Introducing Strain

Interestingly, when the enediyne part is coupled with a moderately bulky group attached at the mefa-position through an alkyne Unker, the photochemical BC became quite efficient (44%). This result still awaits a proper explanation. 

SCHEME 30.4 Effect of annealed ring size on the photochenucal Bergman cycUzation. 

SCHEME 30.5 The photoreactivity of cyclopentene diynes is drastically increased upon methyl substitution at the alkyne termini. 

SCHEME 30.6 Competition between photo-Bergman cycUzation and retro-Beigman cleavage in photochemistry of cyclodec-3-ene-l,5-diyne. 

SCHEME 30.7 Photoreactivity of cyclic enediynes with polyaromatic core. 

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Rings have a profound influence on many properties of a molecule small rings introduce. strain into a molecule, aromatic rings dramatically change its physical and chemical properties, rings present particular problems in syntheses, etc. Thus, a knowledge of the I ings contained in a molecule is important in many applications in chemoinformatics. 

This stereoelectronic requirement would lead to a large distortion of the normal geometry of a five-membered ring and introduce strain. It is this distortion and strain that disfavor the 5-endo-trig cyclization. In contrast, 5-endo-dig cychzation is feasible because the acetylenic system provides an orbital that is available for a nearly planar mode of approach. 

Competition with endemic strains, which may eventually outnumber and eliminate the introduced strains. 

The bond distortions that are required to transform the bound ground state substrate to its transition state structure and then to the product state structure(s) are commonly facilitated by introducing strain into the substrate molecule, through confor-... 

For pulses shorter than v lps the sepctrum will be broader and less pronounced that for pulses longer than 1 ps. Otherwise stated, the electron states immediately after excitation find themselves in a "foreign lattice" and this introduces strains which take finite time to diffuse before the ir-electrons accomodate themselves in a new lattice. These initial strains broaden the spectrum in the initial stage of excitation (up to 1 ps) for time longer than 1 ps the spectrum coalesces to the one observed with very long pul-esor stationnary sources. 

The composition of 2-C-spirocyclopropyl-2-deoxy-L-arabinose was found64 (by l3C-n.m.r. spectroscopy) to be pyranoses, furanoses and acyclic forms in the ratios of 10.6 2.9 1.0. This is, actually, not a sugar having a fused ring but one with a spiro structure. The proportion of the acyclic form is surprising apparently the spiro arrangement introduces strain into both the pyranose and the furanose forms. 

If some of the binding energy were used to introduce strain or distortion within the enzyme or substrate molecules, such that tighter binding of the transition state were achieved, then the binding affinity of the enzyme for the substrate would be reduced. 

Small rings introduce strain inside the ring and higher s character outside it... 

Introducing strain into the Si—Si bond renders it more reactive to reactions with transition metal species. Thus, Tamao et al. (47) have observed the redistribution of Si for Si shown in Eq. (75). Fluorine substituents on... 

Some methods introduce strain directly by generating the new C-C bond in the cyclization step [3]. Wurtz-type cyclizations (modification by Miiller-Roscheisen... 

As some of the examples show, extremely strained molecules are easily accessible and fascinating distortions will be achieved by introducing strain by sulfone pyrolysis. 

Figure 24 Plot of B (cos9)/X as a fimction of (sm9)/X for (a) oxides ball milled to introduce strain and (b) the same oxides heated to 1350°C to remove the effects of stain. (Reprinted from Langford and Louer. Pearson Prentice HaU)...

As more methionines are substituted, the loss in stability is less than the sum of the stability loss for the constituent single replacements (Gassner et al., 1996). The difference between the sum of the losses in stability for the single methionine variants and the loss of stability for the ten methionine protein is 2.5 kcal/mol. As judged by the cavity calculations described above, some of the single mutants lose stability because of introduced strain. In the multiple methionine mutant. 

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