Propellenes

Stmcturally novel pyrazole derivatives include the propellene 2,3,4,5,6-pentakis(pyrazol-r-yl)pyridine 1 and the corresponding 3 ,5 -dimethylpyrazole derivative 2<96T11075>. Poly(pyrazol-l-ylmethyl)benzenes, such as 3, have been prepared as multidentate ligands <95AJC1587>. Solid phase synthesis of structurally diverse 1-phenylpyrazolones was reported, with application to combinatorial synthesis <96SL667>. 

We tried in 1968, as it turned out, unsuccessfully, to prepare a propellane having the [4.2.2] nucleus (actually it was a [4.2.2]propellene). But we were most successful in preparing the isomeric dispiran 7. None of the desired propellane was formed (see reaction scheme) ... 

Further, a very recent paper has reported a cascade rearrangement, under acidic conditions of the simpler educt, a dispiro[3.0.3.3]undecane derivative 93 to the dehydrated isomeric propellane 94 40). It is somewhat reminiscent of the analogous case of 26 where silver ion is the catalyst10). Treatment of the dispiro-alcohol 78 when heated for 2 hrs at 70 °C with p-toluenesulfonic acid in benzene gives in quantitative yield the [3.3.3]propellene 94. The following cascade is proposed to explain the rearrangement. 

The conformational properties of aromatic heterocycles are related to those of phenyl rings but with the added perturbations due to heteroatoms. We have carried out a systematic study of what we called aromatic propellenes, the heterocyclic analogs of hexaphenylbenzene (13) [27, 28], which possess interesting conformational [29] and metal complexing properties (14) [30],... 

The role of secondary orbital interactions in control of the cycloaddition to substrates (Table 4, entries 12-14) has been invoked. Hence, the attractive interaction between the it orbitals (LUMO) of the carbonyls in the reference ring and the n -antisymmetric combination of the nitrogen lone pair orbitals stabilize the syn transition state for dienophilic attack at the syn cyclohexadiene face18,22 23. This view is supported also by the finding that maleimidcs and benzoquinone, which do not have lone pairs on the pertinent carbon atoms, add to propellenes (Table 4, entries 12 and 13) by exclusive anti addition (below the plane of the molecule)24. 

Steric effects are superimposed upon the slereoeleclronic one in the case of propellenes (Table 1, entries 18 and 20), carrying large nitrogen substituents, resulting in nonstereoselective cycloadditions. 

A propellene has been converted to a conjugated propelladiene 10,10-dibromo[4.3.1]pro-pell-3-ene reacted with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone to give 10,10-dibro-mo[4.3.l]propella-2,4-diene in 35 /o yield. -... 

In contrast to the [5.3.1]propellanes, reaction of chloro[4.3.1]propellene 33 or dihalo[4.3.1]propellane 34 with t-BuOK yielded Dewar benzene isomer 35 of... 

Claramunt et al. dedicated several publications to the study of the structure of polyazolyl-benzenes (propellenes) (97M12). These compounds present several N-C bonds between five (azoles) and six-membered rings (benzenes). Besides the dihedral angles the position (up or down) of the azolyl residues are characteristic of these compounds hexa(3,5-dimethyl-pyrazol-l-yl)benzene (16, R = 3,5-dimethyl) (95JCS(P2)1359), hexakis(pyr-azol-l-yl)benzene (16, R = H) (96JPOC137, 02EJ1C3178), 1,2,4,5-tetrakis (pyrazol-l -yl)-3,6-bis(3",5"-dimethylpyrazol-l -yl)benzene (16, R = H... 

Ene Reactions.—Conia, who has done so much to demonstrate the synthetic utility and versatility of unsaturated carbonyl compound cyclizations, as exemplified by the prototype conversion of(271) into (272), has reviewed recent developments which includes applications to bridged-ring compound synthesis. The same group have reported on the double thermal cyclization of 3,3-dibutynylcyclopentanone (273) to give the [3,3,3]propellenes (274 60%) and (275 5%), and the monocyclization product (276 10%). Since (274) is stable under the reaction conditions, (275) is presumably formed from (276) by isomerization prior to a second ene cyclization step. 

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