Ring expansion, cyclopentanone

Some straightforward, efficient cyclopentanellation procedures were developed recently. Addition of a malonic ester anion to a cyclopropane-1,1-dicarboxylic ester followed by a Dieckmann condensation (S. Danishefsky, 1974) or addition of iJ-ketoester anions to a (l-phenylthiocyclopropyl)phosphonium cation followed by intramolecular Wittig reaction (J.P, Marino. 1975) produced cyclopentanones. Another procedure starts with a (2 + 21-cycloaddition of dichloroketene to alkenes followed by regioselective ring expansion with diazomethane. The resulting 2,2-dichlorocyclopentanones can be converted to a large variety of cyclopentane derivatives (A.E. Greene. 1979 J.-P. Deprds, 1980). 

Oxidative rearrangement takes place in the oxidation of the 1-vinyl-l-cyclo-butanol 31, yielding the cyclopentenone derivative 32[84], Ring contraction to cyclopropyl methyl ketone (34) is observed by the oxidation of 1-methylcyclo-butene (33)[85], and ring expansion to cyclopentanone takes place by the reaction of the methylenecyclobutane 35. [86,87]... 

The Pd(0)-catalyzed rearrangement of the iV-allylenamine 800 in CF3CO2H affords the (5, -unsaturated imine 801, which is hydrolyzed to give the 7, 8-unsaturated aldehyde 802[498]. The vinyloxaspirohexane 803 undergoes rearrangement-ring expansion to give the cyclopentanone 804 in the presence of 1 equiv. of p-nitrophenol[499]. 

The reaction of methyl propiolate (82) with acyclic enamines produces acyclic dienamines (100), as was the case with dimethyl acetylenedicarboxylate, and the treatment of the pyrrolidine enamines of cycloheptanone, cyclooctanone, cycloundecanone, and cyclododecanone with methyl propiolate results in ring enlargement products (100,101). When the enamines of cyclohexanone are allowed to react with methyl propiolate, rather anomalous products are formed (100). The pyrrolidine enamine of cyclopentanone forms stable 1,2-cycloaddition adduct 83 with methyl propiolate (82). Adduct 83 rearranges to the simple alkylation product 84 upon standing at room temperature, and heating 83 to about 90° causes ring expansion to 85 (97,100). 

The addition of diphenylcyclopropenone to pyrrolidinocyclohexene and ring expansion of the adduct gave an aminocyclononadicnone (330,331), A hereas the corresponding cyclopentanone derived enamine yielded a bieyclic enone. 

Cyclic ketones react through ring expansion to yield lactones (cyclic esters). For example cyclopentanone 7 can be converted to 3-valerolactone 8 ... 

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

The construction of cyclopentanones can take several avenues based upon the concepts evolving from use of small ring conjunctive reagents. A very simple one is a-cuparenone (183) which can be thought to derive by a ring expansion of a cyclobutyl carbinyl system as in 184 (Scheme 8) 110,1U). If X. is an anion stabilizing... 

The ring expansion of cyclobutane derivatives to other carbocycles remains to be one of the most powerful tool in synthetic organic chemistry. Cyclobutanones are exceptionally facile starting materials for the preparations of y-lactones as well as cyclopentanones. 

A similar example is the ring expansion of cyclobutanone via l-tris(methylsulfanyl)methyl-cyclobutanol (6) to 2,2-bis(methylsulfanyl)cyclopentanone (7).43 The use of Af,A-diisopropyl-ethylamine is unneccessary in this reaction as the hydroxy group has already been deprotonated with butyllithium. Further examples of this type of reaction can be found in refs 43 and 44. 

Condensation Reaction and Ring Expansion 2,2-Bis(methylsulfanyl)cyclopentanone (7) Typical Procedure 43... 

A new method for the synthesis of 2-substituted, as well as 2,4- and 2,5-disubstituted, cyclopentanones in 53-93% yield has been reported.81 For example, the Lewis acid catalyzed transformation of l-propanoyl-l-(4-tolylsulfanyl)cyclobutane gave 2-ethyl-2-(4-tolylsulf-anyl)cyclopentanone (1) in 93 % yield. The formation of the cyclopentanone is best explained by a mechanism which involves initial coordination of aluminum trichloride to the carbonyl oxygen, followed by ring expansion to form the sulfur-stabilized carbocation. Finally, migration of the ethyl group to the carbocation center regenerates concomitantly the carbonyl function.81... 

Cyclopentanone annelation. a,a-Dichlorocyclopentanones, obtained by addition of dichlorokctcnc to cycloalkcnes followed by ring expansion with diazomethane (9, 133-134), are versatile intermediates to a variety of interesting products. Some of the transformations are indicated in Scheme (I) and equation (I).1... 

TMS)3SiH has also been applied for kinetic studies of one-carbon ring expansion in the cyclopentanone series for both primary and secondary alkyl radicals100. 

The accessibility to 2,3-disubstituted — and 4,5-disubstituted 2-cyclopentanones by this thermal C3 -< C5 ring expansion has been illustrated by the syntheses of dihydrojasmone 92), cis-Jasmone 92) and dicranenone A188). 

This reaction applied to allenylcyclobutanols allowed the synthesis of a-sub-stituted cyclopentanones by ring expansion of the four-membered ring of the 7r-allylruthenium intermediate [31] (Eq. 21). 

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