Base-induced fragmentation

Reactions of butenylidene-bis-phosphonium salts with PCl3/triethyl-amine under similar conditions proceeded predominantly via base-induced fragmentation to triphenyl phosphine and polymeric products of unknown constitution (Scheme 2). Monocyclic bis-phosphonio-phospho-lides formed only as spectroscopically detectable but hardly isolable byproducts [18, 19]. 

Hydroxynitrosamine Base Induced Fragmentation Products, Yields, Reaction Times, and Half-Lives. 

Eschenmoser and collaborators (28) have reported the base induced fragmentation of a.B-epoxy-tosylhydrazones to produce acetylenic ketones (e.g. 69 71). Interestingly, in this fragmentation, both the triple-bond and molecular nitrogen are produced from the key intermediate 70 via an anti mode. The decarboxylation of nitrobenzisoxazole carboxylate (72) into 2-cyano-5-nitrophenol (73) can also be viewed as a trans-elimination (29, 30). 

An intramolecular [2+2] photocycloaddition of allyl ethers with dioxinones followed by a base-induced fragmentation leads to substituted tetrahydropyran-4-ones <1997TL5579>. A one-pot scandium triflate catalyzed diastereoselec-tive cyclization between aldehydes and (3-hydroxy dioxinones 1046 followed by alkoxide addition to the resulting bicycles 1047 leads to 3-carboxy-substituted tetrahydropyran-4-ones 1048 with high levels of diastereoselectivity as a mixture of keto/enol tautomers (Scheme 268, Table 49) <20050L1113>. 

A synthesis of 2-cyanocyclohexanone 4.45 from cyclohexanone is shown below. Formylation of cyclohexanone produces a mixture of keto/enol tautomers 4.42 and 4.43, the equilibrium lying to the side of the enol 4.42. Treatment with hydroxylamine affords isoxazole 4.44, and base-induced fragmentation of the isoxazole ring affords 4.45. Explain the regioselectivity of the isoxazole formation, and the mechanism of the fragmentation process. 

Unfortunately, the dependence of the rates on pH cannot be measured by our method since acidimetric titrations are not possible in buffered solutions. However, in weakly acidic and neutral solutions, there cannot be much influence because otherwise no rate constants could have been obtained. In alkaline solution, on the other hand, a base induced fragmentation occurs, which is faster by some powers of 10 than the solvent fragmentation. Thus, in 0.025N methanolic NaOH at 0°C, the half lifetime of stilbene ozonide is only 2 minutes. 

The past few years have witnessed a considerable effort by several research groups to effect stereoselective syntheses of appropriately substituted hydroazulenes. Recently some new approaches to this problem have been reported. Marshall and Huffman, for instance, have found that treatment of the enone-mesylate (395) with diborane followed by base-induced fragmentation of the intermediate boronate (396) yielded the cyclodecadienol (397) in 60 % yield. Solvolysis of the p-nitrobenzoate of (397) in aqueous dioxan containing sodium bicarbonate afforded the hydroazulenol (398) in 70 % yield. The high degree of stereoselectivity in this reaction is rationalized on the basis of the preferred mode of cyclization of the allylic cation (399). To date, the solvolytic routes to hydroazulenes have... 

A rather different approach to the hydroazulenic synthesis has been adopted by Hendrickson and Boeckman. Treatment of 1-cyclopentenylcarboxalde-hyde (411) with the pyrrolidine enamine of cyclopentanone (412) yielded the thermodynamically most stable adduct (413) which, on quaternization and base-induced fragmentation, gave the acid (414) in 25% yield. Furthermore, the acid (414) has been converted into the epoxide (415) and the (5-lactone (416), both of which are potential synthetic precursors of guaiane-type sesquiterpenoids, e.g. pseudoguaianolides. 

An intriguing combination of two cyclopropyl systems allows cyclobutanone formation according to equation 105 base-induced fragmentation of an intermediate cyclobutanol stereoselectively forms a functionalized olefin . 

Base-induced fragmentation of 20-hydroxy-16a- and -16)3-sulphonyIoxy-pregnanes affords a variety of products in addition to the expected androst-16-ene. ... 

Regioselectivity in the context of substituted alkenyl anion has also been addressed. It is known that the base-induced fragmentation of tosylhydrazones (Shapiro reaction) derived from a-substituted ketones leads predominantly to the "kinetic" (cf. ketone enolates)... 

The direct conversion of thiols into chlorides has been effected by a soft base-induced fragmentation (56) of the acyl chloride (4). The formation of (4) from thiols also involves a specific soft-soft interaction. 

Photocycloaddtion of the cyclopentyl derivative 23 also proceeds regioselectively to give the cis- and /rans-cyclobutane 24 and 25 in 70% and 12% yield, respectively. Base-induced fragmentation of both isomers furnishes a single diketone 26 in excellent yield. 

Irradiation of enol acetate 51 affords the exclusive straight adduct 52 as a 3 1 mixture of methyl epimers. Base-induced fragmentation and subsequent spontaneous retroaldolization-realdolization sequence generates 54, which has been converted to daucene. 

A reinvestigation of the base-induced fragmentation of chloroacetyl-hydrazide (559) has shown that the originally postulated mechanism involving concerted elimination to give di-imine and keten is not acceptable. An alternative route involving formation of an intermediate A-amino-a-lactam (560) is proposed. 

Irradiation of the diazepines (165) gives the isomeric diazabicyclo[3 0]hepta-dienes (166). These are thermally unstable, reverting to the diazepine. Base-induced fragmentation of (166) gives rise to the substituted butadiene (167). 

Base-induced fragmentations of y-hydroxyammonium compounds can give poor yields when the product is an aldehyde. However, flash thermolysis is now reported to give good yields of unsaturated aldehydes [equation (20)]. 

Finally, base-induced fragmentation of (234) produces (235), but this cannot be isolated because of further reactions leading to dimer . However, (235) can be trapped by in situ reduction to give (236) in high yield. 

The first total synthesis of the epoxyisonitrile natural product (64) involves a base-induced fragmentation reaction to provide the unsaturated ester fragment (Scheme 49).The stereospecific rearrangement of 2,2-dimethylcyclobutanol into optically active 1,2-c/s-disubstituted cyclopropanes has been reported (Scheme 50). 

The anti-leukaemic and anti-tumour compound eriolanin (30) has been synthesized by Schlessinger and Roberts by a route in which the key step is a base-induced fragmentation of (31) to (32). ... 

Shultz and Berger have reported, in a preliminary note, the results of base-induced fragmentation of methoxymethyl esters. The products of fragmentation recombine to form jS-hydroxyesters (116) together with the untrapped esters (117), the relative amounts depending on the nature of the substituents (Scheme 37). Analogous reactions of a-bromo-methoxymethyl esters i.e. Rj = Br in the scheme) lead to glycidic esters (118). 

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