Cyclopropyl ketenes

The bicyclooctadienone in (3.26) is converted into a cyclopropyl ketene via a [3,3]sigmatropic rearrangement341). 

Herz and Iyer have proposed an a-cleavage mechanism with the intermediacy of a cyclopropyl ketene for formation of the polycyclic acetal 42 from photolysis of the rigid cyclopentenone 4339 The cleavage reaction is thought to occur via an n,7r triplet state of unusually high energy for a cyclopentenone (75 kcal/mole). 

The photochemistry of a,p nsaturated ketones has attracted much attention and is still a field cld of current interest. 1/Numer-ous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition ructions and rearrangements. For example, cyclopentenones / and/readily rearrange to cyclopropyl ketenes upon irra/idiation. Recently, the related cyclohexadienone/butadienyl ketene rearrangement has been shown to be a highly useful tool in the synthesis of natural products and macrocydic lactone /... 

Photolysis of bicyclo[3.2.2]octa-3,6-dien-2-one derivatives, for example the adduct of tropone with benzyne 33 a in water gave the three-membered ring derivative 35 a, while photolysis in acetonitrile gave the cyclopentanone derivative 37 a which must arise from the 1,3-shift product 36a.This different behavior in these two solvents was rationalized in the following way the efficient [3,3] shift of 33 a to cyclopropyl ketene 34 a is thermally reversible, so that in the absence of a nucleophile 35a is not observed. In that case, the less efficient 1,3-shift to form 36a and then 37a is the only detectable reaction. The ketene 34a was detected as the major product of photolysis of adduct 33 a at low temperature methanolysis of 34 a at — 80 "C gave only the enfifo-configurated ester 35 a. 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

On the other hand, l-(phenylethynyl)cyclopropanol 9 (R = Ph) underwent a C3 -> C4 ring expansion and subsequent decarboxylation when treated with MCPBA to yield the 2-phenylcyclobutanone 47, likely via the intermediate 2-(l-hydroxy-cyclopropyl)-2-phenyl ketene 44, formed by migration of the cyclopropyl group in the vinyl cation 43. The ketene 44 thus resulting could be attacked by a second equivalent of MCPBA and ring expanded to the (3-ketoacid 46 which would easily decarboxylate to yield 47, Eq. (15) 14>. 

The transition metal-catalyzed allylation of carbon nucleophiles was a widely used method until Grieco and Pearson discovered LPDE-mediated allylic substitutions in 1992. Grieco investigated substitution reactions of cyclic allyl alcohols with silyl ketene acetals such as Si-1 by use of LPDE solution [95]. The concentration of LPDE seems to be important. For example, the use of 2.0 M LPDE resulted in formation of silyl ether 88 with 86 and 87 in the ratio 2 6.4 1. In contrast, 3.0 m LPDE afforded an excellent yield (90 %) of 86 and 87 (5.8 1), and the less hindered side of the allylic unit is alkylated regioselectively. It is of interest to note that this chemistry is also applicable to cyclopropyl carbinol 89 (Sch. 44). 

A unique thermal cleavage of certain cyclopropyl ketoesters has been reported by Berkowitz ". The intermediacy of a ketene and its rearrangement together with carbon monoxide loss is postulated. This sequence has led on to the preparation of ds-jasmone (equation 58) ... 

A successful trapping reaction of a cyclopropyl ester enolate with trimethylsilyl chloride (TMSC) was first performed by Ainsworth and coworkers . In the reaction of 232 with lithium diisopropyl amide at — 78°C, followed by addition of TMSC, the ketene acetal 233 was formed in 10% yield as well as the silylated cyclopropane 234 (40%). Ketene acetals other than 233 are formed in yields > 90 %. 

In general, cyclobutanones are synthesized by either ketene cycloadditions or by ring expansions of cyclopropyl precursors. For the synthesis of simple a-substituted monocyclic cyclobutanones, the latter method is usually employed, and a variety of approaches have been used to prepare the required eyclopropyl intermediates. 

The transition metal catalyzed decomposition of a-diazo ketones in the presence of alkenes gives access to a variety of cyclopropyl ketones. In contrast to the thermal or photochemical methods, Wolff rearrangement resulting in ketenes is normally not competitive. 

Photochemical conversion of an amino(cyclopropyl)carbene (obtained from cyclopro-pyl(methoxy)carbene with optically pure D,i.-erythro amino alcohols) gave a cyclopropyl-substituted lactone, e.g. formation of 13, which upon hydrogenolysis gave a chiral cyclopropyl-glycine system.The reaction proceeds via a ketene complex. 

With Lewis acid activated ketene acetals 11 acting as electrophiles, the subsequent ring-closure reaction proceeded via attack of the terminal carbon atom on the donor-substituted cyclopropyl carbon atom which resulted in cyclopentenone derivatives. ... 

Cyclopropylation. The complex formed from bis(allylpalladium chloride) by treatment with AgBp4 and 2-(2-pyridyl)imidazole mediates cyclopropylation of ketene silyl acetals with allyl acetates. 

Cyclopropyl aldehyde (ll) has a tertiary alkyl group next to the carbonyl group and might be made by contraction of a four-raembered ring (Chapter 31) such as (12), where X is a leaving group. If we put X—Cl, we could make (12) by reduction of an a-chloro cydobutanone (13), a ketene cydoadduct. 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

Further cyclopropyl aryl ketimine rearrangements have been noted, and bicyclo[3,l,0]hex-3-enones undergo rearrangement via keten intermediates. ... 

A variant of the Staudinger (3-lactam synthesis by Hodous and Fu involves the use of electrophilic imines 16, with umpolung ketene reactivity generated from the addition of a Lewis basic catalyst 18 to form an enolate [12]. In this reversal of reactivity, excellent enantiomeric induction (typically >90% ee) can be achieved via the use of 4-(pyrrolidino) pyridine (PPY) derivative 18 as a chiral Lewis basic catalyst (Scheme 3.3). Notable in this methodology is the broad scope of the imine component, encompassing aryl-, vinyl-, and cyclopropyl A-tosyl imines. Equally, the use of disubstituted alkylarylketenes 15 provide C(3)-quatemary and C(4)-tertiary centered lactams 17 both in good yield and... 

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