Ring expansion Subject

The foregoing designs were discussed as ring expansion joints by Kopp and Sayre, Expansion Joints for Heat Exchangers (ASME Misc. Pap., vol. 6, no. 211). All are statically indeterminate but are subjected to analysis by introducing various simplifying assumptions. Some joints in current industrial use are of lighter wall constniction than is indicated by the method of this paper. 

Intramolecular acylnitrene-induced ring expansions of arenes are of great interest for the synthesis of novel 1//-azepines. Benzyl azidoformates, when subjected to spray-vacuum pyrolysis (SVP),152 yield l//,3//-oxazolo[3,4-a]azepin-3-ones which dimerize spontaneously 153,154 however, 2,6-dichlorobenzyl azidoformate yields the thermally stable (< 100 C) 5,9-dichloro-1 //,3//-oxazolo[3,4-a]azepin-3-one (8).154... 

An elegant extension of these intramolecular acylnitrene-induced ring expansions has been used for the synthesis of cyclopent[h]azepines.2 2-Haloindan-l-yl azidoformates 14 (X = Cl, Br), when subjected to pyrolysis at 300 °C in a hot tube packed with calcium oxide and copper turnings, produce cyclopent[6]azepine (15), as a dark turquoise oil, in excellent yield. Lesser yields (30 and 50%, respectively) of the 4-bromo and 3-methoxy derivatives can be similarly obtained. 

Another longtime subject of debate in C7H6 chemistry is the role of cy-cloheptatrienylidcne (4a). As already mentioned, 4a was initially viewed as the product of ring expansion of la. Attempts were also made to generate 4a directly. Upon pyrolysis or photolysis of the sodium salt of tropone tosylhy-drazone (5), W.M. Jones and co-workers isolated heptafulvalene, the formal product of dimerization of 4a (Scheme 8).21 In the presence of alkenes, decomposition of 5 or dehydrohalogenation of halocycloheptatrienes afforded mainly spirocyclopropane products, presumably formed by addition of 4a to the alkene (Scheme 8).22... 

Reduction of the ketone (147), followed by elimination gave the olefin (148). The olefin (148) was subjected again to a second annelation, and as expected, dichloroketene addition, ring expansion and zinc reduction gave the tricyclic compound (149). Compound (149) could be converted to ( )-hirsutic acid C (150) 52K... 

The cyclization of 8, s-unsaturated acyl radicals has been the research subject of several groups [27]. The propagation steps for the prototype reaction are illustrated in Scheme 7.4. The 5-exo 6-endo product ratio varies with the change of the silane concentration due to the competition of hydrogen abstraction from the silane with the ring expansion path. 

Like their monocyclic analogs (see Section 5.09.2.2.7) bicyclic azetidinium salts are subject to nucleophilic ring opening. Quaternary salt (27) can react with a nucleophile either at C-5 or C-7 producing the ring expansion (28) or direct substitution products (29), respectively (64HCA745, 69AG(E)962. ... 

The ring expansion of aziridines has been reported in 2001 as a well established protocol [144] for preparing p-lactams in a regioselective manner [145]. A variety of aziridines with different substituents and stereochemistry was subjected to cobalt carbonyl-catalyzed carbonylation to give p-lactams. The ring expansion to... 

Oxidation reactions on the sulfur atom of penicillins remain the most important reactivity of S-1 encountered in the literature. Penam sulfoxides and sulfones are indeed important compounds as they confer to the skeleton an ease of thiazolidine ring opening by weakening the C(5)-S(l) and S(l)-C(2) bonds (see Section 2.03.5.9) <2004CHE816>. In particular, the former constitute key intermediates in ring-expansion transformations from penams to cephems (see Section 2.03.5.9), while the latter have a special biological interest as /3-lactamase inhibitors (e.g., sulbactam, tazobactam see Sections 2.03.1, 2.03.5.2, and 2.03.12.4). Since CHEC-II(1996) covers all the aspects of these oxidation reactions on the S-1 atom of penicillins, this section focuses on the most relevant recent papers. As there is no particular change in the subject, only a few articles have been released since 1995. 

