Heterocyclic ring formation

Some examples of ring opening reactions with carbanions leading to five-membered heterocyclic ring formation are shown in Scheme 85. Pyrrole syntheses from functionally substituted oxiranes and amines are often described and typical examples are shown in Scheme 86. 

Once formed, 7 and 8 undergo a Michael reaction that gives rise to ketoenamine 9. Ring closure, to form 10, and loss of water then afforded 1,4-dihydropyridine 11. The presence of 9 and 10 could not be detected thus ring closure and dehydration were deduced to proceed faster than the Michael addition. This has the result of making the Michael addition the rate-determining step in this sequence. Conversely, if the reaction is run in the presence of a small amount of diethylamine, compounds related to 10 could be isolated. Diol 20 has been isolated in an unique case (R = CFb). Attempts to dehydrate this compound under a variety of conditions were unsuccessful. Stereoelectronic effects related to the dehydration may be the cause. In related heterocyclic ring formations, it has been determined that dehydration (20 —> 10) is about 10 times slower than diol formation (19 —> 20). Therefore, one would expect 20 to... 

Heterocyclic block copolymers, 282-284 Heterocyclic diamines, rigid, 281 Heterocyclic polymers, structure-property relationships in, 273-274 Heterocyclic ring formation, PQ and PPQ synthesis by, 309-310 Hexadecyltrimethylammonium bromide (HTMAB), 549-550 Hexamethylene diisocyanate (HDI), 199, 210. See also HDI trimer Hexamethylenediamine-adipic acid salt, 169, 170... 

A further application of ring-closing metathesis in seven-membered heterocyclic ring formation is in the synthesis of the trans-fused oxpane systems. This process involved tandem RCM/allylstannane-aldehyde cyclizations and interaction of the process provides access to trans-fused polyoxepanes <00S883>. 

Flavonoids conjugate addition and heterocyclic ring formation... 

Cyclopentenylacetic acid when treated with sulfur monochloride, Hiinig s base and N-chlorosuccinimide in tetrahydrofuran gave trichlorocyclopenta[l,2]-dithiole ester 104, a product of heterocyclic ring formation, chlorination and dehydrochlorination and, unexpectedly, the conversion of the acid in THF into its 4-chlorobutyl ester (1999JCS(P1)1023 Scheme 52). 

The chromenes and benzofurans are rather simple compounds built from acetate and Isoprene metabolites. Heterocyclic ring formation gives rise to 2,2-dlmethyl chromene or 2-isoprophenyl benzofurans. The majority of known chromenes and benzofurans exhibit a methyl ketone moiety at a position para to the oxygen of the heterocyclic ring. Constituents esterified with phenolic acids or lacking methyl ketones are rare. 

Six-Membered Heterocycle Ring Formation. Heterocycle formation involving diketene usually involves acetoacetylation of a substrate, followed by intramolecular condensation. Diketene itself readily dimerizes through self-condensation forming mainly dehydro acetic acid [771-03-9] (DHA) (13). Dehydroacetic acid and sodium dehydro acetate [4418-26-2] are used as preservatives for foods and cosmetics. DHA is found as an unwanted by-product in many diketene reactions, but can be obtained intentionally by dimerizing diketene in the presence of pyridine [110-86-1] in benzene, diazabicyclo[2.2.2]octane [280-57-9] (DABCO), and other basic catalysts. 

The power of the IMSC methodology has been demonstrated in numerous total syntheses in which a highly stereoselective heterocycle-ring formation by IMSC is generally one of the key steps. 

A double heterocyclic ring formation was also described. Treatment of 2,2 -dibro-modiphenylacetylene 35 with tert-butyllithium followed by tellurium insertion resulted in intramolecular ring closure to afford 111 ben/otel I uro 3,2-/ ] [l]benzotel I urophcnc 36 (Scheme 5). Similarly, 1I bcnzoseleno 3,2-/ I bcn/osclcnophcnc 37 and [l]benzothieno [3,2- ][l]benzothiophene 38 were also obtained [15]. 

Cyclative cleavage strategies (e.g., metathesis and heterocycle ring formation), traceless linkers (e.g., Si, Ge linkers or via chemical reactions such as desulfurization, decarboxylation and cycloreversion) and cleavage steps designed to liberate an increasing diversity of functional groups are illustrated in Section 2.4.4. 

An alternative attachment is through a heteroatom when the heterocyclic ring formation is the final step - it is often easy to incorporate a final cyclisation (heterocycle formation) step in such a way that it results in cleavage of the prodnct from the support at the same time. ... 

A reaction in the absence of a metal ion may differ completely from what occurs in the presence of a metal ion, as exemplified in Figure 6.6. The linear organic molecule formed in the presence of stoichiometric amounts of a metal ion is totally absent in the metal-free chemistry, where small heterocyclic ring formation occurs. 

Heterocyclic ring formation can then lead to a piperidine or a pyridine-type ring system, giving rise to the two principal groups of monoterpene alkaloids ( fig. 9 ). 

Heterocyclic Ring Formation-H202, in the presence of isocyanate as CO-reactant, yields epoxides from olefins in 50-75% yield. This system is advantageous when reactions must be conducted in neutral medium. 1,2-Ditosylates afford... 

Other 0-heterocyclic ring formations from substructure F (Figure 6.4) concern the preparation of isochromanes [104] or the study of the effect of ether versus ester tethering during the cyclization [105]. 

Triazine and Other Heterocyclic Ring Formation. Several types of reactions can be used to form heterocyclic rings in which multiple C-N bonds contribute high thermal stability. When these are used to cross-link heat-stable oligomers, the resulting thermoset polymers may have high thermal stability and other useful properties. These include cyanate/cyanurate, isocyanate/isocyanurate, hexaazatriphenylene trianhydride, and phtha-lonitrile/phthalocyanine. 

Heterocyclic Ring Formation - Aziridines can be formed in a regio- and stereoselectivemanner by LAH reduction of the -iodoazides obtained by addition of INCO to olefins. The reaction has been used to fom 163,173-aziridines in steroids. Treatment of olefins with acetonitrile and... 

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