1-cyclopropyl- 1-alkene 3-alken

In a similar but more complex reaction, dienes carrying a (3-diketo side chain have been alkylated with various vinyl bromides and cyclized to the corresponding alkylated cyclopropyl alkenes in the presence of silver carbonate (Scheme 10.27). With a relative cis stereochemistry for the newly created C-C bonds, the mechanism is... 

Table 11. 1-[l-(Trimethylsilyl)cyclopropyl]alkenes from Ketones and a-Bromovinyltrimethylsilane (Method A) or l-Bromo-l-(trimethylsilyl)cyclopropene (Method B) ... 

Table 12. l-[l-(Phenylsulfanyl)cyclopropyl]alkenes by Dehydration of /J-Hydroxy Sulfides with Thionyl Chloride in Pyridine at 0°C ... 

A similar strategy was used for the final coupling in a synthesis of okadaic acid, which involves an olefination reaction (eq 39). More recently, the same reaction conditions have been used for the preparation of another cyclopropyl alkene fragment in the total synthesis of ambruticin S (eq 40)7 ... 

Scheme 22 One-pot Diels-Alder reaction of tetrasubstituted thiophene 1 -oxides and cyclopropyl-alkenes derived from cyclopropanone hemiacetal [14]...

Skeletel Rearrai ments.—Catalysed rearrangements of cyclopropyl alkenes have been reviewed. [RuCl2(PPh8)s] and [RhCl(PPh8)8] catalyse the dimeriza-... 

Reactions of cyclopentyne with alkenes gives [2+2] cycloadduct with complete retention of stereochemistry (Scheme 24) [120], Laird and Gilbert observed the expected [2+2] cycloadduct along with the polycyclic adduct in the reaction of norbomyne with 2,3-dihydropyran (Scheme 24) [121], and located a cyclopropyl-carbene intermediate [122],... 

PTC has been extensively used for making cyclopropyl derivatives. The most common reaction involves generation of dichlorocarbene from chloroform, using NaOH and a quaternary ammonium hydroxide. The carbene subsequently reacts with an alkene in high yield. Hydrolysis of dichlorocarbene, normally rapid in the presence of water, is minimal. An interesting and very efficient example of a Michael addition to produce a cyclopropyl derivative is shown in Scheme 4.26. 

The complex [Fe(D4-TmAP)Cl] with Halterman s porphyrin ligand can effect asymmetric alkene cyclopropanation with diazoacetate in high product yield and high stereoselectivity [57]. The reaction occurs smoothly at room temperature without the need for addition of CoCp2, affording the cyclopropyl esters... 

Photoelimination of nitrogen from diazirines, for example, proceeds via carbene intermediates. 3-tert-Butyldiazirine (395) is converted into the cyclopropane 396 and the alkene 397 with the formation of the carbene insertion product being favored from the singlet state.328 3-Cyclopropyl-3-chlorodiazirine (398) has similarly been converted to the carbene 399 which undergoes both rearrangement to l-chlorocyclobutene(400)329 and addition... 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

As invented by Wender,196,197 a variant of the second transformation can take place if the alkene partner is substituted by a participating group such as a strained cyclopropyl or a cyclobutanone (in the case of a 1,6-diene).198 The whole process, which mainly relies on the use of rhodium or ruthenium complexes,1 9 results in the formal... 

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

Reaction of bis(pinacolato)diboron to methylenecyclopropanes proceeds with cleavage of the proximal G-C bond of the cyclopropyl ring, giving 2,4-diboryl-l-alkenes (Equation (14)).74... 

The analogous process involving allylic epoxides is more complex, as issues such as the stereochemistry of substituents on the ring and on the alkene play major roles in determining the course of the reaction [38]. Addition of the Schwartz reagent to the alkene only occurs when an unsubstituted vinyl moiety is present and, in the absence of a Lewis acid, intramolecular attack in an anti fashion leads to cyclopropane formation as the major pathway (Scheme 4.10). cis-Epoxides 13 afford cis-cyclopropyl carbinols, while trans-oxiranes 14 give mixtures of anti-trans and anti-cis isomers. The product of (S-elimi-... 

Cyclohexylidene carbene and cyclopentylidene carbene have been generated by the base-induced decomposition of the appropriate 1 -(A-acety 1-jV-nitroso-aminomcthyl)cycloalkan-l-ol in the presence of Aliquat [47-49] and they have been shown to react in high yield with electron rich alkenes (see Section 7.3). Cyclopropylmethylidene, di(cyclopropyl)methylidene, and isopropylidene carbenes have been generated by an analogous route [49]. 

The reductive cyclization of readily available enol phosphates of 1,3-dicarbonyl compounds bearing pendant olefinic units has been explored [66,67]. The chemistry is exceptionally interesting, and provides a unique route to structures possessing a cyclopropyl unit which is suitable for structural elaboration. The reaction occurs in a manner wherein the phosphate-bearing carbon behaves like a carbene that adds to the pendant alkene to form a cyclopropane. While this provides a useful way of viewing the transformation, mechanistic studies indicate that a carbene is not an actual intermediate. Examples are portrayed in Table 11. 

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