1.2.4- Triazole-3,5 dione, formation cycloadditions

Several relevant papers and review articles have appeared recently. These contain reports on the mechanism and kinetics of the ene reaction of ADC compounds,243-245 examples of four-membered ring formation,246-247 other cycloadditions of ADC compounds,248-252 the synthesis of azoalkanes,253 the use of chiral l,2,4-triazole-3,5-diones,254 and the use of the DEAZD/PI13P reagent in organic synthesis.255... 

Dipolar cycloaddition of 3//-1.2.4-triazole-3,5(4// )-diones to strained bicyclic alkenes generally leads to the formation of rearranged 1,2,4-triazolidines (Section7.2.10.1.), although bicyclo[2.2.2]-octene does not react and cyclopentadiene dimer gives exclusively the cne reaction product24. 

The cycloadditions of diethyl diazenedicarboxylate and 3//-1,2,4-triazole-3,5(47/)-diones to 1,3-dienes where one only double bond is part of a fused or bridged bicyclic structure occurs with complete diastcrcoselectivity, since the diazene approaches the diene from the least hindered side, e.g., formation of 22 and 33. 

The cycloaddition of 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione to 2-ex0-3-exo-bis(chloromethyl)-5-[( )-methoxymethylidene]-6-methylidene-7-oxabicyclo[2.2.1]heptane occurs exclusively, within the limit of detectability by H NMR, on the e.vo-face, i.e., from above the plane of the molecule, to give ll6. This is in contrast to the endo-face selectivity reported for a large number of cycloadditions of dienes grafted on to the bicyclo[2.2.1] skeleton (see also Section 7.2.10.3.1). The formation of 7-oxabicyclo[2.2.1]heptane-diazene charge transfer complexes, i.e., the assistance of the ethereal bridge in the cycloaddition, has been invoked to explain the exo-face selectivity. 

Since vinyl azides like 34 are electron-rich olefins, [2 + 2] cycloaddition with electron-deficient alkenes such as diphenylketene could lead to azidocyclobutanes. " The stability of the cycloadducts 211, prepared from 34 or 52 and tetracyanoethene (TCNE), allowed characterization in solution but not isolation of these products because rapid ring-expansion regioselectively afforded the dihydropyrroles 212 already at room temperature (Scheme 5.25). "" A similar mechanism via [2 + 2] cycloaddition and quick ring-enlargement may perhaps explain the formation of 213 from 52 and 4-phenyl-l,2,4-triazole-3,5-dione (PTAD). In this " " and other " " cases, however, different interpretations were offered. The 2-azidobuta-l,3-dienes 92a,b underwent [4 + 2] cycloaddition in the... 

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