Decatetraenal

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

E,4Z,6Z,8 )-2,4,6,8-Decatetraene has been cyclized to give 7,8-dimethyl-1,3,5-cycloodati icne. Predict the manner of ring closure—comoiatory or disrotatory— for both thermal and photochemical reactions, and predict the stereochemistry of the product in each case. 

Attractive Compounds. The male-produced pheromones of sap beetles, known so far, show the rather stereotypic structures 125-147 (Scheme 15) methyl- and ethyl-branched hydrocarbons with three or four (T)-configured conjugated double bonds [4]. Up to now, 23 compounds could be identified, forming species specific mixtures. Major components in the bouquets are (2E,4E,6E)-5-ethyl-3-methyl-2,4,6-nonatriene, 128, in Carpophilus davidsoni [268] as well as in C.freemani [269], (2 ,4 ,6 )-4,6-dimethyl-2,4,6-nonatriene, 129,in C. truncatus [270], (3 ,5 ,7 )-5-ethyl-methyl-3,5,7-undecatetraene, 132, in C. mutillatus [271],(2 ,4 ,6 ,8 )-3,5,7-trimethyl-2,4,6,8-decatetraene, 134,in C. hemipterus [272] as well as C. brachypterus [273], (2 ,4 ,6 ,8 )-3,5,7-tri-... 

The mechanism of the degenerate [5,5]-sigmatropic rearrangements of 5,5a,10,10a-tetrahydroheptalene and (z, z)-decatetraene-l,3,7,9 has been explained. A stepwise diradical mechanism has been predicted for both reactions. ... 

The male-produced volatiles eficiting responses from male and female antennae of the sap beetle, Colopterus truncatus Randall (Coleoptera Nitidulidae), were identified as (2A ,4A ,6A )-3,5-dimethyl-2,4,6-octa-triene, 2E, AE, 6T )-4,6-dimethyl-2,4,6-nonatriene and 2E, AE,6E,SE) 3,5,7-trimethyl-2,4,6,8-decatetraene. A fourth male-specific compound... 

Figure 6.6 Structures of 2,3-dihydro-2,3,5-trimethyl-6-(1 -methyl-2 -oxobutyl)-4H-pyran-4-one (stegobinone), pheromone of Stegobium paniceum (L.) (Anobiidae) (Kuwahara et al., 1978) and (2E,4E,6E,8E)-3,5,7-trimethyl-2,4,6,8-decatetraene, pheromone component of Carpophilus hemipterus (L.) (Nitidulidae) (Bartelt et al., 1991). The carbon skeletons are identical, and likely derived through similar pathways (see text).

CoCl2 and Pl K yield 13 in which the pentadienyl groups have dimerized. The monomer CoPl 2 is a probable intermediate. In the decatetraene ligand the two butadiene units are conjugated (133) in contrast to the dimer formed on reduction of pentadienyltricarbonyliron cations (Section IV,D) in which the two units remain unconjugated (134). 

Figure 7. Time-resolved mass spectrometry. AU-trcms-(2, 4, 6, 8) decatetraene was excited to its 5 2 electronic origin with a femtosecond pulse at A-pump — 287 nm. The excited-state evolution was probed via single-photon ionization using a femtosecond pulse at ApIObe = 235 nm. The time resolution in these experiments was 290 fs (0.3 ps). The parent ion CioH signal rises with the pump laser, but then seems to stay almost constant with time. The modest decay observed can be fit with a single exponential time constant of 1 ps. Note that this result is in apparent disagreement with the same experiment performed at Xprobe — 352 nm, which yields a lifetime of 0.4 ps for the S2 state. The disagreement between these two results can be only reconciled by analyzing the time-resolved photoelectron spectrum.

Ea, activation energy AEreac reaction energy. For trans,cis,cis,trans-2,4,6,8-decatetraene [34]. 

The decatetraene cyclizes at — 10°C. by a conrotatory motion, to produce the cyclooc-tatriene with the two methyl groups in a trans orientation. When the cyclooctatriene is warmed to 20°C, it undergoes a thermally allowed disrotatory ring closure to give the final product. 

The predicted conrotatory nature of this electrocyclization has been definitively established. The conrotatory mode was found to operate in the individual thermal electrocyclization of the stereoisomeric decatetraenes (430), (431) and (432). The former pair (430 and 431) afforded the rranr-cyclooctatriene (433), while the latter (432) led to the dr-cyclooctatriene (434). Both cyclooctatrienes (433) and (434) were observed to be in equilibrium with the corresponding 6e electrocyclized derivatives (435) and (436), respectively. 

Problem 29.3 Thermal ring closure of three stereoisomeric 2,4,6,8-decatetraenes (1, II, and III) has been found to be in agreement with the Woodward-Hoffmann rules. 

Two of these stereoisomers give one dimethylcyclooctatriene, and the third stereoisomer gives a different dimethylcyclooctatriene. (a) Which decatetraenes give which cyclooctatrienes (b) Predict the product of photochemical ring closure of each. 

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