Preparative organic synthesis was investigated in aqueous media at temperatures up to 300 degrees C. Experiments were conducted with a recently disclosed pressurized microwave batch reactor (MBR) or in conventionally heated autoclaves. Thirty-six examples are presented. Among these, methods were developed for a Fischer synthesis, an intramolecular aldol condensation that was scaled up, decarboxylation of indole-2-carboxylic acid, Rupe rearrangement of 1-ethynyl-1-cyclohexanol, isomerization of carvone to carvacrol, and conversion of phenylacetylene to acetophenone. The applicability of high-temperature water was also demonstrated for biomimetic processes important in food, flavor, and aroma chemistry and for tandem reactions such as formation of 2-methyl-2,3-dihydrobenzofuran from allyl phenyl ether. When addition of acid or base was necessary, less agent was usually required for high-temperature processes than for those at and below boiling, and the reactions often proceeded more selectively. In some instances the requirement was orders of magnitude lower, with obvious consequences for safe, economic processing and for lowering costs of effluent disposal. The diversity of reactions indicates that high-temperature aqueous media could play an increasingly important role in the development of new preparative processes.

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In a systematic study, allyl phenyl ether (1) was heated in water for 1 h at temperatures of 180 degrees C and above. Parallel experiments were conducted with a conventionally heated autoclave and a recently developed microwave batch reactor. Relatively modest temperature differences resulted in diverse product distributions, and these were independent of the method of heating. Maximum conversion of 1 to 2-allylphenol occurred at 200 degrees C (56%) and to 2-methyl-2,3-dihydrobenzofuran at 250 degrees C (72%). Although 2-(2-hydroxyprop-1-yl)phenol comprised less than 1% of the product mixture at both 180 and 260 degrees C, it accounted for 37% at 230 degrees C. The reaction sequence was investigated by heating intermediates individually at selected temperatures up to 290 degrees C. Hydration of 2-allylphenol to 2-(2-hydroxyprop-1-yl)phenol was partially reversible. The work showed that high-temperature water constitutes an environmentally benign alternative to the use of acid catalysts or organic solvents and offers scope for interconversion of alcohols and alkenes.

A moderately yielding (50±4%) method for obtaining 3-methylcyclopent-2-en-1-one from hexane-2,5-dione in ≥99·5% purity was developed, using 2·1% (w/v) of aqueous KOH at reflux. However, the yield was increased to 80±5% by using considerably lower concentrations of base (<0·1%) and higher temperature (200°C) with a conventionally heated autoclave or a microwave batch reactor. The preparation was scaled up with a larger autoclave and also with a continuous flow microwave reactor. The dione impurity was removed from the enone by preferential adsorption on an anion-exchange resin in the bisulfite form. The product was recovered from the aqueous phase with a hydrophobic resin. This environmentally benign, non-extractive isolation method has potential for broad applicability in cleaner production.

N,N-Dialkyl-N′-chlorosulfonylchloroformamidines 1 reacted with 2-aminopyridines 2 to give novel pyrido[1,2-b][1,2,4,6]thiatriazine dioxides 3 and pyrido[2,1-c][1,2,4,6]thiatriazine dioxides 4. Reaction of 1 with 3-aminopyridazines 5 afforded pyridazo[3,2-c][1,2,4,6]thiatriazine dioxides 6 and a pyridazo[2,3-b][1,2,4,6]thiatriazine dioxide 7. The compounds 6 and 7 are derivatives of new ring systems.