Cycloadditions triene formation

The reaction with furan forms a mixture of the two products with a preference for the [3+4] cycloadduct 88. Similarly, the reaction of furan with the vinylcarbenoid of si-loxy-substitilted vinyldiazoacetate 87 yields 90% of the [3+4] cycloaddition product with no triene formation. 

The terpenes a- and 6-myrcene are classical examples of substrates controlled by factor (ii). In these polyenes the relative reactivity of the nonconjugated C=C double bond (di- versus trisubstituted) controls the first reaction event. 6-Myrcene, a monoterpene triene with a trisubstituted nonconjugated double bond, undergoes preferentially the ene reaction with subsequent [4 + 2]-cycloaddition,whereas a-myrcene 57 solely reacts in a [4 + 2]-mode to give the endoperoxide 58. Alkoxy-substituted 1,3-dienes, which cannot undergo ene reaction, have been shown to be excellent substrates for [2 + 2]-cycloaddi-tion. The l,4-di-t-butoxy-l,3-butadienes were the first substrates that showed nonstereoselective [2 + 2]-cycloaddition reactions. Formation of the endoperoxide 60 was observed only from the ,Z-isomer 59, which indicates a pronounced activation effect by the alkoxy groups for dioxetane formation. 

The intramolecular cycloaddition has proven to be the method of choice for the preparation of steroids. A diastereomeric mixture of 204, prepared from 191 and tosylate 203 has been cleanly converted to dl-estra-1,3,5(10)-trien-17-one (205) in 85% yield (equation 130). A second example of the intramolecular cycloaddition reaction is the formation of the cycloadduct (209), the key intermediate in a synthesis of the As-pidosperma alkaloid aspidospermine, upon heating 208 at 600 °C (equation 131)124. The sulfone 208 can be prepared by reaction of 3-ethyl-3,4,5,6-tetrahydropyridine (206) with the acid chloride 207. 

The formation of bicyclo[3.1.0]hex-2-enes is a common reaction of cyclic and acyclic cZt-l,3,5-trienes. While it has been frequently depicted as a [n4 + tt2]-cycloaddition and it often exhibits the stereochemistry expected of a concerted [zr4s + rr2a] process109, it is well known that it just as often does not. The stereochemistry, conformational requirements and scope of the reaction have been thoroughly studied by a number of workers, and have been extensively reviewed116,128,170 172. 

Irradiation of the bis-alkene 210 brings about the formation of the bishomocubane 211 in good yield98. The triene 212 is of interest and has been shown to be photochemically reactive, yielding the adduct 213 on irradiation. Several approaches to 212 have been reported over the years. One such approach follows the path of photocyclization of 214 to yield 215 that can be converted to the desired product 21299. A variety of sensitizers can be used for the excitation of alkenes to bring about the (2 + 2)-cycloaddition. Commonly, acetone has been used but, in at least one case, the formation of the cage compound 216 from the diene 217, tetraphenylporphine has been found to be of use100. 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

The nature of the dipolar intermediate is crucial in the reaction with furans, since the product distribution is greatly influenced by the substituents present on the furan and vinylcarbenoid (Tab. 14.10) [84—86]. Electron-rich 2-methoxyfuran leads to exclusive formation of triene 89 with vinyldiazoacetate 42, whereas the corresponding reaction with 2,5-dimethylfuran affords only the [3+4] cycloaddition product 88 in 70% yield. 

The formation of the cyclooctatrienes was quenched in the presence of low concentrations of 2,5-dimethylhexa-2,4-diene or 2,3-dimethylbuta-l,3-diene. The photocyclization of the trienes was, however, unaffected by the presence of the dienes. The authors conclude that the primary photoreaction of intramolecular ortho cycloaddition involves the 3 tt, tt state of the arene and the subsequent cyclization of the cycloocta-l,3,5-triene arises from the singlet state. 

Irradiation of conjugated trienes such as 51 can also lead to the formation of bicy-clo[3.l.0]hexanes (52 or 53) via a process that can be considered as a formal [ r4s + 7r2a] or [7r4a r2s] cycloaddition (Scheme Thus, triene 51 affords 52 and... 

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