Ring electrocydic

Of note, Quintela and coworkers used a related aza-Wittig/l,5-electrocydic ring-closure process to obtain access to multiheterocydic structural elements such as py-... 

Sorensen and coworkers used a domino conrotatory electrocydic ring-opening/ 6ji-disrotatory electrocyclization for the formation of ring C in the total synthesis of ( )-viridin (4-327) (Scheme 4.72) [112]. Heating 4-325 in the presence of a base followed by in-situ oxidation with DDQ afforded the tetracyde 4-326 in 83% yield. 

Thiemann and coworkers [68] sought novel types of steroids with different biological activity, and in doing so prepared areno-annulated compounds such as 6/1-133 (Scheme 6/1.35). This is achieved with a Heck reaction of 6/1-132 with an acrylate, followed by an electrocydic ring closure of the formed hexatriene. The reaction is then terminated by removal of the nitro group, with formation of the aromatic ring system. 

Although not involving a pure hydrocarbon, the thermal isomerization of the methylenecyclobutene 268 to the benzocyclobutene 271 shows some typical allene hydrocarbon behavior (see below). In the first step of this sequential reaction, the substrate opens to give a vinylallene , 269, which, at the same time is also a derivative of the (Z)-bisallene 230 already referred to. As such, it readily electrocydizes to the o-xyly-lene 270, which, in a last and also characteristic step, ring closes to 271 [113]. 

The initially generated o-diallenylarene 273 electrocydizes to the annelated o-xyly-lene 274, which can either ring close to a naphtho[fo]cyclobutene or be trapped with a dienophile to yield the Diels-Alder adduct 275. Instead of the terminal hydrogen atom, the ethynyl functions can also carry alkyl and aryl substituents. 

The best known example is the thermal isomerization of 1,5-hexadiyne, which involves a [3.3]-sigmatropic shift to 1,2,4,5-hexatetraene (12) followed by an electrocydic ring closure to 3,4-bismethylencydobutene (324) for reviews, see (a) W. D. Huntsman, in The Chemistry of Ketenes, AUenes and Related Compounds, S. Patai (ed.), Wiley Chichester, 1980, Vol. 2, Chapter 15, pp. 521-667 (b) A. Viola, J. J. Collins, N. Fillip, Tetrahedron 1981, 37, 3765—3811 for quantum chemical calculations on this and related isomerizations, see K. A. Bleck,... 

An interesting possibility for the construction of a tetracyclic system 174 with two cyclobutane rings arises by addition of lithiated alkoxyallenes 120 to 173 followed by two consecutive electrocydic reactions. Products such as 174 are useful precursors for benz[a]anthracene-7,12-diones (Scheme 8.43) [110]. 

The unsymmetricall allene 189 dimerized to give pyranopyran 190. A hetero-Diels-Alder reaction was followed by the subsequent electrocydic ring closure reaction [8]. 

Vinylallenes and bisallenes participate in the Diels-Alder-type cycloaddition as the diene component, providing a powerful tool for the construction of complex ring systems. They also undergo thermal electrocydic ring closure to form methylenecy-clobutene derivatives. 

The electrocydic ring opening of the cydobutene 196 at 140 °C in 1,4-CHD produced in situ the enyne-allene 197 (Scheme 20.41) [78]. Cycloaromatization to the biradical 198 followed by hydrogen-atom abstractions then produced the phenol 199. 

Photochemical electrocycltc ring-closure in a 4-electron system works well for many acyclic dienes (2.17) and related cyclic systems 12.18). The situation with conjugated trienes is more complex, and they can act as 6-electron systems (2.19) leading to cydohexa-1,3-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond Iwhich must be c/s if a 6-electron electrocydic ring-closure is to take place). 

An important group of conjugated diene/triene systems are those in the vitamin D series. The key reactions in the commercial manufacture of vitamin D (and probably also in its formation in skin exposed to daylight) are a photochemical, conrotatory electrocydic ring-opening in the provitamin, and a thermal 1.7-shift of hydrogen in the previtamin so formed (2.23). High conversions to the vitamin are not normally possible because all three species absorb appreciably at the... 

Vedejs, E. Monahan, S. D. Metalation of oxa-zole—borane complexes a practical solution to the problem of electrocydic ring opening of 2-lithiooxazoles./. Org. Chem. 1996, 61, 5192— 5193. 

Fig. 10. Shair s asymmetric synthesis electrocydic ring-closure and introduction of the carboxylic acid side chain. MOM = methoxymethyl.

The 6-electron system required for electrocydic ring-closure can be made up from any combination of aromatic and alkene double... 

Electrocydic reaction A ring closure that involves cyclic movement of electrons. 

Another reaction that can be described in terms of orbital symmetry is the ring closing reaction of cis—1,3—butadiene to produce cyclobutene. The reaction is an example of a class of reactions known as electrocydic reactions. There are two different pathways that can be imagined for this reaction, which can be illustrated as shown in Figure 9.5. Both mechanisms involve rotation of the terminal CH2 groups but in different ways. 

The simplest class of reaction that is conveniently treated by the Woodward-Hoffmann rules is that termed electrocydic. An electrocydic reaction is defined as the formation of a single bond between thejends of a line system of tt-electrons and the reverse process. It is examplified by the thermal ring-opening of cyclobutenes to butadienes ... 

The photochromic compound appears to interconvert in the solid state to a bright blue azomethine-yUde (I) by an electrocydic ring cleavage. An ylide is a neutral species whose Lewis stmcture contains opposite charges on adjacent atoms. The atoms involved are carbon, and an element from either group 5A or 6A of the periodic table, such as N, R or S. Of considerable interest is the fact that the photochemical process has been shown to give exclusively the syn isomer. 

All cw -cyclononatetraene undergoes a spontaneous electrocydic ring closure at 25°C to afford a single product. Suggest a structure for this product. Also, describe an alternative symmetry-allowed electrocydic reaction that would lead to an isomeric bicyclononatriene. Explain why the product of this alternative reaction pathway is not formed. 

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