Five adjacent ring hydrogen atoms

In certain very limited cases the 750 cm band shows splitting into a group of weaker bands at the same frequency. This occurs in the cw-forms of phenyl propenyl carbinol and styryl methyl carbinol [37], although the trans forms have only a single band of normal position and intensity. The reasons for this are not known, but it may be connected with steric interferences in the cis-compounds. Benzoyl derivatives also give rise to distortions in this region. The 750 cm band is not seen in the Raman spectrum and the 690 cm band appears only weakly. 

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Now for the ring, and you are advised to make some deductions before you start thinking about the conformations of five-membered ring systems. We have a double bond and a ketone in this structure. These both confer planarity on adjacent atoms. As a result, we must conclude that the carbon atoms of the ring must all be in one plane. Only the hydrogen and hydroxyl substituents will be out of plane. The stereodrawing, therefore, requires a planar five-membered ring seen in perspective, as shown. 

The spectrum of one of the SMA copolymers investigated (Figure 2) is also consistent with that of published spectra (33). The strong absorption band at approximately 706 - 764 cm"l are indicative of aromatic rings containing five adjacent hydrogen atoms, while the absorption bands at 1780 and 1860 cm l are consistent with those of a saturated five membered cyclic anhydride. The two shoulders on the 1780 cm" peak possibly indicate some hydrolysis as the result of processing. 

Phospholanes are five-membered rings containing phosphorus atoms. At the beginning of 1990s, Burk and co-workers (155) prepared the trans-2,5-disubstituted phospholanes DuPHOS and BPE, depicted in Figure 42. These compounds have been successfully used as ligands in Rh-catalyzed hydrogenations (156). The carbon atoms adjacent to the phosphorus that are... 

Pyrrolizidine alkaloids (PAs) contain two fused five-membered rings in which a nitrogen atom is coirunon to both rings. The precursor is ornithine. A hydrogen atom nsnally occurs opposite the nitrogen atom in the a position, with a hydroxymethyl substitute at the adjacent C-1 position. Most pyrrolizidines have diester groups at C-1 and C-7—these may be non-cyclic, for example echimidine, or macrocyclic, for example senedonine. Toxicity (see below) appears to be mainly linked to the macrocyclic diester type (Denham 1996). The alkaloids also occur as water-soluble nitrogen oxides. 

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