Rearrangements base-catalyzed

These processes take place by a mechanism of /3-or a-elimination. Allyl alcohols may be formed in the course of )3- or a-elimination (Eq. 108). 

If the transannular reaction is hindered and /S-elimination is not possible either, isomeric ketones are produced. The reaction following the carbenium pathway is strongly temperature-dependent. Thi procedure has provided a new possibility of synthesizing bicyclic compounds that are otherwise difficult to obtain. An example is the synthesis of pentacyclododecanol. (Eq. 111).  

Numerous instances have been described in the literature on the base-catalyzed isomerization of oxiranes with various structures. The product in the reaction of benzocycloalkene oxiranes with lithium diisopropylamide depends on the ring size it is either a transannular insertion product or a transannular product and some a-ketone or only a /3-ketone (Eq. 112).  

Studies have been made of the rearrangement of oxiranes containing an acetylenic side-chain, isoprene oxides, and 7,5-unsaturated oxiranes.  

The kinetics of the reaction of fert-BuOK with aryl-substituted oxiranes follow a linear Taft correlation. Steroid 5a,6a- and 5(3,6/3-oxiranes react with pyridine to give 97 (Eq. 113).  

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The base catalyzed rearrangement of a monotosylated 1,2-diol on alumina, followed by immediate condensation of the sensitive ketone with methylenetriphenylphosphorane, gave the exo-methylene compound below (G. Btlchi, 1966B). 

Addition reactions with tetracyanoethane have provided access to 2,5-diamino-3,4-dicyano-thiophene and -selenophene (58JA2775,81ZOR1958). Base catalyzed rearrangement gives the isomeric pyrrolethiol (Scheme 53). 

Treatment of 7V-benzyl-l,2-benzisoxazolin-3-one with base produced a benzoxazine-4-one (see Scheme 88). The base catalyzed rearrangement of the 2-methyl-3-phenyl-l,2-benzoisoxolium salt to an oxazine is believed to proceed via a similar intermediate (67AHC(8)277). A number of other decompositions could possibly proced via this proposed route (74HCA376, 67AHC(8)277), which has also been postulated for the rearrangement of a variety of isoxazolium salts, e.g. the conversion of (200) into (202) (Section 4.16.3.3.2(i)(b)). 

Similar to the thermal rearrangements discussed in the previous subsection, the base-catalyzed rearrangements of cyclic four-membered sulfones to five-membered sulfinates have also been reported. For example, Dodson and coworkers have observed the rearrangement of cis- and trans-2,4-diphenylthiethane 1,1 dioxides to cis- and trans-3, 5-diphenyl-1,2-oxathiolane (2,3)-cis-2-oxides, respectively (equation 35), on treatment with t-butoxymagnesium bromide, which is stereospecific with respect to the phenyl group but stereoselective with respect to the oxygen atoms on sulfur. 

Thus, base-catalyzed rearrangements of one of the heterocycles of difurazanyltriazenes gave the corresponding 2-furazanyltetrazoles (Scheme 7) <2004RCB1121>. 

The carbonium ion rearrangement is essentially an internal displacement reaction in which a carbonium ion becomes bonded to some other portion of the same molecule. Certain base-catalyzed rearrangements have been discussed in the section on carbonium ion rearrangements because they have the distinguishing characteristic of being internal electrophilic displacements by an atom bearing at least a partial positive charge. 

Scheme 4-21 shows the preparation of L-threitol and L-erythritol.38 Epoxy alcohols (2J ,3iS)-61 and (2S,3/ )-61. generated by asymmetric epoxidation, are exposed to sodium benzenethiolate and sodium hydroxide in a protonic solvent to undergo base-catalyzed rearrangement, yielding the threo-diol 62 and erythro-diol 63, which can then be converted to the corresponding tetraacetate of l-threitol 67 and L-erythritol 69 through subsequent transformations. 

The base-catalyzed rearrangement of oxadiazolylureas 184 into ben-zoylamino-l,2,4-triazolin-5-ones 185 has been mechanistically examined by using amines as catalysts in acetonitrile and benzene, and borate buffers at various pS in dioxan-water [90JCS(P2)1289]. For the piperidine-... 

There is kinetic evidence that the migration step in these base-catalyzed rearrangements is concerted with ionization. Thus, in cyclopentane derivatives, the rate of reaction depends on the nature of the trans substituent R, which implies that the migration is part of the rate-determining step.26 27 28 29... 

Scheme 10.3. Base-Catalyzed Rearrangements of a-Halo-ketones...

Mechanisms that are probably associated, respectively, with these processes are (i) the formation of betaine intermediates (306) (Fig. 3) 103,143,149,197,200 homolysis or heterolysis of the X—Z bond (304) or the X—Z bond (305) giving diradicsd (307) or dipolar (308) intermediates, (iv) 1,3-dipolar cycloaddition yielding intermediate adducts (e.g, 309), The base-catalyzed rearrangements (ii) present very interesting mechanistic problems suitable for speculation and experimental enquiry. 

An interesting spiro compound (59) that contains the thietane ring was obtained in minor amounts by dimerization of dimethylketene and subsequent treatment with P2S5. Pyrolysis of the spiro structure produced 60 and the thietanethione 61, which can also be prepared by base-catalyzed rearrangement of 62, a process that can be carried out as well with 63 to give the 2-thietanone 60. The solvent and the basicity of the catalyst are important parameters in this rearrangement. 

It has been pointed out (75TL213) that path A behavior might be more complex in some instances, and involve initial ylide formation followed by intramolecular proton abstraction (Scheme 202). The aryloxy quaternary salt (274), formed by reaction of pyridine 1-oxide with an arenediazonium salt, undergoes an interesting base-catalyzed rearrangement that is believed to take the course (path E) indicated (Scheme 203) (71JA3074). 

The chemical, physical, and thermal properties ana resistance to degradation of polysiloxanes is the result of the high energy (106 kcal/mol) and the relatively large amount of ionic character of the siloxane bond. The ionic character of the Si—O bond facilitates acid and base-catalyzed rearrangement and/or degradation reactions. Under inert conditions, highly purified polydiphenyl- and polydimethylsiloxanes are stable at 350 to 400 °C. 

N-Unsubstituted 1//-azepines are rare since, like the parent system, they tautomerize readily to the 3H-isomers in whose preparation they are often considered as transient intermediates (see Section 5.16.4.1.2(h)). This rearrangement is particularly apparent with 2-amino- and 2-alkoxy derivatives since stabilization of the 37/-azepine is then possible by amidine and imidate type resonance. For the CH2-containing tautomers the order of stability appears to be 3H > AH > 2H, a fact attested to by the facile thermal and base-catalyzed rearrangements of AH- azepines to the 3H-tautomers (72CB982) and the rarity and inherent instability of 2H-azepines. The latter are well established as intermediates in the formation of 3H- azepines (74JOC3070) but have been characterized only as their benzologues. 

The stereoisomeric hexahydrodiazocines, as expected, differ considerably in reactivity. The cis- 3,8-diphenyl Z isomer undergoes base-catalyzed rearrangement to the hydrazone (1,4,5,6,7,8-hexahydroazocine) 1000 times faster than the trans-Z-diphenyl, and the latter is also very much more stable thermally. The least stable isomer is the trans-Z compound, which decomposes explosively at the melting point (62 °C). 

The base-catalyzed rearrangement of the cyclopentadiene-dichloroketene adduct gives tropolone. The reaction course passes through an SN2 displacement of the enol, fragmentative dehydrochlorination, and enolization [85],... 

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. 

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