Rearrangement bicyclo heptanes

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

Formation of the bicyclo[3.2.0]heptane system was visualized as a neopentyl-type rearrangement with the oxoalkyl group acting as the migrating group. However, neopentyl rearrangements occur in acid media via cations. 

There are many examples of 1234-1243 rearrangements. In particular, the reaction provides a useful route to bridgehead-substituted bicyclo[3.2.0]heptanes, but is normally in competition with the homoallyl rearrangement (see Section 2.1.). There are three main possibilities for the preparation of cyclobutanes ... 

Further examples of the preparation of bicyclo[3.2.0]heptan-6-ones by base-catalyzed rearrangement of y-oxo p-toluenesulfonates are listed in Table 4. 

Table 4. Bicyclo[3.2.0]heptan-6-ones and a Bicyclo[4.2.0]octan-7-one by Base-Catalyzed 1234-1243 Rearrangement of y-Oxo />-ToluenesuIfonates...

Oxaspiropentanes have been generated and rearranged in a large variety of different environments. A series of alkylidene- and allylidenecyclopropanes, present in the structures of bicy-clo[3.1.0]hexanes or bicyclo[4.1.0]heptanes, were epoxidized and rearranged in situ to bicyclic ketones with the alkyl or allyl group preferentially to exclusively in the exo position (Table 4).51 This corresponds to a preferential to exclusive epoxidation of the corresponding alkenes from the sterically less demanding exo face. 

Bicyclo[4.2.0]octan-7-ones 7, substituted at C8, resulted from addition of 7-lithio-7-methoxy-bicyclo[4.1.0]heptane to saturated and unsaturated aldehydes and subsequent rearrangement.146148... 

Bicyclo[3.2.0]heptan-6-ones 10 were obtained by transformation of 2-methoxycyclohex-2-enones 8 to l-methoxybicyclo[4.1.0]heptan-2-ols 9 and subsequent rearrangement (Table 13).155 161 For the synthesis of the requisite precursors, a sequence of reduction and cyclopropanation 8 - 9, eventually followed by oxidation and an addition to the carbonyl group 9 — 11 — 12, proved most effective. 

Table 13. Rearrangement of l-Methoxybicyclo[4.1.0]heptan-2-ols to Bicyclo[3.2.0]-heptan-6-ones...

At 150 °C, the analogous 2-methylenetricyclo[4.1.0.01,3]heptane (34) equilibrated with 35 and 36 as products of a methylenecyclopropane rearrangement and a subsequent retro-Diels Alder reaction, respectively, until at 180 °C an irreversible rearrangement to bicyclo[4.2.0]octa-l,5-diene and 3,4-dimethylenehexa-l,5-diene (39) via diradical 37 took place.223,224 Increasing pressure favored the formation of the bicyclic system 38 (50% yield at lOOTorr).223... 

Bicyclo[2.2.1]hept-7-yl cations, formed via the corresponding diazonium ions by diazotization of bicyclo[2.2.1]heptan-7-amine derivatives, rearrange partially to give bicyclo[3.2.0]heptanes (Houben-Weyl, Vol. 4/4, pp 106-107). Related diazonium ions can also be formed by irradiation of bicyclo[2.2.1]heptan-7-one tosylhydrazones in diluted sodium hydroxide and rearrange to form predominantly bicyclo[3.2.0]heptan-e. o-2-ols. On photolysis, the hydrazone 17 in 0.2 M sodium hydroxide gave t> <7o-2-methylbicyclo[3.2.0]heptan-exo-2-ol (18) with 77% selectivity and in 72-78% overall yield (GC).68... 

As discussed, electron-donating groups at Cl in bicyclo[2.2.1]heptane systems promote the rearrangement to bicyclo[3.1.1]heptane derivatives. Photolysis of 1-methyl bicyclo[2.2.1]heptan-2-one tosylhydrazone in 1,2-dimethoxyethane in the presence of sodium hydroxide gave 2-methyl-bicyclo[3.1. l]heptan-2-ol (26a) in 7.5% yield in addition to bicyclo[2.2.1]heptane derivatives, while the 1-phenyl derivative gave the ring-contracted product 26b in only 2% yield.91... 

Very highly constrained bicyclic alkenes rearrange via carbene intermediates to ring-contracted products. Thus, bicyclo[3.3.0]oct-2-ene gave 6-methylenebicyclo[3.2.0]heptane (38), and bicy-clo[2.2.1]hept-2-ene gave 5-methylenebicyclo[2.1.1]hexane (39).113... 

