Heterocycles with Three Heteroatoms

Condensation of substituted aromatic or heterocyclic amines with formaldehyde and urea or phenylthioureas under MWI in an aqueous medium gave 2-(oxo)thioxo-hexahydro-l,3,5-triazines 778 in 92-98% yields within 45-120s, but conventional heating gave 50-58% yields within 10-12 h. Triazines 778 further reacted with chloroacetic acid or hydrazine hydrate to give 779 and 780, respectively. Compounds 780 were obtained in 86-90% yields within 1.5-2.0 min under MWI, but classically 

The traditional thermal conditions were applied to the reaction of heterocyclic acyl hydrazide with heterocycles containing a 1,2-diketone in a 1 1 ratio by using an excess of ammonium acetate in refluxing acetic acid for 10-24h to give 3-het-erocyclo-l,2,4-triazines 784 in less than 30% yield. On the other hand, when the 

The imidazotriazepines 808 were obtained on heating hydrazide 807 and an orthoester. Although the yield of 808 was 74% on reacting 807 with trimethyl orthobenzoate in ethanol for 22 h (84JHC1817), the replacement of the phenyl group of the orthoester by a methyl or an -propyl group led to a low yield of 12% and 8%, respectively. However, MWI improved the reaction leading to imidazotriazepines 

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Table 2 Linearly fused (5 5 5) heterocycles with three heteroatoms...

Five-Membered Heterocycles with Three Heteroatoms 6.24.5.1 1,2,3-Triazoles... 

Five-Membered Heterocycles with Three Heteroatoms 246... 

Applications of five-membered tin heterocycles with three heteroatoms are widespread and the subject of numerous papers and patents. However, practically all publications are devoted to only one type of compound, the readily available 1,3,2-diheterastannolanes. 

All four-membered heterocycles with three heteroatoms can be classified into four types compounds with three similar heteroatoms 1 symmetrical 2, and unsymmetrical 3 heterocycles with two different heteroatoms and derivatives with three different heteroatoms 4 <1996CHEC-II(1B)1189>. 

All four-membered heterocycles with three heteroatoms described in this work belong to the ring systems shown in... 

Other examples of 7-membered ring heterocycles with three heteroatoms in the ring include the chiral titanium complex 311 [01SL1889], the novel phosphoramidite ligand 312 [01SL1375], and the cyclipostin natural products from a Streptomyces species (e.g. Cyclipostin N, 313), which is a hormone-sensitive lipase inhibitor [02JAN480]. 

Semi-empirical and molecular mechanics calculations have been widely used in the study of eight-membered heterocycles with three heteroatoms. Theoretical studies have centered on conformations of this class of heterocines, which usually belong to four low-energy conformational families classified as boat-chair, crown, boat, and chair forms (see Section 14.08.4.2). 

DIMEDONE-ANNULATED FIVE-MEMBERED HETEROCYCLES WITH THREE HETEROATOMS... 

Table 35. Frontier molecular orbital (FMO) energy (eV) gap between the heterocycles with three heteroatoms and acetylene, ethylene, and cyclopropane...

Scheme 6. The heterocycles with three heteroatoms in 1, 2, and 3 positions.

Table 41. Frontier Molecular Orbital (FMO) energy (eV) changes going from reactants to transition state structures with heterocycles with three heteroatoms in 1, 3, and 4 positions as dienes for Diels-Alder reaction. The values were...

Because these transition state structures are symmetric, all previously used approaches for determining relative reactivity of the heterocycles should be applicable in these cases. The cycloaddition is a HOMO dienophile and LUMO heterocycle (diene) controlled cycloaddition reaction with exceptionally low demand for orbital energy changes in reactants to achieve the electronic contribution present in the transition state structure (Table 41). There is no doubt that 1,3,4-oxadiazole is the most reactive of all five-membered heterocycles with three heteroatoms. However, the question remains as to whether this heterocycle is more reactive than, for instance, furan or even cyclopentadiene. To answer this question, the deviation of bond order uniformity in the six-membered ring being formed was computed (Table 42). The bond order uniformity selected 1,3,4-... 

Table 13 UV spectral maxima of eight-membered heterocycles with three heteroatoms.

Application of Ionic Liquids in Heterocyclic Synthesis 4 1 Synthesis of Five-Membered Heterocycles 4.1.1 Synthesis of Five-Membered Heterocycles 443 443 with Two Heteroatoms 4.2.3 Synthesis of Six-Membered Heterocycles with Three Heteroatoms 4.3 Synthesis of Some Other Spiro Heterocycles 468 479 480... 

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