Cyclopropanes ring expansion

Simpkins and co-workers were the first to use an asymmetric catalytic process in (-)-anatoxin-a synthesis (Newcombe and Simpkins, 1995) instead of resorting to the chiral pool strategy. Their total synthesis of (-)-anatoxin-a relied on an enantioselective enolisation reaction of a readily available ( )-3-tropinone (33), by a chiral lithium amide base (34) (Bunn et al. 1993a, 1993b) and subsequent cyclopropanation/ring expansion reaction giving the ketone 37 (Scheme 7.8). 

If ring opening of a fused-cyclopropylcarbinyl radical cleaves the exo-carbon bond instead of the e carbon bond as described above, cyclopropane ring expansion can be achieved by reacting the ring-opened radical with an appropriately positioned radical acceptor (Scheme 17) [24]. 

Gold(I)-catalysed allene-vinylcyclopropane cycloisomerization has been reported to give the tricyclic framework of the protoilludanes in a single step involving cyclopropane ring expansion and a Prins cyclization (Scheme 122). ... 

Sodium hydride dimethyl sulfoxide Mono- and bi-cyclic isocyclics by cyclopropane ring expansion... 

DFT calculations have been carried out to shed some light on the mechanism for cyclobutene formation. No direct pathway for the formation of cyclobutene 11-14 from flnf/-exo-cyclopropyl gold carbene 11-13 was found (Scheme 3.9). In contrast, iyw-exo-cyclopropyl gold carbene 11-13 forms cyclobutene 11-14 by a cyclopropane ring expansion. The formation of iyn-exo-cyclopropyl gold carbene 11-13 has been postulated to occur by a iyn-type attack of the alkene to the alkyne gold moiety in 11-15. However the anti attack is more favorable the syn attack could compete if the substitution at the alkene and/or the alkyne does not favor the skeletal rearrangement [Ref. 15 in Chap. 1]. 

Wassermann has reported (19—21) the synthesis of ( )-3-ANA by way of the P-lactam (25). Various methods were used to prepare (25), such as cyclopropane ring expansion of (24), cyclisation of (26) or acylation of the azetidine carboxylate (27). In another approach (22) the a-methylene P-lactam (28) has also been elaborated to 3-ANA via the use of (29). 

The two general mechanisms that have been proposed for the rhodium-catalyzed [5-1-2] cycloaddition are depicted in Scheme 20.10. One would proceed through initial formation of a metallacyclohexene followed by alkyne insertion and then reductive elimination. A second would involve initial formation of a metallacyclopentene followed by cleavage of the cyclopropane (ring expansion) and then reductive elimination [25]. 

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). 

Interest in this reaction was revived when the relevance of a carbene mechanism was realized, particularly following the demonstration (cf. SectionI,B) of a similar ring expansion of indene to 2-chloro-naphthalene by dichlorocarbene via the cyclopropane adduct. Indeed, at this time Nakazaki suggested that these reactions occurred by the addition of dichlorocarbene to the indolyl anion and subsequent rearrangement to the indolenine and, with loss of chloride ion, to the quinoline [Eq. (12)]. The preference of dichlorocarbene for... 

The cyclopropane derivative 1, formed by the addition of dibromocarbene to 4-ethoxy-l-tosyl-1,2-dihydroquinoline, in refluxing pyridine undergoes ring expansion and aromatization to the 1/f-l-benzazepine 2.168... 

This procedure illustrates a new three-step reaction sequence for the one-carbon ring expansion of cyclic ketones to the homologous tt,/3-unsaturated ketones. The key step in the sequence is the iron(III) chloride-induced cleavage of the central bond of trimethyl-silyloxycyclopropanes which me obtained by cyclopropanation of trimethylsilyl enol ethers. The procedure for the preparation of 1-trimethylsilyloxycyclohexene from cyclohexanone described in Part A is that of House, Czuba, Gall, and Olmstead. ... 

The present procedure for ring expansion has also been applied to l,2-bis(trimethylsilyloxy)bicyclo[n.l.0]alkanes, which are prepared by cyclopropanation of l,2-bis(silyloxy)cycloalkenes. The latter are readily available from acyloin condensations in the presence of chlorotrimethylsilane. " This reaction provides a new route to cyclic 1,3-diketones and macrocyclic compounds containing two 1,3-diketone units in the ring. 

BanweU, M.G., Berak, M., Hockless, D.C.R. (1996) Construction of the Colchicine Framework via Two Consecutive Cyclopropane-Mediated Ring-Expansion Reactions. Journal of the Chemical... 

Fragmentation of cyclopropylcarbinyl radicals has been incorporated into several synthetic schemes.357 For example, 2-dienyl-l,l-(dimethoxycarbonyl)-cyclopropanes undergo ring expansion to cyclopentenes. 

Longifolene has also been synthesized from ( ) Wieland-Miescher ketone by a series of reactions that feature an intramolecular enolate alkylation and ring expansion, as shown in Scheme 13.26. The starting material was converted to a dibromo ketone via the Mr-silyl enol ether in the first sequence of reactions. This intermediate underwent an intramolecular enolate alkylation to form the C(7)—C(10) bond. The ring expansion was then done by conversion of the ketone to a silyl enol ether, cyclopropanation, and treatment of the siloxycyclopropane with FeCl3. 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

Cis- and rranx-cyclopropane-1,2-diamines (both primary and secondary) react with a range of aldehydes, R CHO, to give pyrroles under very mild conditions. NMR has been used to identify the intermediates. The key steps involve ring expansion of the monoiminium ion (22), via an azomethine ylid (23), to yield a dihydropyrrolium ion (24). 

The reaction of vinylcarbenoids with vinyl ethers can lead to other types of [3 + 2] cycloadditions. The symmetric synthesis of 2,3-dihydrofurans is readily achieved by reaction of rhodium-stabilized vinylcarbenoids with vinyl ethers (Scheme 14.17) [107]. In this case, (J )-pantolactone is used as a chiral auxihary. The initial cyclopropanation proceeds with high asymmetric induction upon deprotection of the silyl enol ether 146, ring expansion occurs to furnish the dihydrofuran 147, with no significant epi-merization during the ring-expansion process. 

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