15 октября, 2015 
Pokraskin Nucleophilic transformation at the carbonyl group of DPPs without cleavage of the bicyclic ring system corresponding to a formal replacement of O by S, N, and C have also been investigated [5].
Early attempts at direct reaction of nucleophiles with diaryl-DPPs were foiled by the low reactivity of the carbonyl group. However, replacement of O by S could be achieved by reacting diaryl-DPPs with P4S10 or the Lawesson reagent (Scheme 11.11) [8, 33].
Derivatives of the type 29 have also proved to be more suitable starting materials than the corresponding O-analog for condensation reactions with carbanions like malodinitrile, ultimately enabling the formal replacement of O by C [8].
In order to achieve a formal displacement of the oxygen with a nucleophile, an activation of the carbonyl group with POCl3 is generally necessary. The phospho —
rylated DPP could be transformed to the corresponding DPP imines by reaction with aniline [5, 34].
One example of a direct oxygen displacement is the reaction of a DPP with cya — namide or R3Si-N=C=N-SiR3 under Lewis acid catalysis leading to 1-keto-4-cyani- mide-pyrrolo[3,4-c]pyrroles or 1,4-dicyanimide-pyrrolo[3,4-c]pyrroles [35].
In the presence of POCl3/PCl5, Gompper has transformed the DPP 2 to the corresponding iminium chloride which could react further with N. N-diethyl- aniline [8].
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