Open-chain analogs

Heterocycle (kJmor ) Open chain analog (kJ raor )... 

A dramatic decrease in the magnitude of the magnetic susceptibility anisotropy is observed on going from thiirane to the open-chain analog, dimethyl sulfide, and has been attributed to non-local or ring-current effects (70JCP(52)5291). The decrease also is observed to a somewhat lesser extent in oxirane relative to dimethyl ether. 

Heterocyclic amines are compounds that contain one or more nitrogen atoms as part of a ring. Saturated heterocyclic amines usually have the same chemistry as their open-chain analogs, but unsaturated heterocycles such as pyrrole, imidazole, pyridine, and pyrimidine are aromatic. All four are unusually stable, and all undergo aromatic substitution on reaction with electrophiles. Pyrrole is nonbasic because its nitrogen lone-pair electrons are part of the aromatic it system. Fused-ring heterocycles such as quinoline, isoquinoline, indole, and purine are also commonly found in biological molecules. 

With respect to the nucleophilic addition of organocopper reagents, a sharp contrast between the rigid isopropylidene glyceraldehyde and its open-chained analog, 2,3-bis(benzyloxy)propanal. was observed (compare Tables 15 and 16). With the isopropylidene-protected aldehyde a high syn diastereoselectivity could only be obtained when tetrahydrofuran was used as reaction solvent, and the diastereoselectivity dropped considerably in diethyl ether. In contrast, the latter solvent allows excellent syn selectivities in additions to the dibenzyl-protected glyceraldehyde81. On the other hand, tetrahydrofuran yields better results than diethyl ether in the... 

No compound other than the methyl ester of N-benzoyl-Lphenylalanine, 33, is an obvious choice for an open-chain analog of the locked substrate 25 but D-24, on the other hand, may be a locked analog of either N-benzoyl-D-alanine methyl ester 34 or of N-formyl-D-phenylalanine methyl ester 35 (75). If 24 is an analog of 34 rather than 35, the comparison of the two locked analogs made in Section V.B. is not valid the phenyl of 24 would then correspond to the benzoyl phenyl of 34. 

In contrast, substitution of oxygen for —NH— in open-chain compounds causes a great decrease in activity. He states that the heterocyclic ring of D-24 locks the carbomethoxy group at the active site and thus accomplishes the same amount of restriction as does binding of both the aromatic and acylamido groups of the open-chain analogs (82). 

The UV spectra of 4,5,7,8-tetrafluoro[2.2]paracyclophane (26) 18> and of the octafluoro compound 27 54> reveal the close relationship of these compounds to unsubstituted [2.2]paracyclophane (2). The absorption bands occurring between 286 and 291 nm, like those in the spectra of 2 can be attributed to deformation away from planarity of the aromatic rings. Compared with the fluorine-substituted open-chain analogs, these absorption bands are likewise bathochromically shifted by about 25 nm. 

The unsaturated cyclic compounds 32 and 33 show none of the absorption characteristics of the open-chain analog, cis-stilbene, since the 7 electrons of the benzene rings and the C=C bonds cannot overlap because of the rigid architecture of the molecules. 

The binary system of 21a and 21b has been studied and compared with that of the open-chain analogs. Both 1,2,3,4-tetrahydrodibenzo-thiophene 5-oxide and octahydrodibenzothiophene 5,5-dioxide have been studied as engine oil lubricant additives, although the preparation of neither of these compoimds has been recorded. Dibenzothiophene 5-oxide (21a) is reported to he phytotoxic. ... 

The chemistry of cyclic hydrocarbons and their corresponding open-chain analogs is similar. Exceptions are the cyclopropanes, whose strained rings open easily, and the cyclobutanes, whose rings open with difficulty. The larger rings are stable [see Problem 9.5(c)]. 

