Further Rearrangements

Further Rearrangements.— Acetic anhydride-pyridine rearranges both cis-and trans-4-acetamido-5-phenylisothiazolidin-3-one 1,1-dioxide [(61) and (64)] to 4-benzylidene-2-methyl-2-oxazolin-5-one (67). The isomers are convertible stereospecifically into the corresponding cis- and trans- parent 

The isothiazolone (68) undergoes base-catalysed rearrangement in di-methyl-acetamide or -formamide, yielding (70), presumably by way of the intermediate acylimine (69). The observation is of significance in penicillin chemistry, since compounds (68) and (70), amongst others, are by-products of the transformation of penicillin V sulphoxide methyl esters into cephalosporin derivatives/  

Complex Formation.—Suitably substituted 2-alkylisothiazolium salts yield crystalline complexes with heavy-metal salts, including mercury(ii) chloride, lead nitrate, and antimony trichloride.  

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Basically, the term [fi(a /2E)Ih is the visco-plastic dissipation term and the term [1/2ctoLc/] is the intrinsic interface strength term. This equation can be further rearranged to... 

Further rearrangement in terms of die data in die table gives... 

Equation 11-17 is refeiTed to as a Lineweaver-Burk equation involving separate dependent and independent variables 1/v and 1/Cg, respeetively. Equation 11-17 ean be further rearranged to give... 

In the case of an appropriate substrate structure, the carbenium ion species can undergo a 1,2-alkyl shift, thus generating a different carbenium ion—e.g. 4. The driving force for such an alkyl migration is the formation of a more stable carbenium ion, which in turn may undergo further rearrangement or react to a final product by one of the pathways mentioned above—e.g. by loss of a proton to yield an alkene 3 ... 

Steroids are heavily modified triterpenoids that are biosynthesized in living organisms from farnesyl diphosphate (Cl5) by a reductive dimerization to the acyclic hydrocarbon squalene (C30), which is converted into lanosterol (Figure 27.12). Further rearrangements and degradations then take place to yield various steroids. The conversion of squalene to lanosterol is among the most... 

Experiments with 14C-labelled substrates also demonstrated conclusively the intramolecularity of the rearrangement. The generally accepted scheme involves the formation of the dienone LXXXV which can lose a hydrogen atom if R = H, to form the ortho product or if R H further rearrangement to LXXXVI occurs with subsequent formation of the para product, viz. 

Tetrasubstituted pyrroles were also obtained in a coupled domino process carried out under solvent-free conditions on silica gel (Scheme 9). The process involved the transformation of the alkynoate 27 into the 1,3-oxazoUdine 28 that could be further rearranged (through loss of one molecule of water)... 

Condensation of phenylalaninenitrile, prepared from phenyl acetaldehyde, with 1-oxime of pyruvaldehyde afforded the pyrazine N-oxide, which was further rearranged to give the N-acetyl pyrazinone. The acetyl groups were removed by treatment with hydrazine to give the target 6-aminopyrazinone. 

It has been established that these abnormal products do not arise directly from the starting ether but are formed by a further rearrangement of the normal product ... 

It is interesting to observe that the product in the above reaction sequence will add another isocyanide ligand to give [Pt(PPh3)2(CNCH3)- C(=NCH3)R ]X but that this product does not further rearrange, at least under the relatively mild reaction conditions used 144). 

Compare this step to the mechanisms we saw in the previous section syn dihydroxy-lation), and you should see striking similarities. The initial product (shown above) is called a molozonide, and it subsequently undergoes further rearrangements, before ultimately giving the product upon treatment with dimethyl sulfide (DMS). The structure of DMS is ... 

Exactly such steric repulsion seems to be responsible for the further rearrangements observed with heptaphenylborepin (109 or 114 with R = Ph). The end result of a reasonable, but amazing sequence of pericyclic reactions is 1,2,3,3a,4,5-hexaphenyl-5-bora-3a,4-dihydro-5//-benz[>]in-dene (119), whose generation is initiated with 109 and continues through 116-118 (Scheme 11). The last step, 118-119, is an impressive intramolecu-... 

This prediction was borne out for reactions of 2,3-dioxabicyclo[2.2.2]octane 2 27) and the 7-substituted-[2.2.1]-peroxides 10201 and 1121) which afforded 62b-d respectively upon treatment with either methanolic KOH or triethylamine. The bismercurated-[3.3.2]-peroxide 47 reacted similarly with aqueous NaOH, but the resultant 5-hydroxycyclooctanone underwent further rearrangement to the internal hemi-ketal.49)... 

The reaction with trans-zndiyne 109 proceeds without further rearrangements giving rise to bis-3-borahomoada-mantane 79 (Scheme 42) <2002CEJ1537>. 

A different P-hydrogen can be removed from the carbocation, so as to form a more highly substituted alkene than the initial alkene. This deprotonation step is the same as the usual completion of an El elimination. (This carbocation could experience other fates, such as further rearrangement before elimination or substitution by an S l process.)... 

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. 

By the same type of epoxide rearrangement, other bicyclic and tricyclic orthoesters can be synthesized [80]. However, the orthoesters are only the kinetic products and, if not sufficiently inert, can further rearrange under reaction conditions to more stable tetra-hydrofuran derivatives (cf. Scheme 8.40). In many cases, the tetrahydrofurans are the... 

As illustrated in Scheme 5, ( )-31 directly gives cation 37 via phenyl participation, while (Z)-31 provides 38 more slowly via methyl participation. Cation 37 can further rearrange to more stable 38 by 1,2-hydride shift, but 38 cannot isomerize to less stable 37. As a result, ( )-31 can afford not only 33,35, and 36 but also 34, but (Z)-31 only gives 34 and 35 depending on the nucleophilicity of the solvent. The unrearranged product 32 is formed via inversion only from (Z)-3I in a more nucleophilic solvent. This must result directly from the SN2 reaction of (Z)-31. 

This can be further rearranged to give an expression in terms of G (co) ... 

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