Ring expansion substitution

Ring-expansion substitution reactions of 1,3-dithiolanes and 1,3-dithianes with N-halo- and IV-cyano-succinimide give mono-halo- and -cyano-l,4-dithiins and -1,4-dithiepins (Scheme 19). With NBS and BU4NN3 and NBS and NH4SCN, the azido and thiocyanato analogues are formed, respectively.35... 

Synthesis From Other Ring Systems. These syntheses are further classified based on the number of atoms in the starting ring. Ring expansion of dichlorocyclopropane carbaldimine (53), where R = H and R = ryl, on pyrolysis gives 2-arylpyridines. Thermal rearrangement to substituted pyridines occurs in the presence of tungsten(VI) oxide. In most instances the nonchlorinated product is the primary product obtained (63). 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

Ring expansion of activated aziridines (43) with sulfur ylides also provides a synthesis of azetidines (75JOC2990, 58BSF345, 81CC417). The highly reactive sulfonium methylide (44 R = R = H) undergoes further reaction with the azetidines (46), but the reaction is satisfactory for substituted methylides. The less reactive sulfoxonium methylide (45 R = R = H)... 

Ring expansion of haloalkyloxiranes provides a simple two-step procedure for the preparation of azetidin-3-ols (Section 5.09.2.3.2(f)) which can be extended to include 3-substituted ethers and O-esters (79CRV331 p. 341). The availability of 3-hydroxyazetidines provides access to a variety of 3-substituted azetidines, including halogeno, amino and alkylthio derivatives, by further substitution reactions (Section 5.09.2.2.4). Photolysis of phenylacylamines has also found application in the formation of azetidin-3-ols (33). Not surprisingly, few 2-0-substituted azetidines are known. The 2-methoxyazetidine (57) has been produced by an internal displacement, where the internal amide ion is generated by nucleophilic addition to an imine. 

Ring expansions of suitable /3-lactams can also be achieved on treatment with base rearrangement of the Af-substituted azetidin-2-ones (133) occurs in the presence of LDA to give (134) (72JA9261). Aminolysis of the /3-lactam epoxide (135) and the aldehyde (137) affords (136) and (138) respectively (81JHC1239). 

Cyclopropyl methanols when treated with a combination of hydrogen fluoride, pyridine, potassium hydrogen fluoride, and diisopropylamine undergo fluonnation and rearrangement to give excellent yields of homoallylic fluorides Chlorobenzene substituted cyclopropyl methanols at low temperatures leads to ring expansion to give... 

The corresponding 4-(iodomethyl)-l,4-dihydropyridine undergoes rapid transformation to the 4//-azepine 2 under milder conditions (e.g., on a silica gel column or on a TLC plate).120 4//-Azepines are also formed by the ring expansion of 4-(chloromethyl)-l,4-dihydropyridines with a variety of other nucleophiles, but, in general, are not isolated since they readily suffer addition of the nucleophile to yield 4-substituted 4,5-dihydro-l//-azepines.120 121-132... 

In contrast to the acyl- and sulfonylnitrenes described in this section, arylnitrenes produced thermally or photolytically from aryl azides, including those bearing strongly electron-withdrawing substituents (e.g., CN, N02, CF3), fail to promote ring expansion of arenes to 1H-azepines, although intermolecular substitution of electron-rich substrates, e.g. mesitylene and A.TV-dimethylaniline, have been noted.167... 

Azidoquinoxaline behaves in a similar manner to the azidoquinolines and on photolysis in strongly basic solution yields the 5//-pyrazino[2,3-c]azepine 30.153 Similarly, 2-azidophenazine undergoes ring expansion to the azepino[3,4-6]quinoxaline31, but only in poor yield (15 %).155 Ring expansions of other 1-substituted 2-azidophenazines are more complex.156... 

It has been proposed that the ring-expansion reaction is initiated by attack of the base at position 2 of the quinazoline oxide, which is rendered electron deficient by the inductive effect of the (V-oxide group. In fact, 2-(Chloromethyl)quinazolines, compounds that lack this feature, react with amines to give only simple substitution products.1133... 

Treatment of phenylchalkogen substituted alkenyl alcohols with /-BuOK provided useful tetrahydrofurans stereoselectively <96JOC8200>. A concise synthesis of cis- and trans-theaspirones via oxonium ion-initiated pinacol ring expansion was developed <96JOCl 119>. 

In a series of papers culminating in the report of a total synthesis of hemibrevetoxin B, Nakata and co-workers utilised a zinc catalysed ring expansion of poly-substituted tetrahydropyrans, exemplified by the transformation of 34 to 35 (Scheme 7) <96TL213, 96TL217, 96TL6365>. 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

Since aromatic substitutions, aliphatic substitutions, additions and conjugate additions to carbonyl compounds, cycloadditions, and ring expansion reactions catalyzed by Fe salts have recently been summarized [17], this section will focus on reactions in which iron salts produce a critical activation on unsaturated functional groups provided by the Lewis-acid character of these salts. 

Scheme 2.64. CsF-initiated ring-expansion reactions of substituted pyridines 2-272 and 2-275.

A combination of a Pd-catalyzed nucleophilic substitution by a phenol and a ring expansion was described by Ihara and coworkers [127] using cis- or trans-substituted propynylcyclobutanols 6/l-262a or 6/l-262b. The product ratio depends on the stereochemistry of the cyclobutanols and the acidity of the phenol 6/1-263. Thus, reaction of 6/l-262b with p-methoxyphenol 6/1-263 (X = pOMe) led exclu-... 

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

Figure 13 Ring expansion of a haloboron(III) subphthalocyanine (49) to give an unsymmetrically substituted phthalocyanine, here the zinc(II) benzonaphthoporphyrazine (51).252,253 As usual, the product is...

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