Forming six-membered nngs

Reaction type 3 (equation 10), where the complete hetero-l,3-diene skeleton is incorporated into the newly formed ring system, occurs with compounds having both a nucleophilic center and an electrophilic center If these two functionalities are in positions 1 and 2, various types of six-membered ring systems become accessible 4,4-Bis(trifluoromethyl)-I,3-diaza-1,3-butadienes require only room temperature to react with acetyl cyanide to yield l,4,5,6-tetrahydropynmidin-6-ones [96] Likewise, certain open-chain 1,3-diketones (acetylacetone and acetoacetates) and the heterodiene form six-membered nng systems [97] (equation 19)... 

Each of the following may be represented by at least one alternative resonance structure in which all the six membered nngs correspond to Kekule forms of benzene Write such a resonance form for each... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

The enol form is stable because of the conjugation and because of intramolecular CH3 hydrogen-bonding in a six-membered nng. 

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