Carbocyclic synthesis five-membered-ring

Dipolar cycloaddUions. Interest in 1,3-dipolar cycloadditions increased dramatically during the past 20 years, largely because of the pioneering studies of Huisgen [7, 2] The versatility of this class of pericychc reactions in the synthesis of five-membered-ring heterocyclic compounds is comparable with that of the Diels-Alder reaction in the synthesis of six-membered-ring carbocyclic systems (equation 1)... 

Hydrosilylation of 1,6-dienes accompanied by cyclization giving a five-membered ring system is emerging as a potential route to the synthesis of functionalized carbocycles.81,81a,81b 82 As its asymmetric version, diallylmalonates 86 were treated with trialkylsilane in the presence of a cationic palladium catalyst 88, which is coordinated with a chiral pyridine-oxazoline ligand. As the cyclization-hydrosilylation products, //ww-disubstituted cyclopentanes 87 were obtained with high diastereoselectivity (>95%), whose enantioselectivity ranged between 87% and 90% (Scheme 25).83 83a... 

Of special interest is the use of protected cyanohydrins in the formation of carbocyclic rings. Ring closure of an acyclic intermediate to form a five-membered ring (75-85%) has been Ascribed in the synthesis of prostaglandins (equation 17). In addition this method is applicable to the formation of cyclopropyl, cyclobutyl and cyclohexyl rings (60-70%). ... 

In the laboratory of S.L. Schreiber, the total synthesis of (+)-epoxydictymene was accomplished by the tandem use of cobalt-mediated reactions as key steps. The eight-membered carbocycle was formed via a Nicholas reaction, while the five-membered ring was annulated by the Pauson-Khand reaction. Several P.-K. conditions were explored and the best diastereoselectivity was observed when NMO was used as a promoter. The annulated product was isolated as an 11 1 mixture of diastereomers. 

Due to the growing interest in cyclopentanoid natural product synthesis, numerous methods for the construction of carbocyclic five-membered rings have been developed1- 4. Among these, [3 + 2] cycloaddition methodologies play a prominent role. 

Another example from the Spanish authors [14, 35] is especially interesting, as the cycloaddition leads to the formation of a compound with a seven-membered ring (Scheme 9). The literature shows that in recent years a tremendous amount of effort has been expended in trying to prepare carbocyclic five-membered rings by a route as simple and elegant as the Diels-Alder reaction for the synthesis of six-membered rings. [4c] Is a counterpart for... 

Synthesis of Five-Membered Carbocycles An annulation of chromium arylcarbenes by alkynes without CO-insertion leads to five-membered rings. It has been observed as a side reaction along the benzannulation which may become predominant depending on the nature of the carbene heteroatom substitution pattern, [72] the metal [40] and its coligands [73] and the solvent [43] used. In this respect, indene derivatives have been obtained from arylcarbene complexes of chromium and tungsten. [73,74]... 

Shibasaki and co-workers used a ring-closing metathesis approach to prepare a number of five-, six-, and seven-membered rings from electron-deficient olefins. Treatment of acyclic enol ether 18 with 7 mol % of 3 in refluxing benzene provided the corresponding cyclic enol ether 19 in 94% yield. Deprotection of the silyl ether 19 (not shown) resulted in the corresponding cyclic ketone, a valuable synthetic intermediate in natural products synthesis and a number of industrial processes. The authors reported additional examples of the synthesis of five-membered ring carbocycles as part of this study. 

Undheim and co-workers applied their olefin metathesis strategy for the synthesis of five-membered ring spirocyclic carbocycles to synthesize six-membered ring spirocyclic carbocycles for use as templates in natural product synthesis. Reaction of 198 with 2 mol % of 3 furnished the corresponding spirocycle 199 in greater than 95% yield. 

Saturated Carbocyclic Ring Synthesis 3 Five-membered Rings... 

The Pauson-Khand reaction (PKR) [96] consists of the synthesis of cyclopen-tenones by reaction of an alkene with a dicobalthexacarbonyl complexed alkyne (Scheme 57) and has recently emerged as one of the methods of choice for the obtainment of five-membered carbocyclic rings [97]. Its unique atom connectivity, which involves the two unsaturated carbons of the reagents and the carbon atom of a carbon monoxide ligand of cobalt usually in a regioselective manner (Scheme 57), has brought to refer to PKR as a [2 -I- 2 + 1] cycloaddition. 

Synthesis of Four-, Five- and Six-membered Rings from Carbocyclic or Heterocyclic Precursors 540... 

Synthesis of Five-Membered Carbocyclic and Heterocyclic Ring Systems... 

The foregoing methods have been successfully applied to the synthesis of carbocyclic cytochalasans and the macrolide methynolide (191 equation 22). Iterative ring expansion of the five- and eight-mem-bered ring sulfonium salts (187) and (189) leads to the 11-membered ring sulfur heterocycle (190). Sulfonium salt (189) is a 1 1 mixture of stereoisomers that gives (190) with 16 1 selectivity in 76% yield. 

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