Five-Membered Rings with More Than Two Heteroatoms

Five-Membered Rings with More Than Two Heteroatoms 

When treated with either sulfur monochloride or thionyl chloride, AAs and 2,3-diaminopropionic acid afforded neither l,2,5-thiadiazole-3-carboxylic acid or 3-alkyl-4-hydroxy-l,2,5-thiadiazoles in fair to low yields (66JOC1964 67JOC2823). 

A-Protected sulfonamides are formed from AA esters and chlorosulfonyl isocyanate. After deprotection, the product was cyclized in the presence of sodium methoxide or TFA to give 36 (89JOC4471 91TL6545). 

Various AAs as such or their amidines react with triethylborane, triphe-nylborane, 9-borabicyclo[3.3.1]nonane (9-BBN), or with KBp3Ph to give the corresponding boroxazolidones 37. The reaction has synthetic utility for the simultaneous protection of the a-amino and a-carboxyl groups (83T2995 93JA11612). 

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IV. Five-Membered Rings with More Than Two Heteroatoms... 

Volume 4 Five-membered Rings with More than Two Heteroatoms and Fused Carbocyclic Derivatives... 

Pace A, Buscemi S, Vivona N (2005) The synthesis of fluorinated heteroaromatic compounds. Part 1. Five-membered rings with more than two heteroatoms. A review. Org Prep Proc Int... 

Five-membered ring systems with more than two heteroatoms 3.4.1. Triazoles... 

The aromaticity of five-membered rings with two or more heteroatoms was discussed in detail in earlier reviews.52 100 111 In a comprehensive survey on the quantitative measurements of aromaticity,112 it has been shown that basicity-based quantification of aromaticity gave more reproducible resonance energies than other methods, such as heats of formation, ring currents, magnetic susceptibilities, and theoretical indices. 

Benzoheteropine Rings with Two Heteroatoms on the Heteropine ring Systems with More than Two Atoms on the Heteropine Ring and Miscellaneous Ring Systems Reactivity of Benzoheteropines with Fused Five-Membered Rings... 

Volumes 3 and 4 cover five-membered rings with two heteroatoms, or more than two heteroatoms, respectively, each with their fused carbocyclic compounds. 

Five-membered rings with two or more heteroatoms are usually good at electrophilic substitution as one of the heteroatoms must be either O or S or a pyrrole-like nitrogen atom any of which supply lone pair electrons. Pyrazole 12, imidazole 13, oxazole 17, thiazole 18, isoxazole 19 and isothiazole 20 are examples. Multiple substitution can be a problem but is less so than for pyrrole because there are fewer carbon atoms available and the pyridine-like nitrogen atoms deactivate the ring. 

Finally, we note that all of the discussed heterocyclic derivatives of indane and indene have heteroatoms only in their five-membered ring. There are many other heterocycles related to indane that have the heteroatoms located elsewhere. However, the thermochemistry of such species is essentially uncharted. The enthalpy of formation of purine (XLVIII), with its four nitrogens dispersed through both rings, has been measured in recent times [78], and chronicled in the archive [15] with yet a later value [79], These two values inexplicably differ by over 2000 kJ mol-1. In the absence of a value for the enthalpy of sublimation we are hesitant to discuss this species further, other than to note two estimates from a paper more than 100 years old [80] that straddle the results in [78] by ca. 20 kJ mol-1. 

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