A broad, structureless fluorescence emission is observed for [2.2], [3.3], and [4.4] paracyclophane, but only structured monomer emission is seen in [4.5] and [6.6] paracyclophane. The fluorescence properties of the [2.3], [2.4], [3.4], [3.6], [4.6], [5.5], and [5.6] paracyclophanes have not been reported, although the latter three would be expected to yield only monomer emission. The UV absorption spectra of all of the above paracyclophanes have been reported, and all [m.n] phanes for which both m and n are 4 have absorption spectra that are identical to 1,4-bis (4 -ethylphenyl)butane, the open-chain analog. The UV absorption spectra of other paracyclophanes become increasingly red-shifted and broadened in the order [3.6], [3.4], [2.4], [3.3], [2.3], and [2.2] paracyclophane. 

The naphthalenophanes that have been synthesized to date are listed in Table 6, in order of their discovery. The [m.n] isomers for which m,n > 3 have not yet been synthesized. References for the UV absorbance, fluorescence, and other properties of existing naphthalenophanes are given in Table 6. The UV absorption spectra of all the naphthalenophanes are red-shifted and broadened relative to their respective open-chain analogs, similar to the [2.2] and [3.3] paracyclophanes. Moreover, broad and structureless emissions have been observed for the naphthalenophanes in all references cited in Table 6 except one.107) The structural aspects of naphthalenophane photobehavior will be discussed in detail in the following paragraphs. 

The naphthalenophanes that are fully eclipsed, i.e. the sj>n-[2.2](l,4), achiral [2.2](1,5), achiral [3.3](2,6),. n -[3.3](l,4), and syn-[2>,2] A) isomers, share certain traits in absorption and fluorescence. The UV absorbance spectra of these compounds between 260 and 310 nm retain all of the structure shown in the spectra of the open-chain analogs. Also, new absorption shoulders not seen in the open-chain spectra appear strongly at 245 and weakly at 340 nm. The fluorescence peak of these fully eclipsed naphthalenophanes occurs near 22,000 cm-1, as seen in Table 7. This represents a red shift of 2600 cm-1 relative to the solution excimer of the dimethylnaphtha-lenes.71)... 

The remaining naphthalenophanes in Table 6, which are noneclipsed, show little vibrational structure in their UV absorption spectra relative to the open-chain analogs. 

Heterocycle Proton affinity (kJ mol-1) Open chain analog Proton affinity (kJ mol-1)... 

Cycloalkenes and cycloalkadienes are about as reactive chemically as their open-chain analogs. Cycloalkenes can undergo addition reactions in which the double bond is eliminated and also can undergo cleavage reactions which cause the ring structure to be opened into a chain. 

Carbon-13 shift of common non-aromatic heterocycles with endo- and exocyclic double bonds are reviewed in Table 4.66 [416-432], - Deshieldings of / -carbons induced by carbonyl groups in heterocyclic a, /1-enones due to (—)-M electron withdrawal (e.g. 2-pyrones, coumarins) and shieldings of [ carbons in cyclic enol ethers arising from (+ )-M electron release (e.g. 2,3-dihydrofuran and oxepine derivatives in Table 4.66) fully correspond to the effects described for the open-chain analogs. Outstandingly large shift values are observed for the lithiated carbon in cyclic a-lithium enol ethers (Table 4.66). In terms of its a and / carbon-13 shifts, 2,7-dimethyloxepine is also a typical enol ether [420], Further, 2,6-dimethyl-4-pyrone [421] and flavone [422] display similiar shift values for the a, /1-enone substructure. 

Since 1-azirines are well known, it does not seem reasonable to attribute the nonexistence of 2-azirines exclusively to strain energy. An unfavorable electronic situation is most likely responsible for their alleged instability. As mentioned previously, the 2-azirine ring system is a cyclic conjugated necessarily planar n system containing four 7r electrons and Hiickel s rule would not predict it to be stabilized by delocalization. In fact, HMO theory predicts that delocalization results in a less stable 7r system than the open chain analog.5 Such systems are predicted to be relatively unstable and have recently been designated as antiaromatic.8,9... 

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