Electrocydic ring opening

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. 

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. 

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. 

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 ... 

Electrocydic reactions have proved especially suitable as a structural unit for photochromic systems. Scheme 3 shows four different types of ring-opening reactions. However, only a few of the many possible electrocyclizations are important with regard to application. 

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]. 

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