Fluorogenic substrates incorporating the sequence Asp-Glu-Pro-Asp-Ser were able to quantify caspase-3 activity without notable caspase-7 and cathepsin B cross-reactivity.

Flash vacuum pyrolysis (FVP) at 500-600 °C of 1-substituted pyrazolylaminomethylene derivatives of Meldrum's acid provides 1-substituted pyrazolo [3,4-b]pyridin-4-ones in high yields. If the 1-substituent is a tert-butyl group, FVP at 750-850 °C causes elimination of 2-methyl-1-propene to give the parent pyrazolo [3,4-b]pyridin-4-one.Key words gas-phase reactions, pericyclic reactions, heterocycles, Meldrum's acid, medicinal chemistryThere are very few references to 1-unsubstituted pyrazolo [3,4-b]pyridin-4-ones 1 in the literature 2 and all known derivatives except the parent compound 1 (R = R′ = H) have a substituent in the 6-position. Potential functionalization of the 4-position (e.g., via the triflate or the 4-chloro compound) would provide 4-substituted pyrazolo [3,4-b]pyridines 2 (Figure 1), which have shown diverse application in medicinal chemistry.3 On the other hand, substitution at the 1-position generally results in loss of biological activity due to the disruption of the hydrogen bonding regime. Scheme 1The present work therefore had a range of objectives. First, 3 (R 1 = alkyl or aryl) were synthesized and pyrolyzed to ensure that, in the absence of the pyrazole NH, cyclization onto the adjacent carbon atom to give 1 (R = 1-alkyl or 1-aryl) would take place (Scheme 1), as observed in many related systems.6 Second, we explored the design of a thermal N-protecting group, which would remain at low furnace temperatures, but be selectively removed at higher furnace temperatures to provide N-unsubstituted pyrazolopyridinones 1 (R 1 = H). If the previous stages were successful, we aimed finally to functionalize the 4-position of the pyrazolo [3,4-b]pyridin-4-ones to establish that the route has significant potential for the synthesis of pyrazolo [3,4-b]pyridines 2.