Small R group

Directed ortV o-methyl lithiation/cyclization of AT-acyl-o-toluenesulfonamides 229 provides a second approach to the synthesis of 1,2-benzothiazine 1,1-dioxides 230 via creation of the bond between C-3 and C-4 (Scheme 31) <1999CPB1730>. The best yields of cyclization products are achieved when the R group on the amide 229 is bulky. In the case of substrates with small R groups (Me, Ph, etc.), products 231 and 232 are formed from attack of the Bu"Li on the carbonyl carbon. 

These acidic and basic properties can be used to help identify these functional groups. If an unknown compound is water soluble (has a small R group), then the pH of an aqueous solution of the compound provides an important clue to the functional group that it contains. If the unknown is not soluble in water, its solubility behavior in aqueous acid and base provides the same information. Consider, for example, a carboxylic acid that is not soluble in water. If this acid is added to an aqueous solution of sodium hydroxide, the following reaction occurs ... 

Alternatively, a protein might have a pleated sheet conformation, more common with sequences of amino acid units with small R groups on the a-carbon (see Figure 3-lb). Thus, the secondary structure for a given protein depends in large part upon the tendency of the R groups to attract or repel each other along the chain. In other words, the secondary structure is dependent upon the primary structure. 

The (3-strand sequences are stretched out conformations of these polypeptide sections and are typically stabilized by inter-strand hydrogen bonds between keto (C = 0) oxygens and peptide bond NHs, the strands being arrayed in an antiparallel fashion. This type of secondary structure is favoured by amino acid residues with small R groups (such as Gly, Ala and Ser) that minimize steric overlap between chains. Thus a well-known protein having this type of secondary structure is silk fibroin that has a high proportion of repeated sequences involving Gly, Ala and Ser and an extensive antiparallel (3-pleated sheet structure. The macroscopic properties of silk fibroin (flexibility but lack of stretchability) reflect this type of secondary structure at the molecular level. 

Physical properties of a number of organobismuth complexes have been compiled. The triaUcylbismuth compounds are generally very oxygen sensitive, though stable to moisture. Many triarylbismuth compounds are air stable as well as stable to moisture. The triaUcyls with small R groups are colorless liquids, which may be distilled imder vacuum. Triphenylbismuth is a commercially available, air stable, white crystalline solid. 

Silicon is known to have a high oxophilicity and we thought that the use of (/Bu2SnO)3 in reactions with R2SiCl2 and RSiCb, respectively, would allow the controlled synthesis of Si-0 rings and cages of defined size. However, for small R groups, such as Me and Ph, mixtures of various products were obtained, which have not been separated yet. 

With a, p-un saturated esters, competition between cydopropa nation (l,4addition) and acylation (1,2-addition) becomes acute. With ethyl acrylate (63) and ethyl crotonate (64), the small R groups on the p-carbon atom permit facile conjugate addition leading to good yields (60-65%) of the corresponding cyclopropane derivatives (67) (Scheme 25). On the... 

Because of conflicting reports or inadequate controls, the question of kinetic or thermodynamic control of stereochemistry for reported Reformatsky reactions often has no satisfactory answer. Jacques and co-workers have concluded that Reformatsky reactions of benzaldehyde in refluxing benzene can be completed with kinetic stereoselection. The relatively high syn.anti ratios they observed, at least with small R groups (equation 36 and Table 4), are not those expected for equilibrated zinc chelates. 

When R = (CH3)2CH and (CH3)3C the angles between the two chelate ring planes in the crystalline materials are 60° and 54°, respectively, and there are reasons to believe these distortions persist in solution. However, in most cases, with small R groups, the coordination is planar. Again, in the dipyr-romethene complex, (25-H-VII), steric interference of methyl groups renders... 

The secondary structure is also determined by interactions, both attractive and repulsive, between amino acid side chains ( R ) for example, small R groups enable protein molecules to be hydrogen-bonded together in a parallel arrangement, whereas larger R groups produce a spiral form known as an a-helix. 

Two general approaches were used to synthesize 2-cyano-l-indenones with different alkyl groups at the 3-position. The first was an expansion of the Campaigne synthesis (Figure 1, Scheme A), which required the s mthesis of substituted ylidenemalononitriles, 2, and their sub uent cyclization in concentrated sulfuric acid. One limitation of this procedure in the expansion of the SAR was that for small R groups (i.e. lower alkyl), the cyclization did not occur. 

In the middle ground—secondary substrates— we might expect to be able to find both reactions. Small R groups, use of a powerful nucleophile, and a solvent of proper polarity for the charge type of the reaction favor the Sn2 reaction. Large R groups, a weak nucleophile, and a polar solvent favor the SnI reaction (Fig. 7.66). 

Small R groups enable protein molecules to be hydrogen-bonded together in a parallel arrangement whereas larger R groups produce a spiral form known as an alpha-helix. 

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