Ring contraction thermal

Heating of thietanes can often lead to a ring contraction. Thermal treatment (150°C, in vacuo) of the tosylhydrazide salt 213 gives the allene epi-sulfide 214, a unique synthesis for this molecule. The mechanism is suggested to proceed either via the intermediate bicyclobutane ylide 215 or the mesomeric structure 216. Dodson et al. have noticed the rearrangement... 

We took advantage of this valence isomerization method we prepared the precursor 8b efficiently by using enone-olefin photocycloaddition and subsequent ring contraction. Thermal isomerization of 8b proceeded quantitatively, giving methoxycarbonyl-substituted cyclophane lb (Scheme 3) [5]. Bis(methoxycarbonyl) derivative Ic [6] and diketo derivative le (Structures 2)... 

Ring contraction of 2-thiocephems has also been examined as a route to penems. Desulfurization of (82, n = 0) using triphenylphosphine gave mixtures of 5(R)- and 5(5)-penems (121). The stereochemical problem was neatiy overcome by regioselective oxidation to the thiosulfonate (82, n = 2) which underwent stereospecific thermal extmsion of sulfur dioxide (122) to give the S(R)-penem (83). 

Pyridazinones may undergo ring contraction to pyrroles, pyrazoles and indoles, the process being induced either by an acid or base. The structure of the final product is strongly dependent on the reaction conditions. For example, 4,5-dichloro-l-phenylpyridazin-6(lFT)-one rearranges thermally to 4-chloro-l-phenylpyrazole-5-carboxylic acid (12S), while in aqueous base the corresponding 4-hydroxy acid (126) is formed (Scheme 40). 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

The 2a,7b-dihydrocyclobuta[7>]benzofuran structure, obtained by addition of activated acetylenes to benzofurans (see Houben-Weyl, Vol.E6b, p 148), isomerizes thermally to a 1-benz-oxepin182 184 185 which reacts further to 1-naphthol derivatives at higher temperature182,185 (see Section 1.2 ). Under photochemical conditions, the 1-benzoxepins undergo ring contraction to the starting material.182 184... 

In general, 1//-azepines are fragile ring systems which undergo facile ring contraction to arene derivatives under acidic, thermal, and photolytic conditions. 

Azepines, although more stable than 1/f-azepines. readily undergo ring contraction to pyridine derivatives under acidic, basic, thermal, and photolytic conditions. 

For the thermally induced ring contraction to pyrazoles, see Houben-Weyl, Vol. E8b, p 565 ff. 

More recently, several similar thermal rearrangements of other cyclic sulfoxides involving reversible intramolecular cycloelimination of sulfenic acid have also been reported . A detailed study of this rearrangement which, in certain cases, is accompanied by ring expansion or ring contraction has been performed by Jones and coworkers. These authors found that at 140 °C in xylene, conditions under which acyclic sulfoxides readily decompose , thian 1-oxide (180a) was inert after 6 days. 

Diazocarbonyl compounds readily undergo [3 + 2] cycloaddition to electron-poor alkenes 139). The 1-pyrazolines thus formed usually tautomerize to 2-pyrazolines if there is a hydrogen in an a-position to one of the nitrogen atoms otherwise, thermally induced ring contraction with evolution of nitrogen to give cyclopropanes can occur (Scheme 18). 

The anodic oxidation of 7-MeO-CHT in MeOH results in the formation of benzalde-hyde dimethyl acetal through a ring contracting rearrangement, whereas 3-MeO-CHT and 1-MeO-CHT are prepared by thermal rearrangement of 7-MeO-CHT and afford 7,7-diMeO-CHT in 83% and 85% yields, respectively, upon the anodic oxidation. The hydrolysis of 7,7-diMeO-CHT in 5% aqueous H2SO4 gives tropone in 85% yield (equation 17). 

Diastereoselective intramolecular 1,3-dipolar cycloadditions of alkylidene-cyclopropyl nitrones provide spirocyclopropylisoxazolidines. These compounds have been shown to undergo either thermally induced ring expansion to octahydro[l]pyrindin-4-ones or to acid induced ring contraction into fS-lactams with concomitant loss of ethylene (Scheme 2.218) (710-716). Use of chiral auxiliaries, that is (L)-2-acetoxylactate can lead to enantiomerically enriched heterocycles (715). 

Ring contraction of fused tetrazoles to iV-cyanopyrazoles under thermal conditions has been elaborated by Simoni et al. <2000TL2699>. The results are compiled in Table 4. 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

It is of some interest that the ring contractions giving rise to pyrimidine or pyridine derivatives may occur as simple thermal processes, for example (77TL2709 83LA1230). 

Paquette has used the chloroketal derivatives of thietane 1,1-dioxide as an especially well-suited model for the theoretical study of the thermally induced intramolecular six-electron ring contraction rearrangement, which is pointed out in Eq. (54) for the ketals of 2-bromocyclobutanone that give cyclopropylcarboxylates. Desulfurization of thietanes to cyclopropane did not prove too successful. ... 

Substituted quinazoline derivatives can be prepared by thermal ring contraction of 3/7-1,4-benzodiazepines <1999H(51)2407>. Both 5-methoxy 981 and 5-diethylamino-3/7-l,4-benzodiazepines 983 have been converted to the analogous 4-substituted quinazolines 982 and 984. 

In a novel total synthesis of the tricyclic sesquiterpene (—)-longifolene, an intramolecular diazoalkane cycloaddition to a cyclohexadienone ring followed by thermal ring contraction of the resulting pyrazoline gave the tricychc vinylcyclo-propane 261 and this constitutes the key steps in this synthesis (314) (Scheme 8.63). The interesting features of this sequence are the separation of dipole and dipolarophile by five atoms and the formation of a seven-membered ring in the cycloaddition step. 

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