Interaction, five-membered

This is an addition-elimination reaction of methyllithium with iV-methyl-iV-methoxypropionamide forming 2-butanone. As illustrated below using arrow pushing, methyllithium initially adds to the amide. Unlike the process illustrated in Problem 6(f), a second methyllithium does not add and an alcohol is not formed. This is explained by the ability of lithium to coordinate between the two present oxygen atoms. The first is the oxygen of the former carbonyl and the second is the oxygen associated with the methoxy component of the illustrated amide. Due to the stability of this type of five-membered interaction, initial... 

For example, cyclohexanone is reduced by sodium borohydride 23 times faster than cyclopentanone." The explanation for this difference lies in the relative torsional strain in the two systems. Converting an sp atom in a five-membered ring to sp increases the torsional strain because of the increase in the number of eclipsing interactions in the alcohol. A similar change in a six-membered ring leads to a completely staggered (chair) arrangement and reduces torsional strain. 

Weak intramolecular interactions between sulfur or selenium and nitrogen are a recurrent phenomenon in large biomolecules. They may occur in the same residue or between neighbours of a peptide chain. The formation of four- or five-membered rings of the types 15.1 and 15.2, respectively, is most common. A feature that is unique to proteins is the participation of sulfur atoms in bifurcated N S N contacts. 

When formation of either the five- or six-membered ring was possible for N-chloroamine 37, only the five-membered ring was conducive under the Hofmann-Ldffler-Freytag reaction conditions, forming exclusively 6-ethyl-6-aza-bicyclo[3.2.1]-octane (38). No 2-ethyl-2-aza-bicyclo[2.2.2]-octane (39) was observed. On the other hand, 2-methyl-2-aza-bicyclo[2,2.2]octan-6-one (41) was installed by UV irradiation of a solution of A -chloroamine 40 in TFA. Ironically, when the ketone functionality on 40 was protected as its ethylene ketal group, the resultant steric interactions completely prohibited the classic Hofmann-Loffler-Freytag reaction. 

This type of reaction is represented by 1,3-dipolar cycloadditions, corresponding to one of the Huisgen categories (69MI1). The 1,3-dipolar cycloaddition corresponds to the interaction between a 1,3-dipole and a multiple system five-membered ring closure (Scheme 1). 

The interaction between a 4-iodopyrazole-3-carboxylic acid and copper ace-tylides having both donor and acceptor substituents at the triple bond generated six- rather than five-membered lactones, as in the aromatic series (Scheme 117). 

The preparation and characterization of diastereomeric A-iminodiacetatoplatinum(II) complexes of (R,S)- and (R.R)-, 2-chxn have been reported,373 where the substituted iminodi-carboxylato ligand is bonded to the metal center in a tridentate manner (159). One arm of the ligand forms a stable five-membered A,O-chelate, and the remaining carboxylate group interacts with the positively charged metal center to form a zwitterionic species. Unequal proportions of the diastereomers are formed in solution. 

Construction of isolated or benzannulated five-membered rings of NHPs can be accomplished by means of various condensation or cycloaddition reactions all of which involve interaction of an electrophilic Pj and a nucleophilic C2N2 building block. Salts containing 1,3,2-diazaphospholide anions or 1,3,2-diazaphospholenium cations can be directly accessed by some of these reactions but the products are in most cases neutral 1,3,2-diazaphospholes or NHP. A particularly concerted effort has been directed toward the synthesis of P-halogen-substituted NHP which are capable of undergoing further reactions via halide displacement or halide abstraction and serve thus as entry points for the preparation of a wide variety of neutral and cationic NHP derivatives. 1,3,2-Diazaphospholide anions are normally accessed by deprotonation of suitable iV-H-substituted precursors. 

The high reactivity of these fused systems is shown by the ease with which one of the five-membered rings is opened by interaction with an electrophile. So, for example, compound 397 (R = Me) reacts with 3-bromo-l,3-diphenylpentane-2,4-dione, to produce 399, effectively (pathway a in Scheme 119) by reversing the last step in the... 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

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