Formation from anhydrobases

The reason for this relative lack of reactivity of 2-methylthiazoIium is probably due to the too-weak nucleophilic character of its carbon-2. For example, any /S-alkoxyalcene (29) derivatives resulting from the condensation of o-ester could never have been isolated, whereas they constitute the essential intermediate step in trimethine syntheses for rings of acidic character (64). However, even if a negative 5-substituent such as ethoxy-carbonyl increases the yield (61) by promoting independently the possible formation of the methylene base, it may be stressed that the presence of this base is not the essential condition of the reaction, since the isolated anhydrobase itself is not reactive toward the o-ester (Scheme 41). 

Two moles of /3-alkoxyaicene can condense on each other by means of their a- and /3-carbon atoms. The resulting intermediate reacts on the anhydrobase by elimination of a molecule of ethanol resulting in a neocyanine formation (Schemes 59 and 60). Both monoanilino and bis-anilino derivatives resulting from the condensation of dimethylform-amide have been isolated. They are capable of furnishing various condensations on either ketomethylene or another reactive nucleus (Scheme 61). 

Therefore, most probably, the formation of chrysenes 246 from 2-benzopyrylium salts occurs via the process of a-1 dimerization. At the same time, one cannot exclude the action of anhydrobase 267 as dieno-phile, which leads to its addition to positions 1 and 4 of the initial 2-benzopyrylium cation by analogy with [4 + 2] cycloadditions in reaction with ethyl vinyl ether (Scheme 14) (cf. Section III,D,1). 

The formation of acyl-chrysenes 270 from 1-methyl-substituted ben-zo[c]pyrylium salts 266 occurs not only on heating in alkaline solutions, but also in acidic nucleophilic media (cf. Section III,C,4,b,i). However, in the latter case, together with acylchrysenes 270, their deacylated analogs 273 are formed (85KGS910). Under these conditions, the same results were obtained for conversions of diketones 29, therefore it is difficult to conclude which compound (anhydrobase 267 or diketone 29) is the intermediate in the formation of chrysenes 270 and 273 from salts 266 in acidic nucleophilic medium. It was not possible to trap or detect the dimeric pseudobase 269 under these conditions. However, the latter compound, under the described conditions or on heating in acetic acid, forms the mixture of the same products (270 and 273). 

The formation of an anhydrobase, rather than the pseudobase, from heterocyclic cations bearing activated alkyl groups is usually readily determinable from an analysis of the PMR spectrum of the cation in basic methanol.41 -65-66 70 The appearance of vinylic protons in the PMR spectrum in basic solutions is quite characteristic of anhydrobase formation. Spectral evidence for the rapid formation of a pseudobase, followed by a slower conversion to the thermodynamically more stable anhydrobase, has also been presented.41... 

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