VCP-CP rearrangement

CHAPTER 11 VINYLCYCLOPROPANE-CYCLOPENTENE REARRANGEMENT Scheme 11.1 VCP-CP rearrangement. 

Scheme 11. S4 NifOVIPr-catalyzed rearrangement of activated and unactivated VCPs. Scheme 11. SS Mechanism for NifOVcatalyzed rearrangement of dienylcyclopropanes. Scheme 11. SB Mechanism for NiiOVIPr-catalyzed VCP-CP rearrangement.

Scheme 11.61 Mechanism of CnthfacacW talyzed rearrangement of heteroatom-variant of VCP-CP rearrangement.

Since the discovery of the carbon VCP-CP rearrangement, tremendous progress has been made in this field using thermal, acid-mediated/catalyzed, photochemical, and metal-catalyzed reaction conditions. This chapter will focus on the developments made in VCP-CP rearrangements, their heteroatom-substituted analogues, and the application of these rearrangements in syntheses. 

Although the previously mentioned heteroatom-substituted VCPs (see Schemes 11.3-11.6 underwent conversion to cyclopentenes at somewhat lower tenperatures than the original chloro-substituted and all-carbon systems, lowering reaction tenperatures even further would make the rearrangement synthetically more useful. The Danheiser group demonstrated a significant rate enhancement of the VCP-CP rearrangement by the use of lithium salts of vinylcyclopropanols. Cyclopentenol formation proceeded effectively at room tenperature in low to excellent yields (Scheme 11.7). ... 

The Hudlicky group also took advantage of anion-accelerated VCP-CP rearrangements by treating (siloxyvinyl)cyclopropanes to the tetra-butylammonium fluoride (TBAF) or trimethylsilyl iodide in the presence of hexamethyldisilazane (TMSI/HMDS). These conditions promoted rearrangement to form [5-5] and [5-6] annulated cyclopentenes at low temperatures with high selectivities (Scheme 11.9T ... 

In 2009, Lambert reported a mild, efficient, and stereoselective synthesis of fused cyclopentenes via Mgl2-promoted isomerization of activated VCPs. The VCPs were prepared by Pd-catalyzed cyclopropanation of 1,3-dienyl p-ketoesters in moderate to high yields and good diastereoselectivities. Typical standard reaction conditions for the VCP-CP rearrangement (i.e., pyrolysis, transition metal catalysis, standard Lewis acid catalysis) were found to be ineffective for these substrates. However, the use of 1.5 equiv. of Mgl2 led to conplete conversion, providing bicyclic cyclopentenes in high yields fScheme 11.15L... 

The activation energy, E, of the thermal parent VCP-CP rearrangement is approximately 50 kcal/mol and is 13 kcal/mol less than the activation energy required for the cis-trans isomerization of the l,2-d2-cyclopropanes. Interestingly, the realization that the resonance energy of the allyl radical (about 13 kcal/mol) might contribute to lowering the for this... 

Heterocyclic variants of VCP-CP rearrangement have also been explored to synthesize five-membered heterocyclic rings, although to a lesser extent than their carbocyclic counterparts. The ring expansion of vinylaziridines to pyrroline products was reported independendy by Atkinson and Rees in 1967 and by Lwowski and coworkers in 1968, whereas the corresponding thermal rearrangement of vinyloxiranes to dihydrofurans was disclosed by Paladini and Chuche in 1971 fScheme 11.231. ... 

The heteroatom variants of the VCP-CP rearrangement generally operate either by a zwitterionic or a diradical-type mechanism The zwitterionic mechanism is more common and involves the heteroatom-assisted C—C cleavage of the three-membered ring to form an ylide, which undergoes electrocyclic ring closure to form the product (Scheme 11.28. Eq. 

In contrast, the mechanism of Lil or TMSI-mediated hetero-VCP-CP rearrangement involves... 

Hudlicky s group applied the VCP-CP rearrangement to the synthesis of a variety of complex natural products. For example, their total synthesis of the terpenoid ( )-hirsutene utilized the thermal rearrangement of activated VCP 34 to diastereoselectively form the advanced tricyclic core 35 of the molecule (Scheme 11.31... 

Hudlicky and coworkers also reported an elegant synthesis of the iridoid sesquiterpene (-)-specionin, by utilizing the low-temperature anion-accelerated VCP-CP rearrangement developed in their lab. The precursor siloxyvinylcyclopropane 38 was synthesized as a mixture of exo/endo isomers by the cyclopropanation of substituted cyclopentenone 36 with the lithium dienolate 37 derived from4-(dimethyl-tert-butylsilo5q )-2-bromocrotonate tScheme 11.321. Rearrangement of the diastereomeric VCP substrates was achieved by the use of TMSI/HMDS at -78 °C to afford the tricyclic ketone in high yield as a mixture of diastereomers, which was then converted to the natural product. ... 

Two years after the discovery of the parent thermal VCP-CP rearrangement in 1960, Frey observed the first photochemical version of this process. " Since then, appreciable efforts have been made to investigate the photochemical behavior of VCPs and to apply these processes in synthesis. The absorption maxima for the n n olefin band in the parent VCP has been 192 nm, which clearly indicates the requirement of additional chromophores for its excitement in the more accessible UV/ s region.The photochemical VCP-CP rearrangement can be achieved by both direct and sensitized methods. Direct irradiation of VCPs having additional chromophores results in relatively low-energy n n excitations and, therefore, is more... 

The photochemical route has also been applied to a heterocyclic variant of the VCP-CP rearrangement. Canpos s group reported an efficient method to synthesize pyrrolines by the... 

Sonawane et al. further utilized the photoinduced VCP-CP rearrangement in a formal s5mthesis of the sex pheromone of the male cotton boll weevil, (ij-grandisol. The toluene-sensitized irradiation of enantiopure (+)-A -carene 69 afforded the annulated cyclopentene ( )-72 in... 

Only a few reports describing the Rh-catalyzed VCP-CP rearrangement are currently in the literature. Grigg et al. developed a Rh(I)-catalyzed rearrangement of 9,9-bicyclononatriene VCPs to form the cis-dihydroindene product predominantly. However, the reaction was limited to the use of activated substrates, that is, dienyl- or trienylcyclopropanes, and the products were often contaminated with low amounts of unidentified impurities tScheme 11.451. ... 

A variety of stoichiometric complexation studies on Rh(I)-mediated VCP-CP rearrangements as well as divinylcyclopropane rearrangements have been reported. In addition, some proposed reactive intermediates have been isolated and characterized. Based on these studies, the proposed mechanism involves the formation of an rj -alkene complex 75 (Scheme... 

A variety of Ni catalysts have been reported in the literature to effect the VCP-CP rearrangement. Murakami s group demonstrated the use of a Ni(0)/P(t-Bu)3 catalyst which was effective only with activated VCPs bearing dienyl or styrenyl substituents. The isomerization of the vinylcyclopentene products to methylenecyclopentenes was also observed in some cases under the reaction conditions (84a and 84b, Srheme 11.52T... 

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