Two Five-Membered Rings

Most work has been concerned with molecules of types 8.185-8.188, and in particular the thienothiophenes, selenolothiophenes, and selenolo-selenophenes, (8.185-8.187). Many MO calculations have been carried out, and all of them are unsatisfactory in some respect. For example, tt densities (calculated by the SCF-CNDO/2 method) indicate that, as expected, the 2-positions of 8.185 and 8.186 should be more reactive than the 3-positions however, the 3-positions are predicted to be deactivated, which is incorrect. Positions a and (3 to selenium were predicted be be, 

The ability of thienothiophenes and selenoloselenophenes to transmit substituent effects has been determined by H and l3C NMR, and also calculated by the CNDO/2 method [82CS(20)208 83CS(22)22]. As expected, resonance effects are better transmitted between the 2- and 5-positions in 8.185 than in 8.186, since only in the former is direct conjugation between these positions possible. Both resonance and inductive/field effects were greater in the sulfur- relative to the selenium-containing compounds, but in general, replacement of sulfur by selenium caused only minor perturbations of data, which follows from the fact that the main mode of transmission of substituent effects in five-membered heterocycles must be via the carbon chain (cf. Chapter 6, Section 9.C). 

For selenolo[2,3-b)selenophene (8.186, X, Y = Se) comparisons of the transmissions of substituent effects between the 2-position on the one hand, and the 3-, 4-, and 5-positions on the other, showed the following generalizations. 

Deuteriation of thieno[3,2-c]isoxazole (8.199, X = S) and selenolo-[3,2-c] isoxazole (8.199, X = Se) occurred at the 3-position in each case [76CS(10)165], This is the expected result because of the following factors. 

Nitration and bromination of (8.199) (X = S) also went into the 3-position, but for the selenium compound, attempted nitration caused decomposition, and bromination produced 2,3-addition [76CS(10)165]. 

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The pyrrolidine enamine of cyclopentanone, in which the double bond is exocyclic to two five-membered rings, shows the vinylic proton at 237 Hz (7S). 

The cis stereochemistry of (7) is irrelevant to (10 or (11), as cyclisation can give only ais fusion of two five-membered rings, but it might be a problem for (9). In any case, no disconnection, other than removing single carbon atoms, can easily be made on (9). Much the same is true of (12), but reconnection of the 1,6-dicarbonyl in (13) provides a good starting material (14). 

Spiro compound (18), also containing two fIve-membered rings, can be made by oxidation of the acyloin (19) (Chapter T24). 

This compound contains two five-membered rings, but it also contains a six-membered ring with all the hallmarks of a Diels-Alder adduct. The real problem is how to make (25). 

Hypervalent Structures with a Pentacoordinated Phosphorus and Two Five-Membered Rings... 

However, compound 57, containing two 1,3-azaphosphetidine fragments, obtained by heating tetrakis(aminomethyl)phosphonium (38) chloride or tris(anilinomethyl)phosphine (40) in ethanol, is described in Eq. (28) (72JOC2752 77JOC4040). A detailed analysis of NMR spectra made it possible to reject a structure with two five-membered rings. 

Scheme 33 illustrates the difference in reactivity between triazolines obtained from cyclohexanone and cyclo-pentanone enamines. Thus, the reactions of azidophosphonates 239 with cyclohexanone enamines produce unstable aminotriazolines 240 that cannot be isolated due to their spontaneous elimination of amines to provide triazoles 241. Contrary to that, triazolines 242, derived from cyclopentanone enamines, are isolated in good yield (76-88%) and cannot be converted to the corresponding triazoles even by thermolysis <1995H(40)543>. Probably, introduction of a double bond between two five-membered rings would involve too much molecular strain. 

While SN4 and STD exhibit essentially D2d symmetry, SN3 has Cs symmetry with a planar diene ring and an envelope-shaped cyclopentene ring. The maximum torsion in the folded ring of SN3 is 20.2°. The spiro-connection of two five-membered ring systems leads to some strain at the spiroatom (101.4° to 101.8° at A1 compared to 109.5° for tetrahedral... 

Is this value of 330 kJmol-1 plausible Were pentalene a normal polyene, we would anticipate an enthalpy of formation of ca 4.52,5 + 5.5 or ca 235 kJmol-1. There is thus ca 100 kJ mol-1 of destabilization. Is this due to antiaromaticity since we recognize pentalene as a derivative of planar [8]annulene We think not, for there are two five-membered rings in pentalene each contributing ca 30 kJmol-1 of strain apiece104. 

If a tether is attached to the C(3) atom of nitronates (214), the reaction gives rise to spiro systems (215). For these systems, generating a linkage containing three atoms between the six-membered ring and the C,C double bond is a case of choice. It necessarily leads to cis -fusion of two five-membered rings. 

The irradiation of benzenes with alkenes provides a fascinating array of photochemical reactions, not least because it converts the aromatic substrates into polycyclic, non-aromatic products. In principle, benzene can undergo reaction across the 1,2-(ortho). 1,3-(meta), or 1,4-(para) positions the 1,3-cycloaddition is structurally the most complex, but it is the predominant mode of reaction for many of the simplest benzene/alkene systems. The products are tricyclic compounds with a fusion of two five-membered rings and one three-membered ring, and an example is the reaction of benzene with vinyl acetate (3.411. For monosubstituted benzenes there can be a high... 

Various alioyolio epoxides have been subjected to the action oJ oarbanionoid reagents. Cyclopentene oxide (Eq. 887) is unique thus far, in that it gives no lactone.870 Lactone formation is prevented b the high activation energy required to establish irwwt-fusion between two five-membered rings.870... 

Van Tamelen 19 proposed that if such a mechanism is operative, one might expect the conversion of cyclopenteno oxide (lie) inlo cyclopentene sulfide (Via) (Eq. 2) to be difficult since it would involve a strained intermediate (IVo possessing two five-membered rings fuscil in a Irons sense. Experimentally, treatment of (lie) with potSHriuni... 

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