Cycloadditions vinylcyclopropane rearrangement

In practice, it was found that whereas the synthesis of hirsutene according to the dual strategy met with success under thermal conditions, but at temperatures as high as 580 °C, under photochemical conditions it afforded the unnatural cis, syn, cis configuration of some intermediates which then need further elaboration. Although the transformations 44 — 43a and 45. — 43a by a [2 + 2] -cycloaddition and a vinylcyclopropane rearrangement, respectively, may involve intermediates with a more or less biradical character, they are not typical radical reactions such as the ones we are considering here. 

When carbene 21 is generated from the diazo precursor photochemically in solution, it reacts with added alcohols by 0,H insertion (equation 55), in contrast to the gas-phase copyrolysis where the silene intermediate is trapped . Similarly, photolysis of 20 in the presence of 2,3-dimethylbutadiene gives mainly vinylcyclopropane 184, while after copyrolysis of 20 and the same diene one finds that most of the vinylcyclopropane rearranged to the cyclopentene, together with the [4-1-2] cycloaddition product of diene and silene 22 °. Furthermore carbene 21, generated photochemically or thermally (at 117-148 °C) in solution, undergoes [1 4-2] cycloaddition to alkynes to give cyclopropenes 185. 

Adducts derived from cyclopropyl-TMM reactions are versatile synthetic intermediates. Alkylidenecyclopropanes have been proven useful in further Pd-cata-lyzed transformations [4], On the other hand, vinylcyclopropanes can undergo smooth thermal ring-expansion to cyclopentenes. Thus, a total synthesis of 11-hy-droxyjasionone (27) was achieved with the cyclopropyl-TMM cycloaddition as the crucial step, and the thermal rearrangement of the initial adduct (28) as an entry to the bicyclo[6.3.0]undecyl compound (29), a key intermediate in the synthetic sequence (Scheme 2.9) [19]. 

The photoindueed 1,7-cycloaddition of carbon monoxide across the divinyl-cyclopropane derivative 32 yields the two cyclic dienyl ketones 34, via the ferracyclononadiene intermediate 33 [18]. (Scheme 11) cyclopentene rearrangement. The dienylcyclopropane 35 is capable of forming the complex 36, followed by ring enlargement to 37 [19]. 1,1-Dicyclopropylethylene 29 is also converted to the 1-cyclopropyl-1-cyclopentene 38. The additional functionality of vinylcyclopropanes is necessary to serve as a 7t-donor... 

Asymmetric induction is possible in the two-step [3-1-2] cycloaddition by starting the sequence with an asymmetric cyclopropanation (Scheme 14.15) [106]. The Rh2(S-DOSP)4-catalyzed reactions gave the desired vinylcyclopropanes 126 with high enan-tioselectivities [106]. Partial or complete racemization occurred in the vinylcyclopro-pane rearrangement of monocyclic vinylcyclopropanes, but the fused vinylcyclopropanes 128-133 rearrange to form 134-139 with virtually no racemization. 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

As a related reaction, the bicyclo[5.3.0]decane derivative 64 was obtained at 30 °C by the Rh-catalysed intramolecular [5+2] cycloaddition of the alkyne with the vinylcyclopropane moiety in 61. The latter behaves as a pseudo-1,3-diene in oxidative addition, and generates 62. This is followed by rearrangement to 63, whose reductive elimination gives 64 [20]. [Rh(CO)2Cl]2 is a better catalyst than RhCUPl+P. The reaction can be extended to alkenes [20a],... 

When the double bond of the enyne possesses a cyclopropyl substituent, an intramolecular [5+2] cycloaddition of alkyne and vinylcyclopropane takes place [75, 76]. The ruthenacycle does not undergo /l-hydride elimination but a rearrangement of the cyclopropane to produce a ruthenacyclooctadiene. Thus, a variety of bicyclic and tricyclic cycloheptadienes were obtained in good yields [75] (Eq. 55). 

In a study of vinylcyclopropanes with tetranitromethane (TNM), from 1,1-divinylcyclopropane 156, a specific 1,4-diene, the unexpected product 118, was obtained. The formation of the nitronic ester was accompanied by homo-allylic rearrangement, followed by the intramolecular 1,3-dipolar cycloaddition (Equation 9) <2002DOC9>. 

The synthetic utility is restricted somewhat by the failure of alkenes activated by only one electron-withdrawing group to undergo the cycloaddition. Instead, vinylcyclopropanes, corresponding to [1 + 2] cycloadducts, are formed. Unfortunately, attempts to affect a vinylcyclopropane-cyclopentane rearrangement were unsuccessful. 2-Acetoxyacrylonitrile fails to participate in either [1 + 2] or [2 + 3] cycloaddition (Schemes 26,21)P ... 

In a novel combination of Pauson-Khand cycloaddition with vinylcyclopropane chemistry, de Meijere has described an entry to linearly fused triquinanes beginning with cyclopropylalkynes. Cyclopentenone formation has been carried out with a variety of substitution patterns on the cyclopropane, and moderate yields achieved with both norbomene and cyclopentene as substrates. Thermal vinylcyclopropane-cy-clopentene rearrangement of the cycloaddition products leads to the final tricyclic system (Scheme... 

With the exception of vinylcyclopropane-cyclopentene rearrangements (cf. Section VII) radical reactions have so far rather rarely been used synthetically. This is also true for pericyclic modes of cyclopropane cleavage which are mainly restricted to divinylcyclopropane-cycloheptadiene type expansions. Cycloadditions, whether concerted or stepwise, occur only with very specific compounds. ... 

The reactivity of these promising building blocks has very recently been reviewed by de Meijere S therefore only a few principles will be discussed in this text. Treatment with n-butyllithium as outlined in equation 187 will give certain functionalized alkynylcyclopro-panes , which can serve as active components in cycloadditions or reactions with olefins in the presence of Co2(CO)g. The latter [2 + 2 + l]-addition provides cyclopropyl cyclopentenones capable of undergoing vinylcyclopropane-cyclopentene rearrangement. 

An interesting rearrangement, which appears to be anion-accelerated, takes place in the enol thioether, anion-terminated vinylcyclopropanes of type 14. ° The rearrangement proceeds at — 78 C and is reasonably stereoselective with regard to the final cyclopentene products (syn selectivity 16 1). Regioisomers are encountered in the formation of the dihydrothiopyran cycloaddition adducts 13 in several instances. The mechanism of this rearrangement appears to involve the enol thioether anion in accord with the well-documented donor acceptor principles " and may be related to similar rearrangements observed with trimethylsilyl enol ether terminated vinylcyclopropanes under fluoride ion or Lewis acid catalysis." " ... 

Three more reactions, which presumably proceed via carbopalladation steps without subsequent dehydropalladation, should be mentioned (Scheme 24). The first one is the diastereoselective formal [3 + 2] cycloaddition of the chiral nonracemic (/3-sulfinyl)vinylcyclopropane derivative 109 onto acrylonitrile (110). The second example is the Pd-catalyzed asymmetric vinylcyclopropane to cyclopentene rearrangement of the chiral nonracemic E)- and (Z)-arylsulflnyl-l,3-butadienylcyclopropane derivatives 112 [66] Plausible mechanisms that can rationalize the stereochemical outcome of the reactions were proposed in both publications. ... 

---