SIX MEMBERED COMPOUNDS

Pyridones and other six membered compounds (functional tautomerism)... 

Pyridones and other six-membered compounds (functional tautomerism). The pyridone /hydroxypyridine tautomerism (76AHCS1, p. 87), especially 2-pyridone (15a)/2-hydroxypyridine (15b), has received more attention from theoreticians than any other example of tautomerism, probably in part because it is a simple model for biologically important molecules such as thymine, cytosine, and uracil (Scheme 8). 

The Diels-Alder reacdon followed by radical denitradon provides a nseftd strategy for constnicdon of six-membered compounds, in which thenitro group accelerates the reacdon and also controls the reglo-chemistry of the addidon fEq. 7.75. ... 

The Diels-Alder reaction followed by radical denitration provides a useful strategy for construction of six-membered compounds, in which the nitro group accelerates the reaction and also controls the regio-chemistry of the addition (Eq. 7.75).95... 

Organolanthanide-catalyzed cydoisomerizations have also been realized [142, 143], Regioselectivity is an issue here with R being H, six-membered compounds are the major products while with R = alkyl, the reaction proceeded via path b affording 284. 

L. C. Behr, 1,3,4-Oxadiazoles, in A. Weissberger, The Chemistry of Heterocyclic Compounds, Vol. 17, Five-and Six-Membered Compounds with Nitrogen and Oxygen, S. 263-278, John Wiley Sons, Inc., New London 1962. 

M11 A. Quilico, in Five- and Six-Membered Compounds with Oxygen and... 

In these examples with one more carbon atom between the allene and the OH or NH group, the corresponding six-membered compounds could be obtained. 

It is noteworthy that the bicyclic three- and six-membered compounds 44 and 45 can serve as precursors for both germylene and digermene at 70 °C21. Such species can be effectively trapped by 2,3-dimethyl-l,3-butadiene to form germacyclopentene 48 and bicyclic compounds 49 and 50 (Scheme 17). 

Primary and secondary y- and 8-aminonitriles may lead to ring closure to form five-and six-membered compounds, respectively. For example, the hydrogenation of 2-nitro- or 2-amino-4,5-dimethoxyphenylacetonitrile with Raney Ni gives 5,6-dimethoxyindole (eq. 7.58).109... 

A. Quilico, in Five- and Six-Membered Compounds with Oxygen and Nitrogen (R. H. Wiley, ed.), p. I. Wiley (Interscience), New York, 1962. 

Biotransformations of hve-membered alicyclic trani-A-protected-amino nitriles proceeded faster than in case of six-membered compounds. The products of the trani-A-protected-amino nitriles (amides and acids) were formed preferentially than the products of the c A-counterparts (only amides). Enantioselectivities were strongly dependent on the structure the trani-hve-membered substrates gave exclusively amides with excellent optical purity (94-99%), in contrast to the tran -six-membered substrates resulted in the formation of the acid with excellent enantiopurity (87-99%). The corresponding c A-compounds yielded much lower enantiomeric excesses. Nitrile precursor of a-methylene-P-amino acids was analogously investigated (Winkler et al., 2005) (Table 17.13). 

Quilico A., in Five- and Six-membered Compounds with Nitrogen and Oxygen, ed. R.H.Wiley, Wiley-Interscience, 1962, p. 1. 

Heteroaromatic six-membered compounds condensed with cyclobutane ring, syntheses and reactivity of 82H(19)2183. 

The acid—base properties of the five-membered heterocyclic dioxa— and dithia-boron systems (similar to 20) with trimethylamine and tri-methylphosphine have been considered,and it was claimed " that there was a reversal of base strength. In this work Shore et al. did not make use of the AB values, although these did in fact correspond to the relative acid—base strengths found from the study of homogenous displacement reactions and from dissociation tendencies. Similarily, from dissociation pressures it was concluded ° that the five-membered dioxaborolane (42) was a stronger Lewis acid than the corresponding six-membered compound (43) the AB values obtained for the complexes of (42 AB= -I-21-2) and (43 AB= -I-9-3) are in agreement with this conclusion. 

Treatment of the /-butyl formamidine (72) with /-butyllithium followed by an electrophile produces the tetrahydroazepine derivatives (73) <85JOC10I9>. Typical electrophiles are n-propyl iodide, trimethylsilyl chloride, and benzyl bromide. 2,3,4,6-tetrahydro-17/-benz((>]azepines <93JOC6538> afford the corresponding 2-alkyl-dihydro derivatives together with the product from o-alkylation in the aromatic ring. iV-Boc derivatives give an even better yield of the 2-alkyl derivative, which is in marked contrast to the five- and six-membered compounds, where the sole product from the iV-Boc... 

In general, all commercialized neonicotinoids can be divided into open-chain compounds (Chapter 29.2.1) and neonicotinoids having ring systems such as five-membered (Chapter 29.2.2) and six-membered compounds (Chapter 29.2.3) that differ in their molecular characteristics. The structural requirements for both neonicotinoids having open-chain stmctures and ring-system containing neonicotinoids consist of different segments Usted below (Fig. 29.2.1, Tables 29.2.1 and 29.2.2) [1, 2]. 

Pharmacophores [-N-C(E)=X-Y] Five- and six-membered compounds (R -R, R -Z-R ) Open-chain compounds (R R2)... 

A double bond in a five- or six-membered compound must have a cis configuration and compounds like these are readily made. Dehydration of this hydroxylactone can give only a cis double bond and ring-opening with a nucleophile (alcohol, hydroxide, amine) gives an open-chain compound also with a cis double bond. 

Cyclopentane Derivatives - The isoxazolidinocarbocyclic derivative 72 was produced (together with a six-membered compound (89)-see Section 2.2) from cyclization of the nitrone 73, itself generated in situ from the iV-benzylhydroxyl-amine derivative of 3-C-allyl-l,2-0-isopropylidene-a-D-riZw>-pentodialdofuranose. The formation of the cyclopentane adduct predominated in non polar and polar aprotic solvents while the cyclohexane adduct predominated in polar protic solvents. ... 

R] Wawzonek, S. In Heterocyclic Compounds, Vol. 2 Polycyclic Five- and Six-Membered Compounds Containing One OorS Atom Wiley New York, 1951, pp. 173-216. 

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