Bicyclic intermediates, formation

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

N3S3F2][MF6] can be isolated from the reaction of 8.12b with MF5 (M = As, Sb), but they decompose to give the bicyclic cation [S4N5] (Section 5.3.8). Treatment of 8.12a with trimethylsilyl azide generates the polymer presumably via the intermediate formation of... 

Antidepressant activity is retained when the two carbon bridge in imipramine is replaced by a larger, more complex, function. Nucleophilic aromatic substitution on chloropyridine 31 by means of p-aminobenzophenone (32) gives the bicyclic intermediate 33. Reduction of the nitro group (34), followed by intramolecular Schiff base formation gives the required heterocyclic ring system 35. Alkylation of the anion from 35 with l-dimethylamino-3-chloropropane leads to tampramine 36 [8]. 

Scheme 13.33 shows broad retrosynthetic formulations of the longifolene syntheses that are discussed in this subsection. Four different patterns of bond formation are represented. In A, the C(7)-C(10) bond is formed from a bicyclic intermediate. This pattern corresponds to the syntheses in Schemes 13.24, 13.25, 12.26, and 13.29. In retrosynthesis B, there is concurrent formation of the C(l)-C(2) and C(10)-C(ll) bonds, as in the synthesis in scheme 13.28. This is also the pattern found in the synthesis in Scheme 13.32. The synthesis in Scheme 13.29 corresponds to retrosynthesis C, in which the C(l)-C(2) and C(6)-C(7) bonds are formed and an extraneous bond between C(2) and C(5) is broken. Finally, retrosynthesis D, corresponding to formation of the C(2)-C(3) bond, is represented by the synthesis in Scheme 13.31.

Reactions involving intermediate formation of bicyclic sulfonium salts... 

The favorable stereochemistry and the displacement of the very good leaving group promoted the formation of a [3.2.0] bicyclic intermediate (see Scheme 2). Owing to the absence of a C2 symmetry axis two different [3.2.0] bicyclic sulfonium salt intermediates, 8a and 8b, can be generated by nucleophilic transannular attack of sulfur on the... 

The reverse process has also been examined. 2-Phenyloxazole is converted in a similar fashion to 3-phenyl-2//-azirine-2-carbaldehyde on irradiation in benzene or cyclohexane.128 Further rearrangement to the corresponding isoxazole can be effected thermally but not photochemically. A competing pathway leading to the formation of 4-phenyloxazole has also been observed and is thought to involve a bicyclic intermediate arising by 2,5-bonding. 

Similarly, in a 1,3-dipolar cycloaddition of DMAD to the conformationally locked cyclic a-alkoxycarbonylnitrone (727), bicyclic ring systems, containing a nitrogen atom at the bridgehead position have been synthesized. A mechanistic interpretation of the origin of the fused pyrroles (729) includes the intermediate formation of the aziridine ring in (728) (Scheme 2.303) (820). 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

Transient digermene 86 was postulated as possible intermediate in the reaction of 1,2-dichlorodigermacyclobutane 87 with Mg/MgBr2 in the presence of an alkene, leading to the bicyclic compound 8878 [Eq. (17)]. However, an electron transfer reaction with the intermediate formation of a germyl radical is also a possibility.78 Similarly, transient 1,2-digermacyclobu-... 

Bazureau and co-workers reported an unusual hydrolysis of the tetrasubstituted oxazoline 323. In this case, 323 was unstable and hydrolyzed quickly to the tetra-hydroisoquinoline amino ester 324 under mildly basic conditions (Scheme 8.97). Although the authors did not discuss the reaction mechanism, it is possible that the facile hydrolysis is facilitated by the formation of the bicyclic intermediate 325 produced via intramolecular trapping of 326. 

While the majority of trapping experiments favors a biradicaloid structure, evidence for a bicyclic intermediate (butalene, 31) was also found. " Breslow et al. " reported the formation of 31 (the possible existence of which had been pointed out by Dewar before ) upon dehydrohalogenation of 3-chloro[2.2.0]bicyclohexadiene... 

This method of nitrogen elimination from tetrahydropyridazines is frequently used in the synthesis of polycyclic systems. The thermolysis, as well as photolysis, of 2,3-diazabicy-clo[2.2.1]hept-2-enes gave bicyclo[2.1.0]pentanes via intermediate formation of cyclopenta-1.3-diyl, from which bicyclic compounds with one three-membered and one four-membered segment are formed this type of reaction is discussed in Houben-Weyl, Vol. E17b, pp 1089-1108 and 1163-1166. 

In 1969 the reaction of 1,2,4,5-tetrazines (39) with imidates (198) was reported. The formation of the 1,2,4-triazine products (199) is explained by a [4 + 2] cycloaddition of the tetrazine with the C=N double bond of the imidate to give the bicyclic intermediate (200) which eliminates nitrogen and alcohol (69JHC497). Further studies on this reaction seem to... 

Reaction of 1,2,4,5-tetrazines (39) with AMhionitrosodimethylamine (217) and N-sulfinylanilines (218) led to the isolation of 4-dimethylamino-4// -1,2,4-triazoles (219) and 4-aryl-4//-1,2,4-triazoles (220), respectively. The formation of these products was explained by a [4 + 2] cycloaddition to give bicyclic intermediates, then elimination of nitrogen to yield the 1,2,4,5-thiatriazines, which form the products by extrusion of sulfur and sulfur oxide, respectively (Scheme 12) (79CZ230, 77AP269). 

The preferred elimination of a molecule of carboxylic acid instead of water in this case may be explained by the intermediate formation of a bicyclic oxetane 289. Along with conversions described here, which have some analogy with what is known from a-1 dimerizations and reactions of monomeric C-adducts, a new transformation has been found in the case of 4-1 dimerization, namely disproportionation, as shown in the following paragraphs. 

A novel reaction of pyroglutamate (6) and an isocyanate promoted by NaH in THF leads to functionalized hydantoins (7) in good yields. The reaction involves the ring closure of intermediate (8) by a nucleophilic attack on the carbonyl of the ester function followed by expulsion of an alkoxide anion resulting in the formation of the bicyclic intermediate (9). The alkoxide anion in turn can open this bicyclic intermediate with formation of anions (10) and (11) leading to the final racemic hydantoins (7) (Scheme 3).8... 

Dithiazolidine 90a reacts with ketene enolates 98 at the imino group with the formation of new dithiazolidine derivative 99 through a possible bicyclic intermediate containing a /3-lactame fragment (Scheme 20) <1998EJ0515>. 

The reaction of diphenylketene with 1,3,2-diazaboroles 234 in hexane at — 20 °C leads to the formation of 1,3,2-oxazaborolidines 238 in 64-70% yield. The reaction proceeds via the coordination of the ketene oxygen to boron 236 followed by a [2+3] cycloaddition yielding the bicyclic intermediate 237. Fission of B-N bond leads to the product formation as yellow to colorless crystals (Scheme 40) <20000M5791>. 

Two additional mechanisms can be considered to reach the intermediate anion (Scheme 8). In one, after deprotonation, a ring-opening step followed by ring closure to the favored five-membered ring can be postulated. In the other, a bicyclic intermediate can be formulated by opening the S-N bond. This is, however, a less probable pathway for explaining the selectivity of the product formation <1997HCA671>. 

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