Cyclopropyl ylides

Substituted cyclopropyl ylides also participate in oxaspiropentane formation (Table 2, entries 4d, 30b, 38, and 39). Of the two cyclopropyl carbons that can move in the rearrangement to cyclobutanones, the carbon that best stabilizes a... 

The reaction of the cyclopropyl ylide (100) with benzonitrile oxide gave the oxazaphosphole (101) in 61% yield.51 Pyrolysis of 101 at 100°-110° (0.1 torr) gave an 84% yield of the relatively stable spiro-azirine (102). 

Cyclization of y-substituted phosphonium " and sulfonium salts by 1,3-elimination followed by proton abstraction affords the corresponding cyclopropyl ylides... 

Several steps are involved in these reactions. First, the enolate of the (1-kelocstcr opens the cyclopropane ring. The polarity of this process corresponds to that in the formal synthon B because the cyclopropyl carbons are electrophilic. The product of the ringopening step is a stabilized Wittig ylide, which can react with the ketone carbonyl to form the carbocyclic ring. 

Cyclopropanation of the 4,6-diene function proceeds selectively at the 5,6-double bond. Thus, reaction of 185 with the ylide from trimethyl sulfonium iodide and sodium hydride, in DMSO, affords predominantly the a-cyclopropyl compound (187) accompanied... 

C-C3H5 = cyclopropyl. m By pyridine ylide method. n By direct observation. 

Placing anion stabilizing groups on the cyclopropane greatly facilitates generation of cyclopropyl nucleophiles. The diphenylsulfonium ylide has proven to be an exceptionally versatile conjunctive reagent25). The sulfonium salt 9, available from either 3-chloro-l-iodopropane (Eq. 24a) or 3-chloro-l-propanol (Eq. 24b), is a nicely crystalline stable salt that can be stored indefinitely26,Z7). While substituted... 

Related to the sulfonium salts and their ylides are the oxosulfonium salts and their ylides. Cyclopropyl (dimethylamino)phenyloxosulfonium fluoroborate 11, available by routes analogous to the preparation of 9, suffers smooth deprotonation to the corresponding ylide upon treatment with base (Eq. 26) 28. ... 

A companion reagent to the ylide is the corresponding metalated sulfide 30 which arises by the lithiation of 29 with n-butyllithium 66). The latter forms by base closure of 28. Since closure of 28 b involves use of n-butyllithium, the cyclopropyl sulfide 29 simply becomes an intermediate which is metalated in situ to give 30 directly67). A non-metalation sequence involves 32 which undergoes reductive... 

The availability of cyclopentenones from butanolides allows the lactone annulation to facilitate the synthesis of cyclopentyl natural and unnatural products. An example that highlights the latter is dodecahedrane (178) for which 179 constitutes a critical synthetic intermediate 136,137). Lateral fusion of cyclopentenones as present in 179 can arise by acid induced reorganization and dehydration of 180. While a variety of routes can be envisioned to convert a ketone such as 182 into 180, none worked satisfactorily137 On the other hand, the cyclobutanone spiro-annulation approach via 181 proceeds in 64 % overall yield. Thus, the total carbon cource of dodecahedrane derives from two building blocks — cyclopentadiene and the cyclopropyl sulfonium ylide. 

The source of the latter from the appropriate ketone 194 using cyclopropyl sulfur ylide chemistry is apparent. The ultimate starting material is the well known diketone 195 which is easily available in optically active form. 

Thus, allyl telluronium ylides generated in situ from the corresponding telluronium salts in the presence of Li salts react with a,)3-unsaturated esters and amides to afford trans-2-vinyl-/rany-3-substituted cyclopropyl compounds in high yieds. 

Miscellaneous Iminium Catalyzed Transformations The enantioselective construction of three-membered hetero- or carbocyclic ring systems is an important objective for practitioners of chemical synthesis in academic and industrial settings. To date, important advances have been made in the iminium activation realm, which enable asymmetric entry to a-formyl cyclopropanes and epoxides. In terms of cyclopropane synthesis, a new class of iminium catalyst has been introduced, providing the enantioselective stepwise [2 + 1] union of sulfonium ylides and ot,p-unsaturated aldehydes.As shown in Scheme 11.6a, the zwitterionic hydro-indoline-derived catalyst (19) enables both iminium geometry control and directed electrostatic activation of sulfonium ylides in proximity to the incipient iminium reaction partner. This combination of geometric and stereoelectronic effects has been proposed as being essential for enantio- and diastereocontrol in forming two of the three cyclopropyl bonds. 

Schier and Schmidbaur93 performed a clever experiment that addressed part of this question does the orientation of the carbanion relative to the phosphorus atom play any role Scheme 2 shows two syntheses of ylides involving cyclopropyl substituents. In the first reaction, since the pKa of cyclopropane is considerably below that of propane, the expected product is the cyclopropylide. However, the isopropylide is the only recovered product. The second reaction also demonstrates the avoidance of the cyclopropylide product. The cyclopropylide possesses a very pyramidal carbanion that is directed away from phosphorus, allowing for minimal orbital overlap. The isopropylide is much less pyramidal and phosphorus can better assist in stabilizing the carbanion. While this stabilization does not require explicit orbital overlap (the electrostatic interaction of the carbanion with the onium is expected to be smaller in the cyclopropylide since it is directed away from P), it does suggest that some orbital interactions are involved. Hence, although the ylene contribution is small, it is unlikely that the ylene contribution is nil. 

A solution of dimethyloxosulfonium methylide in DMSO (30 ml) was prepared under argon from trimethyloxosulfonium iodide (9.6 mmol) and NaH (60% dispersion in oil, 9.6 mmol) at room temperature. After stirring for 90 min (the evolution of hydrogen ceased) the solution of the ylide was added dropwise to a solution of the vinyl sulfoxide (3.2 mmol) in DMSO (25 ml) over a 30 min period. The reaction mixture was stirred at room temperature for 14h, poured into an ice water mixture, and extracted twice with ether. The ether layers were washed twice with water, dried over anhydrous MgS04 and evaporated in vacuo to give a crude cyclopropyl sulfoxide as a white solid. Flash chromatography (silica, ethyl acetate/n-hexane (25 75) gave the (7 c,Ss) isomer (1.02g, 82%), m.p. 227-228°C and the (Sc,5s) isomer (0.17g, 14%), m.p. 202-203°C. 

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