Cycloheptadienes rearrangement

Early synthetically oriented divinylcyclopropane-cycloheptadiene rearrangements were performed with hydrocarbons and were in part due to the interest in syntheses of marine brown algae constituents. The preparation of ectocarpene is displayed in equation 164. The corresponding divinylcyclopropanes have either been constructed by Wittig methodology or, as in the case shown, by addition of diazoalkanes to apt polyenes followed by photolysis or thermolysis . 

The stereocontroiied total synthesis of (+)-gelsemine was accomplished by T. Fukuyama and co-workers using the Knoevenagel condensation to prepare a precursor for the key divinylcyclopropane-cycloheptadiene rearrangement. The use of 4-iodooxindole as the active methylene component allowed the preparation of the (Z)-alkylidene indolinone product as a single stereoisomer. 

One example of an acid-catalyzed divinylcyclopropane to cycloheptadiene rearrangement which has been documented is the formation of tricyclic compound 3. ... 

In accordance with this, the reaction of the electron-donor-substituted butadienes 170 (R=Ph, OMe) with the arylcarbene complexes 163 yields divinylcyclopropane intermediates 168 with high chemoselectivity for the electron-rich double bond in 170, which readily undergo a [3,3]-sigmatropic rearrangement to give the as-6,7-disubstituted 1,4-cycloheptadiene derivatives... 

When ring strain is relieved, Cope rearrangements can occur at much lower temperatures and with complete conversion to ring-opened products. A striking example is the conversion of ris-di vinyl cyclopropane to 1,4-cycloheptadiene, a reaction that occurs readily below -40° C.206... 

The preparation of N-carbethoxy-8-azabicyclo [5.1.0] oct-3-ene (158) from ethyl azidoformate (157) and 1,4-cycloheptadiene through a photolytic reaction, and its palladium(II)-catalyzed multistep rearrangement to N-carbethoxynortropidine (159), has been presented by Wiger and Retting as a new route to the 8-azabicyclo[3.2.1]octene skeleton (87) (Scheme 8). 

The analogous 2,6-cycloheptadien-l-ones display zwitterion-derived reactivity as well52,53. For example, the parent compound 113, upon irradiation in either acetic acid or t-butanol, gave diastereomeric solvent adducts 114 and 115 (Scheme 28)52a. On the other hand, tetramethoxy derivatives 116 furnished rearranged products 117, which are equivalent to the cyclopropyl ketone 1,4-shift products seen with cyclohexadienones52b. 

An interesting application of a 1,2-disubstituted cyclopropanol in a seven-membered ring-annelation methodology has been developed by Cha et al. [82], The cyclopropanol 124, obtained from methyl 1-cyclopentenecarboxylate (123) and 4-(triisopropylsilyloxy)-butylmagnesium chloride, was converted to a 1,2-dialkenylcyclopropanol bis-silyl ether, which, by a subsequent facile Cope rearrangement, afforded the cycloheptadiene-anne-lated cyclopentane derivative 125 in 32% overall yield (Scheme 11.32). 

The l-oxa-2,4,5-cycloheptatrienes 602 and 603 were postulated to be intermediates in the rearrangement of certain (ethynylfuryl)oxiranes to furo[3,4-b]furans [251]. The replacement of the methylene groups of 1,2-cycloheptadiene (465) by SiMe2 groups and the introduction of substituents at the allene moiety allowed the preparation of isolable seven-membered ring allenes. Thus, Barton et ah [177] obtained 604 and Ando et al. [178] 605. A few reactions of these systems have also been studied [177, 252]. Both groups [178, 253] synthesized the [4.4]betweenallene 606 and determined its structure by X-ray diffraction. 

If 1,3-butadienes are cyclopropanated by use of vinylcarbene complexes, the divinylcyclopropanes which result can rearrange to cycloheptadienes [71,241,384 -387] (Figure 2.37). 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

Calculations on the ring opening of fran -cyclopropylidene (113) to 1,3-dimethylallene predicted a barrier of 4.2 kcal mor via initial disrotatory motion of the substituents followed by a change to conrotatory motion. The di-cyclopropylidene rearrangement is barrierless and, in agreement with the elusive nature of 1,2-cycloheptadiene, the barrier to ring opening of bicyclic cyclopropylidene (114 n = 2) cannot be overcome at low temperatures. 

Rhodium(II) (iV-dodecylbenzenesulfonyl)prolinate has been found to act as an effective catalyst for the enantioselective decomposition of vinyldiazoacetates to c -divinylcyclopropanes. Combination of this process with a subsequent Cope rearrangement has resulted in a highly enantioselective synthesis of a variety of cycloheptadienes containing multiple stereogenic centres (see Scheme 40). The tandem... 

The vinylcarbenoid [3-1-4] cycloaddition was applicable to the short stereoselective synthesis of ( )-tremulenolide A 73 and ( )-tremulenediol A 74 (Scheme 14.7) [81]. The key step is the cyclopropanation between the cyclic vinyldiazoacetate 69 and the functionalized diene 70, which occurs selectively at the ds-double bond in 70. Because of the crowded transition state for the Cope rearrangement of the divinylcyclopropane 71, forcing conditions are required to form the fused cycloheptadiene 72. The stereo-... 

The trans compound, in contrast, reacts at much higher reaction temperature (190°C) and undergoes first a radical isomerization to give the cis isomer, which then rearranges to yield 1,4-cycloheptadiene.240 Substituents at the terminal vinylic carbons considerably decrease the rate of rearrangement235,241... 

Cyclodecyl cation hydride bridge, 147 1,4-Cycloheptadiene, 170, 171 Cycloheptatriene, 281 Cycloheptatrienes rearrangements, 290 Cycloheptatrienylidene, 275 interaction diagram, 276... 

The Cope rearrangement of c/j-di vinylcyclopropanes is thermally allowed and offers an attractive stereoselective approach to cycloheptadienes. Cyclopropanation reactions can be used to prepare divi-nylcyclopropanes, as shown in Scheme 23.120 Reaction of ethyl diazopyruvate with butadiene generated... 

Cycloheptadiene (340) is obtained by the Cope rearrangement of cis-divinylcyclopropane (339.) Based on this reaction, highly diastereoselective and enantioselective construction of the 1,4-cycloheptadiene 343 (98% ee) was achieved by domino asymmetric cyclopropanation to generate cA-divinylcyclopropane... 

The presumed Z-enoxysilane 28 undergoes rearrangement prior to warming to room temperature furnishing siloxy cycloheptadiene 29. Hydrolysis of silyl enol ether 29 yields (6S, 8/ )-cycloheptenone 7. 

---