Benzoannelated cis,cis,cis,trans-[5.5.5.6]fenestranes: syntheses, base lability, and flattened molecular structure of strained epimers of the all-cis series

Tribenzofenestranes possessing the strained cis,cis,cis,trans-[5.5.5.6]-fenestrane skeleton have been synthesized from cis-2,6-diphenylspiro[cyclohexane-1,2'-indane]-1',3'-diols by two-fold cyclodehydration, in striking analogy to the strategy used previously to construct the stereoisomeric all-cis-tribenzo[5.5.5.6]fenestranes from the corresponding trans-diphenylspirodiols. In this manner, both of the parent hydrocarbons, all-cis-tribenzo[5.5.5.6]fenestrane 3 and cis,cis,cis,trans-tribenzo[5.5.5.6]fenestrane 4, have been made accessible from the spirodiketones 5 and 6, respectively. The C6-functionalized derivatives of 4-cis,cis,cis,trans-fenestranol 9 and cis,cis,cis,trans-fenestranone 12-were prepared through cis-diphenylspirotriol 8 and cis-diphenyldispiroacetaldiol 11, by using the same strategy. The cis,cis,cis,trans-[5.5.5.6]fenestrane framework readily epimerizes to the more stable all-cis isomers under basic conditions, but is stable under neutral or acidic conditions. For example, cis,cis,cis,trans-fenestranone 12 yielded all-cis fenestrane 3 under Wolff-Kishner conditions, but cis,cis,cis,trans-isomer 4 under Clemmensen conditions. Epimerization was also circumvented by radical-induced desulfurization of fenestrane dithiolane 15 with nBu3SnH/AIBN, producing 4 in excellent yields. A single-crystal X-ray structure analysis of 4 revealed that, in accordance with force field and semi-empirical MO calculations, the extra strain of the benzoannelated cis,cis,cis,trans-[5.5.5.6]fenestratriene framework [Estrain(4)-Estrain(3)=46 kJmol(-1)] is due both to the almost perfect boat conformation of the six-membered ring and to considerable bond angle widening at the central, non-bridged C4b-C15d-C11b unit (121 degrees). H/D exchange experiments with the cis,cis,cis,trans hydrocarbon 4 under basic conditions demonstrated that the strain-induced epimerization to 3 occurs through direct deprotonation of the "epimeric" benzylic bridgehead C7a-H bond, which was found to be more acidic than the two C-H bonds at the benzhydrylic bridgeheads.     

Coriolis coupling constants of cis and trans-1,2-dichloroethylene

Coriolis coupling constants and the inertia defect for cis- and trans-1,2-dichloroethylene have been calculated, using force constants obtained by the iterative consistency method. The results obtained are compared with those of previous workers and it is shown that two sets of force constants which reproduce the vibrational frequencies satisfactorily lead to completely different values for the Coriolis coupling constants.     

Semiexperimental equilibrium structure for cis,trans-1,4-difluorobutadiene by the mixed estimation method

The available experimental rotational constants of cis,trans-1,4-difluorobutadiene do not permit a determination of a complete structure. However, this problem, rather frequent in finding structures, may be solved by the mixed estimation method. The experimental ground state rotational constants are corrected for the rovibrational contribution calculated from an ab initio force field. These semiexperimental data are supplemented by structural parameters from ab initio calculations and a weighted least-squares fit allows us to obtain a reasonable structure. The accuracy of the fitted parameters is checked by optimizing a structure at the coupled cluster level. A good agreement is found between the two methods, validating our procedure.     

Stereochemistry of the [4 + 2] cycloadditions of trans,trans- and cis,trans-2,4-hexadiene to C(60)

The [4 + 2] cycloaddition of trans,trans-2,4-hexadiene with C(60) proceeds via a concerted mechanism with retention of stereochemistry in the cycloadduct 1a. However, when cis,trans-2,4-hexadiene reacts with C(60), isomerization of the cis,trans to the thermodynamically more stable trans,trans isomer occurs. Subsequently, the cis,trans diene isomerized to the trans,trans isomer and cycloadds to C(60), to form adduct 1a. When the reaction is carried out at higher temperatures, the formation of cycloadduct 1b is also obtained. This result is consistent with a concerted cycloaddition of cis,trans-2,4-hexadiene with C(60), which is more reactive at elevated temperatures and leads to the formation of the Diels-Alder adduct 1b.     

顺,顺,顺-3,6,9-二十碳三烯及顺,顺,顺-3,6-9-二十一碳三烯的合成

由于昆虫信息素研究的需要,我们曾于1979年合成了顺、顺、顺-3,6,9-二十碳三烯(1)和顺、顺、顺-3,6,9-二十一碳三烯(2),最近Conner等报道,1和2为鳞翅目昆虫信息素组分,并发表了以亚麻油酸及亚麻油醇为原料的合成路线。本文报道我们的合成路线,以丙炔醇为原料,经2-戊炔醇-[1](3),1-溴-2-戊炔