Ring Expansion by Two Carbons

Ring Expansion by Two Carbons.—Direct irradiation of the py-unsubstituted cyclo-pentanone (92) gave cycloheptenone (93) possibly via a ( 2j -I- process. Photolysis of the 2-alkenyl-2-hydroxycyclohexanone (94) gave only a low yield of 2,2-dimethylcyclo-octa- 1,3-dione.  

9-Pyrrolidinophenanthrene reacts with dimethyl acetylenedicarboxylate to give the dibenzocyclo-octatetraene (97). A mixture of the [2 + 2] adduct and the ring-expanded cyclo-octa-1,3-diene was obtained from the reaction between AT-(cyclohex-l-enyl)-JV-methylaniline and dimethyl acetylenedicarboxylate. iVJV-Dimethyl- 

Cycloheptatrienes were isolated from reactions of 1,2,3-triphenylcyclopropane with cyclopentadienes. Thermolysis of the Diels-Alder adduct of cyclopropene 

The mechanism of the thermal rearrangement of [n,4,2]propellatrienes (100) to cyclo-octatetraenes (101) involves an intramolecular Diels-Alder reaction followed by a ( 2s + 25 + 2s) cyclo-reversion. Rearrangement of propellatrienes (100 n = 4 or 5) to cyclo-octatetraenes (101 n = 4 or 5) is facilitated by molybdenum hexacarbonyl, and under these conditions, at least formally, involves a [l,5]shift under milder conditions the molybdenum complexes (102 n = 4 or 5) have been isolated. 

The trimethylsilyl ethers of cis- and frans-l-vinylcyclodec-3-en-l-ol rearrange at 240—305 C to give mixtures of cis-cyclodec-5-enone, frans-cyclodec-S-enone, and 4-vinylcyclodecanone after hydrolysis ratios of 77 11 12 from the cjs-isomer, and 5 85 10 from the trans-isomer, respectively, were obtained. Biradicals may have been involved in the rearrangements, but at least part of the isomerized product may have arisen from a sequence of concerted reactions. 

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A ring expansion by two carbon atoms was discovered on heating (Z)-l-vinyl-cyclonon-3-en-l-yl-trimethylsilyl ether (V/62), Scheme V/10 [31] [32], Two types of trimethylsiloxy enol ethers were observed, each formed by a [1.3] or a [3.3] sigmatropic shift which, after hydrolysis, gave the ketones V/63, V/64, and V/65. The protection of the tertiary alcohol function is not necessary, if the reaction is done in the presence of potassium hydride [33] [34]. The reaction proceeds when the cyclic 1-vinyl alcohols have either a double bond or a benzo group at the 3-position. Other examples of this reaction type are given in Scheme V/10. 

Enamines of cyclic ketones yield [2 + 2] cycloadducts with dimethyl acetylenedicarbox-ylate, which on heating undergo ring-expansion by two carbon atoms (equation 163). 

The second reaction is a kind of cycloaddition followed by a ring opening with the sequence c intermediates shown here. It leads to a ring expansion by two carbon atoms and allows us to make eight-membered rings easily. 

Cycloaddition using an enol derivative of a cyclic 1,3-dicarbonyl compound, followed by retro-aldol reaction, results in ring expansion by two carbon atoms. Reactions of this kind have been applied to the synthesis of a number of polycyclic natural products. For example, irradiation of the enol benzoate 160 gave the tricyclic product 161 in almost quantitative yield (3.112). Dimethylation followed by hydrolysis and retro-aldol reaction gave the eight-membered ring diketone 162, used in a synthesis of the sesquiterpene ep/-precapnelladiene. 

By Two-Carbon Ring Expansion with Hydrogen Transfer... 

Ring expansion. 2-Cyano-1-vinylcycloalkanols are induced to expand by two carbon units on heating with KH and 18-crown-6 in THF. 

A modern method of expanding medium and large rings by two carbon atoms uses flash vacuum pyrolysis (FVP) of aUyl alcohols. Under the reaction conditions, only traces of the expected [l,5]-hydrogen migration are observed the main product arises presumably from a [l,3]-carbon shift, a process which involves a diradical intermediate. This ring-expansion sequence may also be repeated successfully. [181]... 

In addition to the preceding processes that involve cycloadditions in direct analogy to Diels-Alder-type processes, several formal [4+2] cycloaddition processes have been described that proceed via completely different substrate classes and reaction pathways. In one example, a novel two-carbon ring expansion process was reported by Murakami, wherein the addition of cyclobutanones with alkynes provides cyclohexenones by two-carbon ring expansion of the starting material (Scheme 3-23). The mechanism of this process likely involves oxidative cyclization of die ketone and alkyne with Ni(0) to provide a five-membered metallacycle, followed by a ringexpanding p-carbon elimination as key steps of the process. 

A two-carbon ring expansion of cyclic ketones was achieved by the addition of acetylenic esters and diesters to the enamine derivatives of the ketones, and reported almost simultaneously from several laboratories (337-343). The intermediate bicyclic adduct could be isolated in some cases. 

All three levels of theory predict the ring expansion of singlet phenylcarbene ( A -la) to cycloheptatetraene (3a) to occur in two steps, via bicy-clo[4.1.0]hepta-2,4,6-triene (2a) as an intermediate. The first step is addition of the carbene carbon to an adjacent 7t bond of the ring. The second step involves a six-electron, disrotatory, electrocyclic ring opening, which is allowed by orbital symmetry67 and thus proceeds by a highly delocalized transition state. Fig. 4... 

The rearrangement and formation of 2,2-dimethylbutane—with a quaternary carbon atom—is only possible via the above mechanism. Over platinum, 2,3-dimethylbutane (75) gave more benzene over palladium, 2,2-dimethylbutane (97a) gave more benzene. This is the opposite selectivity, as reported by Muller and Gault, for ring expansion of 1,1,3-trimethyl-cyclopentane (57). This may be more evidence that at least two different types of bond shift mechanism can occur. 

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