A3-1,3,4-Oxadiazolines

A3-1,3,4-Oxadiazolin-2-imine, N-phenyl-pyrolysis, 6, 437 A2-1,3,4-Oxadiazolin-5-imines IR spectra, 6, 429 A3-1,3,4-Oxadiazolin-2-imines hydrolysis, 6, 438... 

Hoffmann and Luthardt521 thermolyzed and photolyzed A3-1,3,4-oxadiazolines (107), and trapped the oxonium ylid intermediates 108 by... 

The most studied reactions of A3-1,3,4-oxadiazolines are thermal and photochemical ring cleavage, generally with loss of a molecule of nitrogen. 

There are three nonconjugated reduced systems derived from 1,3,4-oxadiazole 1, namely 2,3-dihydro-l,3,4-oxadia-zole (A2-l,3,4-oxadiazoline) 5, 2,5-dihydro-l,3,4-oxadiazole (A3-l,3,4-oxadiazoline) 6, and 2,3,4,5-tetrahydro-l,3,4-oxadiazole (1,3,4-oxadiazolidine) 7. 

In recent years there has been a growing interest in the use of carbonyl ylides as 1,3-dipoles for total synthesis.127-130 Their dipolar cycloaddition to alkenic, alkynic and hetero multiple bonded dipolaro-philes has been well documented.6 Methods for the generation of carbonyl ylides include the thermal and photochemical opening of oxiranes,131 the thermal fragmentation of certain heterocyclic structures such as A3-l,3,4-oxadiazolines (141) or l,3-dioxolan-4-ones132-134 (142) and the reaction of carbenes or car-benoids with carbonyl derivatives.133-138 Formation of a carbonyl ylide by attack of a rhodium carbenoid... 

Thermolysis of 2-acetoxy-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazoline affords acetoxy(methoxy) carbene.60 The thermal rearrangement of acetoxy(methoxy) car-bene to methyl pyruvate was studied by DFT at the B3PW91/6-31G(d,p) level. The conformation of the carbene was considered, as were competing fragmentations to radical pairs. The authors concluded that the reaction is a concerted 1,2-migration rather than a fragmentation-recombination process. 

Bicydopropylidene (1) reacts with dimethoxycarbene generated thermally from the diazirine at 25 °C to give a complex mixture of products [116b]. The cycloaddition of several dialkoxycarbenes generated in situ from the corresponding 2,2-dialkoxy-A3-l,3,4-oxadiazolines of type 134 with bicydopropylidene (1) affords the dialkyl acetals of dispiro[2.0.2.l]heptanone 135 (Scheme 29 ) [125]. 

There is no published example of a cyclopropanation of the double bond in chlorocyclopropylideneacetate 1-Me with retention of the chlorine atom. Thus, attempted cyclopropanations under Simmons-Smith [37] or Corey [38] conditions failed [25]. The treatment of the highly reactive methylenecyclopropane derivative 1-Me with dimethoxycarbene generated by thermal decomposition of 2,2-dimethoxy-A3-l,3,4-oxadiazoline 26 (1.5 equiv. of 26,PhH, 100 °C,24 h),gave a complex mixture of products (Scheme 7) [39], yet the normal cycloadduct 28 was not detected. The formation of compounds 29 - 33 was rationalized via the initially formed zwitterion 27, resulting from the Michael addition of the highly nucleophilic dimethoxycarbene to the C,C-double bond of 1-Me. The ring closure of 27 to the normal product 28 is probably reversible, and 27 can rearrange or add a second dimethoxycarbene moiety and a molecule of acetone to form 33. 

Lactones of azocarboxylic acids are remarkably reactive. In the presence of phenyl isocyanate, the imino isocyanate formed from reactive 2-hydrazono-A3-l,3,4-oxadiazolines via a 1,3-dipolar cycloreversion is intercepted to give [l,2,4]triazolo[l,2-a]-[l,2,4]triazole-l,3,5-triones by means of two subsequent [2 + 3] cycloadditions via azomethine imine intermediates (Scheme 10) (76T2685). 

Isoxazolidi no[2,3-6][ A3-1,2,5]oxadiazoline synthesis, 6, 997 Isoxazolidone, 5-hydroxy-synthesis, 6, 112 Isoxazoline, 5-hydroxy-synthesis, 6, 61 Isoxazoline, 3-nitro-N-oxides synthesis, 6, 102 Isoxazoline, 4-nitro-synthesis, 6, 71 Isoxazoline, 3,4,5-triphenyl-N-oxide... 

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