Heterocycle-forming reactions, categories

Categorization of the reactions within our three subdivisions at times was somewhat difficult. For example, with the heterocycle polymers of category (2), oftentimes the overall reactions occurred in steps with the actual heterocycle-forming reaction occurring from some precursor polymer. Technically, reactions of this sort could be classified as polymer modifications. However, since these latter reactions usually can be accomplished in the same reaction vessel, simply by elevating the temperature, we have chosen to regard them as heterocycle-forming polymerizations. 

In general, reactions under this category involve both the isoxazole ring as well as the substituent ring or chain and thus result in forming new heterocycles. Oximation reaction and consequent... 

Relatively few papers have appeared concerning heterocycles which fall into this category, and only a small number of them merit discussion. Nitrile sulphides can be generated by thermal decarboxylation of l,3,4-oxathiazol-2-ones, and if this is done in the presence of dipolarophiles new heterocycles are formed. Reactions of this type are known to occur with alkenes, alkynes, and nitriles, and they have now been shown to occur with activated aldehydes and ketones. This provides a new route to 1,3,4-oxathiazolines (312) which were previously rather inaccessible. Silylated thioketones (313) are unstable but may be prepared and reacted in situ to provide routes to a variety of new heterocycles, depending upon the dipolar species used. ... 

The dipole is a triad of atoms which has a n system of four electrons and for which a dipolar resonance form provides an important component of its bonding description. The requirements have also been described as atom A must have a sextet of outer electrons and C has an octet with at least one unshared pair ([119], p. 743). The substrate is an olefin and an alkyne or a carbonyl. The reaction therefore is of n4 + n2 type like the related Diels-Alder reaction and has proved to be very useful synthetically for the construction of 5-membered ring heterocycles [268, 269]. Evidence suggests the reaction is concerted and regioselective. Dipoles fall into two general categories [119] ... 

N-heterocyclic carbene catalysis has become one of the major categories in orga-nocatalysis. Azolium salts are ready deprotonated by weak bases to generate a carbene, which then adds to an aldehyde to form an acyl anion equivalent, generally called the Breslow intermediate. The reactive acyl anion attacks an electrophile to promote the various transformations such as benzoin, Stetter, and redox reactions [107]. Recently, an interesting approach for NHC-catalyzed generation of an enol/enolate intermediate was reported. Enantio-enriched (i-amino acid derivatives (217) are formed by the reaction between the a-aryloxyaldehyde 214 and N-tosyl-imines (215) in the presence of phenyalanine-derived azoUum salt 216 as a pre-catalyst and aryl phenoxide as a base (Scheme 28.28) [108]. 

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