Parent monocycles

Structure of Five-membered Rings with One Heteroatom 

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Various methylene derivatives of spiroorthocarbonates and spiroorthocstcrs have been reported to give double ring-opening polymerization e.g. Scheme 4.36). Like the parent monocyclic systems, these monomers can be sluggish to polymerize and reactivity ratios are such that they do not undergo ready copolymerization with acrylic and styrenic monomers. Copolymerizations with VAc have been reported.170 These monomers, like other acetals, show marked acid sensitivity. 

Optimization at the restricted Hartree-Fock (RHF) and DFT (B3LYP) levels in the 6-31G basis set has also contributed to structural clarification of [l,2,5]thiadiazolo[3,4-f][l,2,5]thiadiazole 6 both approaches successfully predict that 6 ought to have planar equilibrium geometry. Direct a3 initio ipsocentric calculations of the total 7t-current density map for 6 permits classification as aromatic according to magnetic criteria. It exhibits a strong and uniform diatropic perimeter circulation, of comparable intensity to the parent monocycle 11 and similar to that of naphthalene <2004JA11202>. 

Deprotonation from the azonium group leaves a lone pair of electrons on the nitrogen atom, and a neutral aza substituent. The known parent monocyclic azines (see Scheme la) include all the possible diazines and triazines, but only one tetrazine, the 1,2,4,5-isomer. Some 1,2,3,5-tetrazines have been reported, but only when heavily substituted or fused. Some aromatic bicyclic 1,2,3,4-tetrazines have been prepared (see Section 4.4.8.2.3) as well as reduced 1,2,3,4-tetrazines (see CHEC 2.21). No pentazines are known. All attempts to prepare hexazine also failed though several claims about fixation of the latter in a matrix have appeared. 

Parent monocyclic Polycyclic derivatives OH-Substituted derivatives NH2-Substituted derivatives compounds... 

The electron-withdrawing effect of the isothiazole unit is evident in the proton chemical shifts of H-3 and H-4 in 5-phenylfuro[3,2-d]isothiazole (15), V-methyl-5-phenylpyrrolo[3,2-chemical shift of H-4 (see structures (14a) and (17)) <82AJC385>. 

CO bands are expected to have absorption frequencies as in the parent monocycle or in their benzo homologues, e.g. (1). The tautomerism of hydroxy and amino compounds largely... 

The amino form is the dominating tautomer in the amines in the same way as in the parent monocyclic rings. 

Hydroxy derivatives are expected to show the same tautomeric preferences for the oxo form in ortho and para positions to the nitrogen as in the parent monocycle. Thus, for the 2-hydroxythiazole (395) comparison of the spectroscopic data with the data for its AT-methyl and O-methyl derivatives is consistent with the oxo form the same preference has been shown for its [5,4-c] isomer (68CJC691). 

High-accuracy molecular dimensions for the parent monocyclic heterocycles have been determined by micro-wave spectroscopy and these can be found in Section 2.3.3.2 (Table 7). The ring dimensions obtained by X-ray diffraction for the 2-carboxylic acid derivatives of furan, thiophene, selenophene, and tellurophene (Table 3) are generally in good agreement with those obtained for the parent heterocycles using microwave spectroscopy (Table 7). 

The proton NMR spectra of the 5 5-fused heterocycles 95-97 have a close similarity to the parent monocycles. The protons at positions a and (3 to the heteroatoms resonate at frequencies analogous to the parent molecules. The similarity of the H NMR spectra of l//,4//-pyrrolo[3,2-A pyrrole 95 (X = Y = NH) and pyrrole suggests that both ring systems have similar ring currents and aromaticity. An upfield shift (0.17 ppm) of the (3-proton in the bicycle 95 (X = Y = NH) reflects the influence of the fused pyrrole ring on electron density <1984TL5669>. 

In fully-conjugated 5 5-fused heterocycles the ring protons are found in the aromatic region but their position is influenced by the adjacent ring. Thus, in the series 111-113 the isothiazole unit has an electron-withdrawing effect and the left-hand ring protons are deshielded relative to the parent monocyclic heterocycles (Figure 18) . 

Only relatively little recent work concerns the parent monocyclic system by far the greater part is devoted to fused ring systems incorporating 1,2,5-thiadiazole. 

Lipton et al. noticed that the switching of cyclization pathways observed by Schmittel takes place due to a combination of effects. The calculations of the energies of diradicals formed by Myers-Saito and Schmittel cyclization indicate that benzannulation of the eneyne-allene as in 15 plays a significant role in promoting C2-C6 cyclization. The energy of diradicals for the benzannulated system is 10.5 kcal/mol less as compared to those for the parent monocyclic diradicals (16 vs. 13, and 17 vs. 14). ... 

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