Ylide compounds five-membered rings

Padwa etal. utilized the ammonium ylide [1,2]-shift in the synthesis of tetrahydroisoquinoline and benzazepine fused with a five-membered ring, a structure found in a cephalotaxine family. When diazo ester 172 is treated with a catalytic amount of Cu(acac)2 in refluxing toluene, 5,7-fused compound 174 is isolated in 73% (Equation (25)). Again, use of Rh2(OAc)4 results in slow reaction and eventually gives a complex mixture of the products after a prolonged reaction time. 

Cycloadditions 1,3-dipoles readily react with the (6,6)-double bond of C o that acts as a highly reactive dipolarophile. Five-membered rings are generated on the fullerene s surface this way, and these can bear a multitude of functional groups. Suitable 1,3-dipoles include, for instance, nitrile oxide, azomethine ylides, alkylazides, diazo compounds or trimethylenemethane (Figure 2.60). 

The chemistry of the cationic [3,2-u] fused compounds is dominated by nucleophile-induced cleavage of the five-membered ring. In the first edition this was clearly documented for the 2,3-dihydrooxazolo[3,2-a]pyridinium system, and has since been shown to occur with the 5,6,7,8-tetrahydro cation (132) on reaction with certain hard nucleophiles (Scheme 26) <92JOC3070>. The involvement of the azomethine ylide (133) was demonstrated by trapping experiments. The 2,3-dihydro-5-one (134) was similarly cleaved on reaction with nucleophiles <84IJC(B)18>. Use of amines resulted in reclosure to imidazo[l,2-u]pyrid-5-ones <84IJC(B)24>. 

On the basis of the 1,3-dipolar cycloaddition of 5-membered ring carbonyl ylides, syntheses of oxabicyclo[2.2.1]heptan-2-one ring systems were successfully accompUshed. For example, the reactions of the cyclopropyl-substituted five-membered ring carbonyl ylide 48 derived from the a-diazo ketone 47 with different dipolarophiles have been investigated [78-80]. The compound 47 undergoes cycloaddition in the presence of Rh2(OAc)4 with dimethyl maleate, dimethyl fumarate, cyclopentenone, 1,1-dimethoxyethylene and bi-cyclopropylidene furnishing the expected cycloadducts 49-53, respectively... 

Synthesis of endo spiro-oxabicyclo[3.2.l]octan-2-ones 85 has been reported based on the reaction of the six-membered ring carbonyl ylide generated from the diazo ketone 80 with cr-methylene ketones (Scheme 25) [88]. Importantly, the reverse stereoselectivity was observed between the reactions involving five-membered ring carbonyl ylide intermediates and 66, which led to exo spiro compounds (see Scheme 19). 

Rhodium-generated bicyclic six-membered ring carbonyl ylides from the diazo ketone 158 with p-quinones have also been studied to yield interesting oxygen heterocycles [133]. In line with the five-membered ring carbonyl ylide reactions (see Sect. 2.1), the a-diazocarbonyl compound 158 furnished oxygen-rich heterocyclic systems 161-163 (Scheme 52). 

Targets with one Heteroatom Five-membered heterocycles with one heteroatom, such as pyrrolidine and tetrahydrofiiran skeletons, exist widely in numerous natural products and bioactive compounds. Therefore, intense efforts have been devoted to the synthesis of these five-membered ring systems. Among the various methods existing for the synthesis of chiral pyrrolidine and proline derivatives, few can match the synthetic potential of 1,3-DC reactions of azomethine ylides with alkenes [3]. Generally, azomethine yhdes are unstable species, so they are normally generated in situ and trapped by unsaturated bonds (Scheme 2.1) [2b]. 

Five-membered Rings.—Pyrrolidines constitute an important class of compounds, and much synthetic effort is devoted to them each year. Cycloaddition reactions figure prominently among the many methods that are available for their preparation, and the number has increased this year. Trimethylsilyl triflate is an efficient catalyst for C-C-bond-forming reactions, and has now been shown to catalyse 1,3-dipolar cycloadditions of the ylide (94) with olefinic and acetylenic dipolarophiles, to give pyrrolidines. The geometry of the products depends... 

The carbonyl-substituted ylides (16) and (17), derived from 1,3- and 1,4-dithians, are crystalline compounds which give substituted cyclopropanes stereoselectively with P-unsaturated carbonyl compounds (Scheme 2). The five-membered-ring analogue (18) may also be used. °... 

The reaction of SiF-coupled six-membered rings with lithium organyls first produces ring contraction and subsequently eliminates LiF. As discussed in Section II, we have now obtained an intermediate ylide that contains silicon atoms with the coordination numbers three and five. This gives rise to a nucleophilic migration of a methanide ion. A fused bicyclic compound is formed (Scheme 24). This compound has also been characterized by an X-ray analysis and the structure is shown in Fig. 13. Both the four-membered rings are planar.42,46,50... 

Heterocyclic Synthesis. - The reactions of phosphorus ylides with phenan-threne-9,10-quinone (113) have been used to prepare phenanthrene [9,10-x]-fused compounds with four, five, and six membered heterocyclic rings. (E)-4-carbethoxymethylene-l,2,3,4-tetrahydro-2-quinolones 114 have been obtained from the stereoselective reaction of 3-hydroxy-1,2,3,4-tetrahydroquinoline-2,4-diones and ethyl(triphenylphosphoranylidene)acetate. A -trifluoroacetylanilines 115 react with Ph3P=C02Et producing enamine derivatives 116 as a mixture of (E)- and (Z)-isomers. Enamines 116 are useful precursors for the synthesis of indoles and quinolones. 

Desmotropy of some five-, six-, and seven-membered heterocyclic rings and of some important pharmaceutical compounds and tautomeric equilibria of heterocyclic carbenes with mesoionic compounds and ylides are described. 

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