Cyclopentene formation

Nair and co-workers have extended the use of enals for homoenolate generation to allow ring annulation with enones [13], Cyclopentene formation is achieved via... 

The first example of a tri-Tr-methane reaction was reported by Zimmerman et al. in their smdy of the irradiation of triene 135 in the solid state that affords cyclopentene 136 exclusively (for reviews, see Refs. 75-78). In this instance the cisoid conformation of the biradical, necessary for cyclopentene formation, is... 

Scheme 37 Proposed mechanism of trisubstituted cyclopentene formation...

The treatment of alkynyliodonium salts not amenable to cyclopentene formation with sodium azide in methanol affords vinyliodonium salts and/or enol ethers (equation 107)". Enol ether formation also occurs when glyme is employed as the solvent (equation 108)". Finally, regeneration of the vinylidene-iodonium ylide, PhC (N3)=C—IPh, from (Z)-(/ -azido-/ -phenylvinyl)phenyliodonium tosylate with potassium J-butoxide in glyme likewise affords an enol ether (equation 109). 

These products emanate from trans to cis isomerization of the methylcyclopropyl allene (206) which competes with cyclopentene formation and a facile [1.7]hydrogen migration ... 

The real difficulties with the biradical interpretation began with the stereochemical studies on the cyclopentene formation. Early work was carried out by Mazzochi and Tamburin and by Doering and Sachdev, then, in 1976, Andrews and Baldwin reported a study that allowed a complete stereochemical quadrisection of the vinylcyclopropane rearrangement of optically active trans, trans-l-methyl-2-propenylcyclopropane. The four stereoisomeric 3,4-dimethylcyclopentenes formed in this reaction (see Figure 23)... 

Allenes 23 can also serve as precursors for allyl complexes which undergo intramolecular allylic alkylation to give vinylcyclopropanes 25. This conversion, however, competes with cyclopentene formation. Several other examples of this type have been reported. ... 

Rhodium-catalyzed cyclopentene formation has been observed in cyclopropanations of enol ethers 8 and found to depend on the nature of the ligand and the ester of the diazo compound 9 42,43 Cyclopentenes 10 were isolated exclusively with 2,6-di-ter -butyl-4-methylphenyl esters. 

Scheme 3-6 Bis-cyclopentene formation via double Michael addition and carbene insertions in the reaction of nucleophiles with bis-iodonium diynes.

Because of the poor migratory aptitudes of both sulfones and the keto as well as the amido moieties, exclusive cyclopentene formation is observed in the reaction of P-ketoethynyl- and P-amidoethynyl-iodonium triflates, 11, with sodium p-toluenesulfinate in anhydrous dichloro-... 

Next to fused bicyclic systems, the domino dienyne metathesis process can also be applied to the construction of bridged bicyclic dienes. Thus, dienyne 110 bearing differently substituted alkene units to ensure a regiocontrolled cyclization readily afforded the taxosteroid hybrid compound 111 without competing cyclopentene formation in the presence of the Grubbs second-generation catalyst 2 (Scheme 2.40) [19]. 

The vinylcyclopropane-cyclopentene rearrangement has been popular for cyclopentene formation over a number of years, and has now... 

Shortly thereafter, the Bode group reported the remarkable finding that a similar cyclopentene formation catalyzed by a chiral triazolium salt led to the formation of cis-disubstituted cyclopentenes in good yield and with... 

Scheme 14.16 Postulated mechanisms for NHC-promoted cyclopentene formation.

A [3,3]-sigmatropic rearrangement/RCM sequence [2] was also used as a key step in the total synthesis of a bioactive spirobenzofuran 7 isolated from the mycelium cultures of Acremonium sp. HKl 0230 [3]. The two consecutive quaternary centers embedded in the 1,6-diene 9 are worthy of note in the context of cyclopentene formation by RCM, and the five-membered ring compound 10 was obtained in high yield (97%) using catalyst [Ru]-I. The same type of strategy was applied for the efficient construction of the two vicinal quaternary carbon atoms present in the herbertanes sesquiterpenes (Scheme 1.2) [4]. 

---