Rearrangement heteroatoms

You have already seen that a carbon-heteroatom bond is easy to make, since we used such bonds as natural places for disconnections (frames 234 ft). It is good strategy therefore to make a carbon-heteroatom bond and then to transform it into a carbon-earbon bond. The Claisen rearrangement is one way to do this an ortho allyl phenol (B) made from an allyl ether (A) ... 

The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

Although evidence is not conclusive, indications are that the rearrangements are concerted. Heteroatom compositions required for the rearrangement are at least one N—O bond in the nucleus of the starting material and the formation of a C—N, N—N or C—S bond in the product (79AHC(25)147, p. 193, 81AHC(29)14l). 

Electron deficient species can attack the unshared electron pairs of heteroatoms, to form ylides, such as in the reaction of thietane with bis(methoxycarbonyl)carbene. The S —C ylide rearranges to 2,2-bis(methoxycarbonyl)thiolane (Section 5.14.3.10.1). A"-Ethoxycar-bonylazepine, however, is attacked by dichlorocarbene at the C=C double bonds, with formation of the trans tris-homo compound (Section 5.16.3.7). 

The advantage of starting with a ring of -1 members lies in the nature of the rearrangements, which proceed through cyclic transition states, so that the system never becomes open-chain — the carbon-carbon bond is broken only while the carbon-heteroatom bond is being made. 

The Cornforth rearrangement involves the thermal interconversion of 4-carbonyl substituted oxazoles, with exchange between the C-C-O side-chain and the C-C-O fragment of the oxazole ring. These reactions generally involve compounds where a heteroatom (-OR, -SR, -Cl) is attached to the 5-position (R2) of the starting oxazole. 

Regarding the series of hetero aromatic pentacyclic compounds with three heteroatoms, an accelerated synthesis of 3,5-disubstituted 4-amino-1,2,4-triazoles 66 under microwave irradiation has been reported by thermic rearrangement of dihydro-1,2,4,5 tetrazine 65 (Scheme 22). This product was obtained by reaction of aromatic nitriles with hydrazine under microwave irradiation [53]. The main limitation of the method is that exclusively symmetrically 3,5-disubstituted (aromatic) triazoles can be obtained. 

Another example of ring closure involving a 1,5-H shift appears to be that provided by Jung,119 who reported that the heteroatom-substituted silenes 144 rearranged to give 1,3-disilacyclobutanes 145 via a diradical intermediate (Eq. 51). When R = Cl the yield was 30%, and with R = MeO the yield of the disilacyclobutane was 44%. 

Reactivity of Substituents Attached to Ring Heteroatoms 12.01.8.1 Rearrangements in A/-Allylquinolizinium Derivatives... 

Cycloaddition of OTr/n-bomyltriazolinedione 706 to 9-thia-l-azabicyclo[4.2.1]nona-2,4-diene 9,9-dioxide 705 gave compound 707 as a diastereoisomeric mixture (Scheme 112). The formation is attributed to the sequential operation of a regioselective [2+2] cycloaddition and a heteroatomic variant of the vinylcyclobutane-cyclohexene rearrangement <2006JOC6573>. 

A mechanistic picture which reconciles the experimental results is given in Scheme 24. It is assumed that both the heteroatom and the double bond of the allyl halide compete for an electrophilic metal carbene. Heteroatom attack yields a metalated ylide 129, which may go on to ylide 131 by demetalation and/or to allylmetal complex 130. Symmetry-allowed [2,3] rearrangement of 131 accounts for product 132, and metal elimination from 130 gives rise to products 132 and 133, corresponding to [2,3] and [1,2] rearrangement, respectively, as well as haloacetate (if R3 = CHc ). 

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