Six- and Seven-membered Rings

Six- and Seven-membered Rings. Full details of the regioselecti- 

The products are 5,6-dihydro-2il-1, 3-oxazines (128) from predominantly endo reaction and yields are high (70-95%). 

The reaction of epoxides with isocyanates is well known, but less well known is the reaction of oxetanes with isocyanates. However, the modified reaction conditions reported for epoxide 

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The possible variations among the three atoms comprising this reactive component make this the largest group and the one with the most potential for further development. They are particularly useful in the synthesis of five-, six- and seven-membered ring systems. They also may be conveniently considered according to their atom composition, and the hybridization state of any carbon atoms involved. 

Dipolarophiles utilized in these cycloadditions leading to five-membered heterocycles contain either double or triple bonds between two carbon atoms, a carbon atom and a heteroatom, or two heteroatoms. These are shown in Scheme 9 listed in approximate order of decreasing activity from left to right. Small rings containing a double bond (either C=C or C=N) are also effective dipolarophiles, but these result in six- and seven-membered ring systems. 

A mode] study has demonstrated the pathways shown in Scheme 4,17. The first cyclization step gave predominantly five-membered rings, the second a mixture of six- and seven-membered rings.155 Relative rate constants for the individual steps were measured. The first cyclization step was found to be some five-fold faster than for the parent 5-hexenyl system. Although originally put forward as evidence for hyperconjugation in 1,6-dienes, further work showed the rate acceleration to be sterie in origin.113-I3j... 

Shorter chain dienes have an increased propensity to form stable five-, six-, and seven-membered rings. This thermodynamically controlled phenomenon is known as the Thorpe-Ingold effect.15 Since ADMET polymerization is performed over extended time periods under equilibrium conditions, it is ultimately thermodynamics rather than kinetics that determine the choice between a selected diene monomer undergoing either polycondensation or cyclization. 

This reaction was also used to close five-, six-, and seven-membered rings. 

Corrugated 6 -net fragments groups connected at 210 pm trigonal prism, Tj Pianar nets of five-, six-and seven-membered rings of B atoms trigonal prism... 

The use of azodicarboxylates as a route to dioxaphosphoranes continues to attract attention. In the most recent contribution, triphenylphosphine and di-iso-propyl azodicarboxylate (43) are shown to react with prcpane-1,3-diol and butane-1,4-diol in THF at 0°C under high dilution conditions to give the expected six-and seven-membered-ring phosphoranes (44 ab)36. In more concentrated solution however, cyclic oligomers are formed. Substituted and ccnformationally restricted 1,3- and 1,4-diols form the expected cyclic phosphoranes without recourse to high dilution techniques. 

Amides and sulfonamides undergo intramolecular chemistry to form aryl amides and aryl sulfonamides (Equations (17)—(19)) in the presence of palladium catalysts ligated by arylphos-phines.35,89 Initially, complexes of P(furyl)3 and P(o-tol)3 were most effective catalysts, but complexes of Hayashi s MOP and van Leeuwen s DPEphos and xantphos have lately been shown to be more active.90 In the presence of catalysts containing one of these ligand systems, five-, six-, and seven-membered rings were formed from halogenated benzamides or from substrates containing an acetamide, an A-carbobenzyloxy, or a t-butylcarbamate substituent tethered to the aryl halide (Equations (18) and (19)) ... 

The two articles in this current volume describe recent developments with small ring compounds which have not teen compiled in such a context before. T. Hirao discusses selective transformations initiated by transition derivatives in the construction of functionally substituted five-, six- and seven-membered rings as well as open-chair compounds. Cycloadditions onto methylene- and alkylidene-cyclopropane derivatives, described by A. Goti, F. M. Cordero and A. Brandi, not only yield products with spirocyclopropane moieties which can be desirable as such or as potential mimics of gem-dimethyl groupings, but also intermediates which can undergo further transformations with ring-opening of the cyclopropane units. 

Condensed 1,2,4-Triazines II. Fused to Heterocycles with Six- and Seven-Membered Rings and Fused to Two Heterocyclic Rings... 

The six- and seven-membered rings S5NR and S6NR are yellow oils that exhibit molecular ions in their electron-impact mass spectra. The nine- and ten-membered rings S8NH and S9NH form photosensitive, pale yellow crystals that are soluble in CS2. The 12-membered ring SnNH has only been characterized by spectroscopic methods.207... 

The new spiroannelation method gives, by use of both the tin and silicon chemistry, ready access to [4.4], [4.5], [5.5], [4.6], and [5.6] spirocyclic systems as well as five-, six-, and seven-membered rings possessing a quaternary center. An in situ oxidation, or protection of an initially formed crude secondary alcohol increases the ease of isolation of the product and leads to improved overall yields. Furthermore, for symmetrical substrates, this chemodifferentiates two oxygen functionalities at... 

The above-described synthetic strategy has also been adapted to the cycliza-tion of acyl radicals [57]. Two examples are given in Reaction (7.47). The intramolecular addition of acyl radicals to the oxauracil moiety is also an efficient reaction for the construction of five-, six-, and seven-membered rings. By replacing the radical acceptor with oxathymine, an additional stereo-genic centre at C5 position is introduced. 

Fig. 33 Six- and seven-membered rings by tandem IMCR-RCM processes...

Six- and seven-membered rings (dihydroisoquinolines 178, isoquinolinones 179, 180 and benzazepines 181) were obtained starting from orf/io-halobenzaldehydes or orf/to-halobenzoic acids [144, 145]. In all the cases, the exo-trig cyclization mode was strongly favoured. However, for 178 and 180, isomerization of the double bond from exocyclic to endocyclic occurred spontaneously under the Heck conditions. 

These peaks are assigned to six- and seven-membered rings (low retention time) and twelve-membered rings (high retention time) since they occur with similar retention times as has been observed for the corresponding homocyclic molecules Sg, S, and... 

Fig. 7. a) Chelation of potassium in the H eis-aconitate salt, b) Chelation of potassium in the H2 potassium ions in H2 isocitrate Apart from the seven-membered ring there is triple chelation giving five-, six-, and seven-membered rings to K+ ... 

Ring-Closing Diene Metathesis 11.07.2.1 Formation of Five-, Six-, and Seven-membered Rings... 

TV-substituted methylenephthalimide derivatives served as the starting material for the preparation of a series of tetracyclic systems. The intramolecular Heck reaction of these compounds (4.10.) led in each case to the formation of a carbon-carbon bond between the exocyclic carbon atom of the olefin bond and the aryl moiety. The formation of five, six and seven membered rings was achieved with near equal efficiency, enabling the preparation of some isoindolobenzazepine alkaloids.11... 

The intramolecular cyclization of a nitro group with a triple bond has been extended to the synthesis of six- and seven-membered ring compounds analogous to isatogens. Thus, the reaction of the acetylene group with the nitro group in 39 yields 2-phenyl-3-oxobenzo[prepared from the corresponding biphenyl derivative.19... 

Anodic oxidation of JV,N-dimethyl-co-hydroxyamides (57) in CH3OH-Bu4NBF4 at a platinum anode leads to formation of N-methoxy-JV-methyl-co-hydroxyamides (58) in high yield.116 The latter could in some cases (formation of five-, six-, and seven-membered rings) easily be transformed to l,3-oxaza-4-oxo heterocyclic systems (59) by acid catalysis [Eq. (49)]. No direct formation of the 1,3-oxazaheterocycles was observed, e.g., 57++59. An intramolecular addition of the hydroxy group to the intermediate acylam-monium ion is believed to be hindered by adsorption phenomena at the anode surface. 

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