Cyclobutene ring 1,3-dienes

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

Discussion. The combination of BCB with BMI significantly improves the properties of the BMI, apparently by tying up the maleimide in a Diels-Alder reaction with the transient diene formed from the opening of the cyclobutene ring. Both the thermal and the thermomechanical properties are improved almost to the level of the BCB... 

Dienes may be involved in electrocyclization reactions as well. Two well-documented examples are the cyclobutene ring opening244 and the 1,3-cyclohexadiene formation245 reactions. Predictions regarding the stereochemical outcome of these rearrangements can be made applying the orbital symmetry rules. The thermally... 

Isomerization reactions of organo-fluorine compounds are included in Houben-Weyl, Vol. 5/3, pp 346-353, where five types of isomerization reactions are covered (1) intramolecular cycli-zation of dienes, (2) opening of cyclobutene rings, (3) opening of cyclobutane rings, (4) isomerization of polyfluorocyclohexadienes, and (5) intramolecular rearrangement of chlorofluoro-and bromofluorocarbons. Much of the work reported is still of fundamental significance and is covered here, where appropriate. Several more later reviews also deal with various aspects of this area.1... 

In this reaction, light of appropriate energy is used to selectively excite 1,3-cyclohepta-diene. The diene closes to a cyclobutene by a disrotatory motion. Although the product, because of its strained cyclobutene ring, is much less stable than the reactant, it is unable to revert back to the diene by an allowed pathway. It does not absorb the light used in the reaction, so the photochemically allowed disrotatory pathway is not available. A conrotatory opening is thermally allowed but results in a cycloheptadiene with a trans double bond. Such a compound is much too strained to form. Therefore, the product can... 

Opening of each cyclobutene ring can occur by either of two conrotatory routes to yield the isomeric dienes. Using B as an example ... 

Acetone-sensitized irradiation of a solution of the pyrone (71) through which acetylene is passed affords the [2 + 2] adduct (72) and the rearrangement product (73). This latter compound is a secondary product and is obtained by further irradiation of the adduct (72), presumably via cyclobutene ring opening and cross-addition of the resultant diene. 

When the cyclobutene ring is transformed into a butadiene, the C-3—C-4 bond breaks. As it does so, there is rotation about the C-1—C-4 bond and the C-2—C-3 bond, so that the substituents on the breaking C-3—C-4 a bond rotate into the plane of the conjugated diene system of the product. In the starting cyclobutene, the methyl and chloro groups lie above the plane of the four ring carbons in the product, these groups lie in the same plane as the four carbon atoms of the butadiene. If all possible rotations about the C-3—C-4 a- bond were allowed, four different products would be formed, two by conrotatory processes and two by disrotatory processes. We will take a closer look at this process. 

An electrocyclic photochemical rearrangement of oxepins to cyclobutene ring-containing valence isomers has also been reported. Thus 2-oxabicyclo-[3.2.0]hepta-3,6-diene has been isolated upon photoisomerization of 1. Similarly, when 1-benzoxepin 100 was irradiated, the valence tautomer3,4-benz-2-oxabicyclo[3.2.0]-hepta-3,6-diene was formed. ... 

The cyclobutene ring first opens in an electrocyclic reaction 152. This must be conrotatory as it is a four electron process but there is no stereochemistry at this stage. Then an intramolecular Diels-Alder cycloaddition 153 closes the new six-membered ring. This is a particularly favourable reaction as the formation of the alkene completes a benzene ring. It would not be possible to prepare such an unstable diene so a tandem process is necessary. 

Two reports have dealt with the result of the photoaddition of ethyne to the hexenulose 70. This reaction occurs stereo-specifically to afford the adduct 71. The adduct can be readily reduced to the corresponding cyclobutane derivative or can be elaborated into optically active tricothecene derivatives. Acetone-sensitized irradiation of the pyrone 72 in the presence of ethyne affords the adduct 73 and the rearrangement product 14 . The formation of this latter adduct presumably involves cyclobutene ring-opening followed by crossed addition of the diene. Cycloaddition of but-l-yne to the quinolone 75 affords the head-to-tail (2 + 2)-cycloadduct 76 while addition to hex-3-yne yields 77 h The route using the adducts 76 has been developed as a synthetic path to 3-substituted quinolones. 

Cyclobutene ring from 1,3-dienes Bicyclic from monocyclic N-heterocyclics... 

Periplanone B is the most active sex pheromone found in the alimentary tract and excreta of the American cockroach Periplaneta americana. An elegant total synthesis of this germacrane sesquiterpene was achieved by SCHREIBER and SANTINI Cyclodecatrienone 1 is an obvious precursor. One of the oxirane rings arises from epoxidation of the enone CC double bond, the other from [2-1-1]-cycloaddition of a carbene to the carbonyl bond of the enone. Oxidation of the methylene group introduces the additional carbonyl double bond. The CC double bond of the enone results from an elimination of HX in the a-X-substituted cyclodecadienone 2, which, on its part, is feasible by substitution of cyclodecadienone 3. An electrocyclic opening of the cyclobutene ring in 4 provides the 1,3-diene substructure in 3. 

Reaction is thought to proceed via initial ring-opening of the cyclobutene ring to give a 3,5-diene-l, 7-diyne. In accord with this, octa-3,5-dienc-l,7-diyne is unstable and is rapidly converted, in high yield, into a dimer of benzocyclobutadiene [103]. 

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