkenes with cyclic a, -unsaturated carbonyl compounds. These reactions will be discussed in more detail in Section 6.4 of Part B. [Pg.771]


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See also in sourсe #XX -- [ Pg.8 ]




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