1.1- Divinylcyclopropane, structure

The molecules taking part in a valence tautomerization need not be equivalent. Thus, NMR spectra indicate that a true valence tautomerization exists at room temperature between the cycloheptatriene 110 and the norcaradiene (111). In this case one isomer (111) has the cw-l,2-divinylcyclopropane structure, while the other does not. In an analogous interconversion, benzene oxide and oxepin exist in a tautomeric equilibrium at room temperature. 

An interesting approach to form a divinylcyclopropane structure capable of rearranging into seven-membered functionalized derivatives consists of the silyloxylation of cyclic ketones 541 followed by a spontaneous Cope rearrangement to produce the cyclic enol esters 542 which then hydrolyzed to ketones 543 (equation 2 1 3)265. 

In principle, the divinylcyclopropane structure discussed here is incorporated into very well known systems such as bullvalene 547, barbaralane 548 and semibullvalene 549, which very easily undergo a Cope rearrangement. 

FIGURE 2. Calculated high symmetry conformations (C2v, C2 and Dyd. S4, respectively) and experimentally determined molecular structures of 1,1-divinylcyclopropane (DVC) and tetravinylmethane (TVM) in Ci presentation with thermal probability plots of 50%... 

An elegant application of this chemistry to the formal synthesis of ( )-quadrone has been reported by Piers and coworkers.78a l2la Decomposition of ethyl diazoacetate in the presence of the bicyclic structure (119) resulted in selective cyclopropanation to form (120). Further modification of (120) generated the /ram-divinylcyclopropane (121), which on thermolysis followed by desilylation produced the tricyclic system (122). Conversion of (122) to the ketone (123) completed the formal synthesis because (123 Scheme 24) had been previously converted to ( )-quadione.l2lb... 

Reaction of vinylcarbenoids with furans offers another level of complexity because the furanocyclo-propanes in this case would be divinylcyclopropanes capable of a Cope rearrangement as well as electro-cyclic ring opening to trienes.27b c As illustrated in Scheme 42, the product distribution was dependent on the furan structure. With 2,5-disubstituted furans [3.2.1 ]-bicyclic systems (202) were exclusively formed, but with furan or 2-substituted furans, trienes (203) were also produced. 

T ABLE 3. Structural parameters of vinylcyclopropane (37) and trans-1,2-divinylcyclopropane (38)" ... 

The bicyclo[S.4.0]undecane sesquitetpenoid ( )-3-himachalene (109) was prepared via a route in which thermolysis of a 3-(2-vinylcyclopropyl) enone played a key role (Scheme 15). Reaction of the P-iodo enone (82) with the stereochemically homogeneous cyclopropylcuprate (106) provides the functionalized trans-divinylcyclopropane (107). Thermolysis of (107) gives exclusively the Cope reanange-ment product (108), which is converted into ( )-P-himachalene (1()9). Notably, in contrast to structurally related systems (vide supra), the p, y-unsaturated ketone function in (108) shows no inclination to rearrange to the corresponding a,P-unsaturated (conjugated) ketone. 

The key intennediate for the synthesis of ( )-sinulaiene (159), a structurally unusual marine natural product, was the bicyclic divinylcyclopropane (160). Successful Cope rearrangement of this substance would be expected to proceed via the endo isomer (161) to produce the bicyclo[3.2.1]octa-2,6-diene (162), possessing the correct stereochemistry at C-4 (exo-isopropyl group) and fimctionality (exo-vinyl group at C-8, enol ether associated with C-6 and C-7) that would allow the straightforward preparation of the tricyclic ring system of ( )-(159). 

Two 1,1-divinylcyclopropanes that have attracted a great deal of interest are 43 and 44. The microwave structure of the first has been obtained and, based on the photoelectron spectrum and STO-3G calculationsa strong electron delocalization from the ring has been implicatedConjugation was also present in 44, but through conjugation of the cyclopropyls was not prominent. Cyclopropylallenes such as 45 behaved similarly to vinylcyclopropanes, with no evidence for interaction of the cyclopropyl with the second double bond ... 

Ozkan, I., Zora, M. Transition Structures, Energetics, and Secondary Kinetic Isotope Effects for Cope Rearrangements of cis-1,2-Divinylcyclobutane and cis-1,2-Divinylcyclopropane A DFT Study. J. Org. Chem. 2003, 68, 9635-9642. 

The present [3 + 4] annulation methodology for the synthesis of seven-membered carbocycles involves a straightforward procedure that also provides the product functionalities (e.g., masked and unmasked ketone carbonyl and trimethylsilyl groups) that can be transformed to hitherto inaccessible or difficult-to-prepare cycloheptane structures. The prior approach,7-9 based on Cope rearrangement of cis-1,2-divinylcyclopropane, bears an intrinsic drawback in that there exist a limited number of methods for stereoselective preparation of the substrate. 

No details were given so far about the structure of these compounds. One of the most interesting applications of the method could be the synthesis of enantiomerically pure, highly functionalized cycloheptane derivatives, since divinylcyclopropanes should be obtainable by the appropriate rearrangement of other primary products. 

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