Nearly all examples of epoxidation of alkylidenecyclobutanes involve 3-chloroperoxybenzoic acid.15 58-70 This is because the conditions are mild, the workup is easy and few byproducts are formed. Generally, dichloromethane or chloroform is used as solvent. Solid sodium hydrogen carbonate is occasionally added to avoid acid-catalyzed rearrangement of the spiro compound. For example, 6-isopropylidene-l,4,4-trimethylbicyclo[3.2.0]heptan-3-one reacted with 3-chloroperoxybenzoic acid and sodium hydrogen carbonate to give 2,2,3, 3, 6-pentamethyl-spiro[3-oxabicyclo[4.2.0]octane-8,2 -oxirane]-4-one (4) in quantitative yield. However, without the use of sodium hydrogen carbonate, substantial amounts of 2,2,6,9,9-pentamethyl-3-oxa-bicyclo[4.3.0]nonane-4,8-dione (5) and 2,2,6,8,8-pentamethyl-3-oxabicyclo[4.3.0]nonane-4,9-dione (6) were also formed.15-64... 

Apart from the rearrangement of the tricyclo[4.1.0.03/7]heptane to the bicyclo[2.2.1]hept-2-ene system under acid catalysis, it can be effected by silver(I) perchlorate [silver(I) assisted ionization]19 and by photolysis at 185 nm.20... 

A similar rearrangement converted a 3-bromopropyl-substituted bicyclo[3.2.0]heptan-6-one 5 to fra i-octahydronaphthalen-l(2//)-one 6.118... 

Bromoallyl)cyclobutanones rearranged to give bicyclo[4.1.0]heptan-3-ones 7 and 8 on reduction with tributyltin hydride.119... 

In the presence of proton and/or Lewis acid and strong nucleophiles bicyclo[3.2.0]heptan-6-ones are converted to 3-substituted cycloheptanones (Table 15). Bicyclo[3.2.0]heptan-6-ones rearrange to give 3-iodocycloheptanones on treatment with iodotrimethylsilane. Zinc(II) iodide or mercury(II) halides as catalysts enhance the rate and the selectivity of the reaction.31 If a second, enolizable carbonyl group is present, an intramolecular alkylation may follow the ring enlargement under these reaction conditions.32 Consecutive treatment with tributyltin hydride/ 2,2 -azobisisobutyronitrile affords reduced, iodo-free cycloheptanones, whilst treatment with l,8-diazabicyclo[5.4.0]undecene yields cycloheptenones.33 Similarly, benzenethiol adds to the central bond of bicyclo[3.2.0]heptan-6-ones in the presence of zinc(II) chloride and hydrochloric acid under anhydrous conditions to form 3-(phenylsulfanyl)cycloheptanones.34... 

Attempts have been made to generate a number of 2-bicyclo[ .l. OJalkyl cations under stable ion conditions55. Ionization of bicyclo[3.1.0]hexan-2-ol 57 in SbF5/S02ClF even at -140 °C gave the rearranged cyclohexenyl cation 59, presumably through the bicy-clo[3.1.0]hex-2-yl cation 60 (equation 40). However, ionization of bicyclo[4.1.0]heptan-2-... 

In the absence of nucleophiles, the anti- and syn-5-methyl-bicyclo[2.1.0]heptanes 26 and 28 rearrange to the isomeric methylcyclopentenes 27 and 29 as major products under photo-ET conditions, although some crossover is detected for 28 (see Scheme 8) [74]. Matrix ESR studies show that bridging-bond stretched intermediate cations are formed, and that substituent shift from the middle carbon to form the... 

An efficient method for the preparation of functionalized seven-membered rings involves the cleavage of the internal bond of a bicyclo [4.1.0]heptane moiety by a thermal homo[l,5]hydrogen shift or enolene rearrangement (equation 25). 

Bicyclo[2.1.0]pentane (62) reacts thermally (120°C, 48 h), with electron-poor olefins via a stepwise, diradical mechanism to afford bicyclo[2.2.1]heptane products derived from the addition of the olefins on the endo side of the bicyclo envelope . On the other hand in the presence of nickel(O) catalysts, the addition occurs under milder reaction conditions and with an alternative stereochemistry, i.e. predominantly on the exo face (equation 42) No class of transition metal catalysed rearrangements has been the subject of more controversy than those of bicyclo butane. A general mechanistic picture, consistent with the experimental facts, has, however, been presented ... 

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