Figure 14.25 shows how the ring expansion of cyclic ketones can be accomphshed without the liberation of molecular nitrogen (in contrast to the ring expansions of Figures 14.21-14.23). A chemoselective monoinsertion of CH2 occurs because the product ketone is never exposed to the reaction condition to which the substrate ketone is subjected. This is a similarity between the present method and the processes described in Figures 14.21 and 14.23, and this feature is in contrast to the method depicted in Figure 14.22. 

Whatever the case, the formation of the cyclooctenone 320 from the cyclobutanone 317 constituted the first example of a one-step C4 — C8 ring expansion. In the same way, the spirocyclobutanone 266 63,73), was treated with vinylmagnesium bromide to generate a mixture of the diastereomeric cyclobutanols 322 and 323 in 78% yield (ratio 21/79). Each diastereomer, separated by HPLC, individually subjected to KH in THF at room temperature, rearranged cleanly to bicyclo[5.3. l]undec-l(l l)-en-4-one in 80% yield, Eq. (87)161>. 

Radical reactions are looked upon by many chemists as uncontrollable processes which yield intractable mixtures of products. Even so, the ingenuity of synthetic chemists has resulted in many useful synthetic schemes via radical pathways. These include chain lengthening, ring formation and ring expansion processes. This synthetic work has seen dramatic steps forward over the last few decades and has been the subject of several general reviews recently664"669. Other notable reviews have also been written10,11,61,670 which cover more specific aspects of radical reactions and the reader is referred to all these sources for further and in-depth information. 

In this section, the subject is subdivided into three parts. These are, first, chain lengthening reactions, second, cyclizations and last, ring expansion processes. These processes may be either chain or nonchain reactions. However, in this review, only the more common chain reactions will be discussed. The reader is referred to the reviews of Curran665 667,669 for details about the less common nonchain processes, and to the chapter by J. Green concerning metal-mediated reactions of halides in the present volume. 

The relationship between the biosynthesis of the penicillin and cephalosporin nuclei [127] is shown in Fig. 8.25. The common intermediate in the biosynthesis of penicillins and cephalosporins is isopenicillin N (IPN), which in Penicil-lium is converted into penicillin G by replacement of the L-2-aminoadipyl side-chain with externally supplied phenylacetic acid, mediated by IPN acyl transferase (IPN AcT). In the cephalosporin-producing Ammonium chrysogenum, IPN is subjected to an enzymatic ring expansion. 

When subjected to the Conia modification of the Simmons-Smith reaction or cuprous chloride-catalyzed diazomethane decomposition, triquinacene was converted to two monocyclopropanated, two doubly cyclopropanated, and two triply cyclo-propanated products.403) Subsequent photochlorination of 454 with r-butyl hypochlorite at —50 °C has been shown to proceed with ring expansion and formation of 455.404) In contrast, //-exo-trishomotriquinacene (456) could be converted to trichloride 457 with retention of the hexacyclic carbon framework 04)... 

The ring expansion of the carbene dioxolane 208 occurred in the presence of Rh(n) catalyst. The reaction proceeded presumably via the intermediate ylide 209, which was subjected to the reaction with benzaldehyde in the presence of Lewis acid to give a mixture of the dioxocine 210, as the major product, as well as dioxocane 211 (Scheme 24) <2003JOC10040>. Compounds 210 and 211 were separated and the yy -conformation of 210 was determined by X-ray crystallographic analysis. The stereochemistry of 211 was also established by X-ray crystallography. 

As a final suggestion for future research, cyclobutanones have also provided the organic photochemist with the opportunity of investigating the existence of unusual and reactive intermediates oxacarbenes, trimethylene biradicals, trimethylenemethane biradicals, acyl alkyl biradicals, and ketenes. Evidence for the intervention of oxacarbenes in the ring-expansion reaction is quite compelling however, their unusual behavior relative to "typical" carbenes (e.g., failure to form cyclopropane adducts with some olefinic substrates) makes them prime subjects for further study and characterization. Unlike oxacarbenes, the existence of acyl alkyl biradicals (e.g., [30]) is tenuous at best. Ideally,... 

Zizaene sesquiterpenes were the subject of considerable synthetic interest in the 1970s and several synthetic strategies to construct the tricyclo[6.2.1.0 ]undecane skeleton involved an intramolecular diazoalkane-carbonyl ring expansion, an intramolecular magnesium-ene reaction, and a titanium-promoted reductive coupling. - ... 

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