New and Novel Cyclisation Reactions of Imidoylketenes
The effect of scale on the conversion to products in flash vacuum pyrolysis experiments was studied using four model reactions. Overall, conversion was dependent on rection scale up to 0.5 g, but above 0.5 g, the effect was minimal and within experimental error. This effect was shown to be due to variations in contact time of the molecules in the furnace tube. Altering the furnace tube diameter had the effect of increasing the conversion to product. The pyrolysis of ortho-anilinomethylene Meldrum’s acid derivatives was investigated. Typically the 8-substituted quinolin-4-one was obtained as the major product, with a few exceptions. Certain substituents (such as nitro) were found to react with the ketene produced in the reaction to give alternative products, while others (such as chloro- and N-unsubstituted amides) gave the 3-substituted quinolin-4- ones via ipso-cyclisation and migration of the substituent. The regioselectivity of the pyrolysis of meta-anilinomethylene Meldrum’s acid derivatives, to give 5- and 7-substituted quinolin-4-ones, was studied. In general a 3:1 – 4:1 ratio of regioisomers was obtained, in favour of the 7-substituted quinolin-4-one. Substituents capable of hydrogen bonding, such as hydroxy-, were shown to give the 5-substituted quinolin-4-one exclusively and DFT calculations were employed to show that, in these examples, the 5-substituted product was favoured energetically. The pyrolysis of the methylene Meldrum’s acid derivative of 3-aminophenol gave 8-hydroxyquinolizinone as the sole product at low temperatures, with 5- hydroxyquinolinone as the major product formed at higher temperatures. The mechanism involves a regioselective electrocyclisation, followed by a hydrogen transfer and generation of a new ketene. Cyclisation of this ketene gives the quinolizinone. The scope of this reaction was explored, with a number of derivatives synthesised, and substitution on the aminophenol and the ketene generator was tolerated. The reactivity of the quinolizinones was also explored. The hydroxygroup was found to be phenol-like and underwent similar reactions, such as alkylations and acetylations. The compound was found to be highly reactive towards electrophiles, reacting in the 1- and 3- positions of the ring system, often in both positions. The pyrolysis of the aminomethylene Meldrum’s acid derivatives of certain pyridazinones was shown to give pyridopyrazidinediones. In some examples, a 4 second product based on a pyrrolopyridazine ring system was observed and DFT calculations show that the mechanism involves probably an electrocyclisation, followed by a decarboxylation reaction. Pyrolysis of amino acid ester derivatives of methylene Meldrum’s acid were shown to give N-unsubstituted 3-hydroxypyrroles and 1H-pyrrol-3(2H)-ones. Different amino acids were tolerated in the reaction, as were different electronwithdrawing groups in place of the amino acid ester. DFT calculations were employed to explore the mechanism of the reaction. 3-Hydroxypyrrole was also synthesised from the pyrolysis of Meldrum’s acid derivative of glycine tert-butyl ester, and the reactivity of the compound explored for the first time. The compound was found to be reactive towards electrophiles, such as diazonium salts, and could be O-acetylated under appropriate conditions.