Diazenes, bicyclic

The treatment of N-aminourazole (38) with two equivalents of BAIB in DMSO results in imidation of the sulfoxide and oxidation of the urazole ring to give the sulfoximine 39 (Scheme 20) [46]. The preparation of 39 from 38 can also be accomplished stepwise, by isolation of the dihydro sulfoximine intermediate and BAIB-oxidation of this compound in acetonitrile. The sulfoximine 39 was not isolated, but was generated in solution and employed for the synthesis of bicyclic diazenes. 

Scheme 30 Thermal dissociations of bicyclic diazenes with vinylcyclopropane substituents...

The cycloaddition to linear polyenes can afford, in principle, different regioisomers, see 25 and 26, but often the diazenes add to the 1,3-diene moiety which is less hindered or less substituted at the terminal carbon atoms. The cycloaddition to cyclic or bicyclic polyenes, however, can proceed by the normal [4 + 2] as well as the [6 + 2] or [8 + 2] cycloaddition mode. 

Phenylhydrazine and symmetrical or unsymmetrical dialkyl hydrazines (acyclic, cyclic and bicyclic) have been oxidized to the corresponding diazenes. 

Bicyclic diazenes, in which the diazcnc group is not activated by acyl substituents, can act as dienophiles in the Diels-Alder reaction with cyclic dienes only in the presence of a stoichiometric amount of tetrafluoroboric acid to give. V,/V"-bicyclic hydrazines 111 . When two diastereomers can be produced, due to the nonsymmetric dienophile (i.e., when n 2), a single diastereomer is obtained ( H-NMR studies). 

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. 

Cyclobutene-fused tetrahydropyridazines 1 and 2 are prepared by the diastereoselective cycloaddition of acyldiazenes to 1,2-diethylidenecyclobutanes. They can be converted to bicyclic diazenes 3, which are readily isomerized to the hydrazones f Pyrolysis of the bicyclic diazenes 3 affords 2,6-octadienes 4 with >99.9% stereospecificity. 

Cycloheptatriene 1 and substituted derivatives thereof do not generally contain detectable amounts of their norcaradiene valence tautomers at equilibrium1-2. However, the bicyclic (tropy-lidene-type) adduct is generally not obtained with diazenes, probably due to the decreased reactivity of the boat conformation of cycloheptatriene. Instead, the tricyclic (norcaradiene-type) adduct 2 is obtained with the most reactive diazenes, i.e., 4-phenyl- and 4-methyl-3W-1,2,4-tri-azolc-3,5(4W)-diones. Ene reaction can also compete. The reaction course is affected by the nature of the diazene, the reaction conditions, the substitution pattern and the nature of the substituents on the cycloheptatriene ring. 

Both the thermally and the photochemically initiated extrusion of nitrogen from bicyclic diazene (1) leads to the formation of a 1,3-diradical (diyl) which is structurally related to trimethylenemethane. As shown in the cascade mechanism illustrated, two forms of the diyl can be intercepted by alkenes bearing electron-withdrawing groups (diylophiles), viz. the open form of the singlet diyl and the triplet diyl which is formed via intersystem crossing from the singlet (Scheme 1). ... 

Although the diazene (8) (Scheme 2) decomposes thermally by loss of N2 to give the bicyclobutane (9) and its ring opened isomer (10) as the major products, photolysis with Pyrex filtered UV light gives mainly the rearranged bicyclic compound (11) and an unidentified hydrocarbon mixture of empirical formula C22H22. ... 

For example, the singlet trimethylenemethane 6.365 can be produced by heating the strained bicyclic hydrocarbon 6.363 or the diazene 6.364, or by photolysis of the latter. It dimerises to give a mixture of at least three hydrocarbons out of the eight possible, of which one stereoisomer of the fused-bridged product 6.366 is the major. The exocyclic methylene carbon is the unique carbon, and can be assigned to be the one with the large coefficient in ip2 and a zero coefficient in The experimental result can then be explained if the former is the HOMO and the latter the LUMO.873... 

A bicyclic Diels-Alder adduct is presumably in intermediate in these reactions although it was not actually observed. Interestingly, ketone hydrazones react with the tetrazine via their enamine tautomers, giving diazenes as products. 

As indicated earlier, heterocyclic systems can be synthesized via intermolecular cycloaddition of a diyl to a carbon-heteroatom Tt-bond (e.g., 19 to 22 and 23 to 25). Certain intramolecular processes also lead to heterocycles. For example, diazenes 86 and 91 are smoothly converted to the bicyclic furans, 90 and 92, in 87 and 70% yield respectively (Scheme 13) [14b]. These reactions are formally intramolecular cycloadditions to the carbonyl n bond. 

The opportunity to conduct a diyl trapping reaction depends upon the availability of bicyclic azo compounds to serve as the diyl precursor. A basic approach to these systems consists of fulvene formation [5], Diels-Alder cycloaddition, selective saturation of the A-5,6 pi bond, and conversion of the biscarbamate to the diazene linkage,[6] as depicted in Fig. (3). 

Diazene 63 was prepared in short order from readily available starting materials. Thus, acid 64, prepared in a standard manner from the isobutyric acid dianion and 3-methyl-3-butenyl p-toluenesulfonate, was first reduced with lithium aluminum hydride and then oxidized by using PCC/Celite to afford the expected aldehyde in 88% yield. In preparation for the Diels-Alder reaction that was to be used to assemble the bicyclic framework found in 63, the aldehyde was converted to fulvene 65. Initially, we attempted to accomplish this objective using methodology... 

Dimethylenecyclohepta-1,3-diene undergoes [8-1-2] cycloaddition with dimethyl diazene-l,2-dicarboxylate to give the bridged bicyclic adduct (Scheme 4.31). 

The tosylhydrazone of an oo-alkenyl ketone or aldehyde on heating under reflux conditions in toluene in the presence of K2CO3 affords the bicyclic diazene. Irradiation of the diazene results in the formation of cyclopropane-embedded substituted cyclopentanes (Scheme 6.35) [39]. 

Miscellaneous Sulphur Heterocycles.—A new hetero-bicyclic system possessing the thiaozonide structure has been reported. The new system was prepared by diazene reduction of thiophen ent/o-peroxides, a highly reactive but as yet unisolated intermediate in the preparation of but-2-ene-l,4-dione derivatives (Scheme 37). ... 

Stereoselective [3-r 2]-cycloadditions of trimethylenemethane equivalents provide an effective route to substituted five-membered carbocycles not otherwise easily prepared [38-41], Trimethylenemethane diradicals have commonly been accessed from bicyclic diazenes such as 127 not otherwise easily prepared [38, 102], Little showcased that with substrate 129, incorporating an embedded stereocenter, tricyclopentanoid 132 was formed with high yield and diastereoselectivity (dr=99 l. Scheme 18.22) [103], The predominant formation of diastereomer 132 was attributed to a kinetically controlled trapping of the energetically favored intermediate radical 130, in which A] 3 interactions involving the silyloxy substituent are minimized. 

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