Oxyanion-accelerated

Many rDA reactions are carried out at temperatures of 150 C or more in solution phase and often at temperatures of 400-600 C using the flash vapor pyrolysis (FVP) method individual conditions are referenced throughout the text. However, an accelerating effect by anionic, cationic and radical substimtion on either the dienophile or at the termini of the diene fragments has been predicted by Carpenter.Experimentally, this prediction has been substantiated only for anionic substitution. In 1967, Hart reported what is likely the first example of an oxyanion-accelerated rDA reaction. Both oxyanionic " and car-banionic substituents accelerate the cycloreversion reaction such that they proceed rapidly at room temperature (for example, equation 3). In addition, acid-catalyzed rDA reactions have been reported in which protonation effectively makes the dienophile fragment of the adduct more electron deficient. Grieco has utilized a room temperature retro aza DA reactitm useful for the N-methylation of dipeptides and amino acid derivatives (equatitm 4). ... 

Oxyaruon-acceleratedrearrangement cf2-vinylcyclobutanol derivatives 8.3.3.13 Oxyanion-accelerated rearrangement cf l-vinylcyclobutanol derivatives... 

As outlined in Scheme 5, tertiary cyclopropanol salts are also subject to the oxyanion-accelerated VCP rearrangement. In this sequence the requisite vinylcyclopropanol salts are conveniently prepared from readily available 1,1-dibromocyclopropanes by sequential halogen-metal exchanges, followed first by alkylation, and then by oxygenation. Products resulting from alternative 1,5-hydrogen shift (retro-ene) pathways were not detected in these reactions. 

Further studies on the scope and stereochemical course of the oxyanion-accelerated vinylcyclopropane reairangement were reported in 1981. This paper introduced a general [4+1] annulation strategy for the synthesis of cyclopentene derivatives in which the anion-accelerated VCP rearrangement functions as the key step. In this report, the accelerated version of the vinylcyclopropane rearrangement was also shown to proceed with remarkably high stereoselectivity, in further contrast to the thermal process. 

The stereochemical course of the oxyanion-accelerated vinylcyclopropane rearrangement can be explained on the basis of several alternative mechanisms. 

Bauld and coworkers have examined the cation radical cycloadditions of 1,3-dienes with electron-rich alkenes and found that, under photosensitized electron-transfer conditions, [2 -i- 2] cycloaddition is in many cases favored over Diels-Alder addition. Thus, as illustrated in equation (30), 1, T-dicyclopentenyl (186) reacts with p-chloroethyl vinyl ether under electron transfer conditions to afford the cyclobutane adduct (187), which was cleaved to the cyclobutanol (188) in 70% yield upon treatment with n-butyl-iithium. Oxyanion-accelerated VCB rearrangement then provided (189) as a mixture of diastereomers in... 

Strategy C in Scheme 21 employs the [2 2] cycloaddition of vinylketenes and 1,3-dienes to generate the requisite 2-vinylcyclobutanone intermediates for oxyanion-accelerated VCB rearrangement. The sequence presented in Scheme 23 illustrates this two-step [4 + 2] armulation strategy, the overall result of... 

In summary then, the oxyanion-accelerated rearrangement of 2-vinylcyclobutanol derivatives is now established as an attractive method for achieving two-carbon ring expansion under relatively mild conditions. In conjunction with the efficient synthetic routes to vinylcyclobutanones outlined in Scheme 21, this version of the VCB rearrangement provides several strategically novel annulation methods for the construction of six-membered carbocycles. ... 

The chemistry of the divinylcyclobutanols derived from 5-methylenespiro[3.5]nonanone (260) proved to be more complicated than the reactions described above. As expected, the oxyanion-accelerated 3,3-sigmatropic rearrangement of the cis isomer (261) was extremely f acile as a consequence of the rigid j-cis orientation of the exocyclic methylene group (Scheme 35). The rearrangement of the trans isomer, however, produced none of the desired Cope product instead this cyclobutanol salt underwent exclusive 1,3-rearrangement to afford the cyclohexenol derivative (264). It is uncertain whether this transformation... 

For other examples of oxyanion-accelerated rearrangements involving 2-vinylcyclobutanol derivatives, see (a)... 

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