Five-membered ring compounds

Five-membered rings. Most five-membered rings arise from the 1,3-dipolar-ophile behaviour of thioketones and studies in this field have been carried out mainly by Huisgen, Mloston, Heimgartner, and their co-workers. An exemplified thioketone reactivity to form five-membered rings with several classes of compounds is shown in Table 2. 

Ring stacking, see Organolithium compounds, ring stacking Ring systems five-membered... 

Photochemical or thermal extrusion of molecular nitrogen from a-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish five-membered heterocycles which... 

This time the spiro-bicyclic diketone (one C atom common to both rings) is preferred to the alternative bridged bicyclic compound because both rings are five-membered. 

A new amorphane sesquiterpene y-lactol derivative, O-methylbaccharocephol, Fig. (18) was isolated from Baccharis sphaerocephala B N [49], This compound possesses a very rare type of carbon skeleton, with two ci.v-fused six-membered rings, a five-membered-ring y-lactone and an epoxide. Chemical analysis of Baccharis salicina Torr Gray afforded two new guaianolides, bacchariolides A, Fig. (19) and B, Fig. (20) [50],... 

Fused-ring bicyclic compounds have been formed by transannular attack of an amino group at phosphorus either to displace a phosphorus substituent, for example, formation of the salt (53), or by valence expansion to produce phosphorane (68). In the latter case the five-coordinate metalloid atom needs to be stabilized by the presence of electronegative substituents and the formation of small rings—preferably, five-membered rings. 

If the monosaccharide ring is six membered, the compound is called a pyranose if the ring is five membered, the compound is designated as a fiitanose. 

Relative Reactivities oe Five- and Six-Membehed Ring Heterocycles Five-membered ring compound (I) Six-membered ring compound (II) i/ n Reference... 

Dialkyl thiolcarbonates, R-S-CO-O-R 1720-1700. 5.81-5.88 vs m See ref. 111 [cyclic compounds (five-membered ring) at... 

The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

Allyl aryl ethers are used for allylation under basic conditionsfh], but they can be cleaved under neutral conditions. Formation of the five-membered ring compound 284 based on the cyclization of 283 has been applied to the syntheses of methyl jasmonate (285)[15], and sarkomycin[169]. The trisannulation reagent 286 for steroid synthesis undergoes Pd-catalyzed cyclization and aldol condensation to afford CD rings 287 of steroids with a functionalized 18-methyl group 170]. The 3-vinylcyclopentanonecarboxylate 289, formed from 288, is useful for the synthesis of 18-hydroxyestrone (290)[I7I]. 

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

This reaction was first described by Gabriel in 1910 (40), when he warmed an acylaminoketone (197a) with an equimolecular amount of phosphorus pentasulfide. The reaction (Scheme 103) is similar to the preparation of other five-membered oxygen- and sulfur-containing rings from 1,4-dicarbonyl compounds. 

A compound (CgHjo) contains a five membered ring When Bt2 adds to it two diaste reomenc dibromides are formed Suggest reasonable structures for the compound and the two dibromides... 

Firefly lucifenn is an example of an azole that contains a benzene ring fused to the five membered ring Such structures are fairly common Another example is benzimidazole present as a structural unit m vitamin B12 Some compounds related to benzimidazole include purine and its ammo substituted derivative adenine one of the so called het erocychc bases found m DNA and RNA (Chapter 28)... 

Hydroxy acids compounds that contain both a hydroxyl and a carboxylic acid function have the capacity to form cyclic esters called lactones This intramolecular esterification takes place spontaneously when the ring that is formed is five or six membered Lac tones that contain a five membered cyclic ester are referred to as 7 lactones, their six membered analogs are known as 8 lactones... 

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