Cyclopropyl unit

As discussed earlier, Ila, Junjappa and coworkers used cyclopropyl units as cation-provider in cationic domino processes. Within their interesting approach, the indole derivatives 1-170 could be converted into the unexpected carbazoles 1-171 with 54-69% yield in a five-step transformation using SnCl4 as reagent (Scheme 1.41) [48],... 

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. 

Pyrethroids are commercially important insecticides that usually contain a cyclopropyl unit that is m-substituted and a cyanohydrin derivative. They are usually sold as a mixture of isomers. However, asymmetric routes have been developed, especially because these compounds are related to chrysanthemic esters (Chapter 12).251 The pyrethroids can be resolved through salt formation or by enzymatic hydrolysis.252... 

Enantioselective Synthesis of 1,2,3-trisubstituted Cycio propanes. The chiral dioxaborolane ligand can also be used to generate 1,2,3-substituted cyclopropyl units when the appropriate 1,1-diiodoalkane is used in the preparation of the zinc reagent (eq 9). The reaction affords 1,2,3-trisubstituted cyclopropanes with excellent enantio- and diastereocontrol, including those obtained from functionalized zinc reagents (eq 10). 

The BH.i/BF.vOEt2 reagent reduces not only aryl ketones and aldehydes, but also cyclopropyl aryl or dicyclopropyl ketones to the methylene compounds without opening or rearrangement in the cyclopropyl unit (equation 21). ... 

FIGURE 17.15 A pseudo peptide is obtained if a cyclopropyl unit is embedded in a peptide, the amide bound is lost but the side chains maintain the spatial disposition of die side chain as in die native peptide. 

Sulfur ylides have been used to generate bicyclic macrolactams, as shown in Scheme 35. Argogel-bound thioacetic acid 130 was synthesized from thioacetic acid esters by alkylation with Argogel-Cl resin and subsequent basic hydrolysis of the ester (not shown in Scheme 35). Esterification with co-hydroxy vinyl ketone 131 gave intermediate 132. Alkylation with MeOTf activated the thioether and reaction of 133 with DBU in CH2CI2 at room temperature provided the macrocycle 134 containing an additional cyclopropyl unit in 52 % yield as the pure trans diastereoi-somer [51]. 

The major alkaloids of K. tenuis are lundurines A-C (220-222), which contain a novel ring system incorporating a cyclopropyl unit. These alkaloids are obviously closely related biogenetically to the lapidilectines and may well be derived from an intermediate such as lO-methoxy-3-oxolapidilectine B 181). 

A transannular 6-endo reaction takes place in the boron trifluoride-diethyl ether complex induced cyclization of a bicyclic aldehyde. After the Markovnikov-orientated ring-closure step, a tertiary cation is generated, which is stabilized by //-proton loss and tetracycle 18, bearing a cyclopropyl unit, is formed in 67% yield24. In this case an ene reaction is impossible. 

One of the double bonds in E,E)-, 5-cyclodecadiene can be replaced by a cyclopropyl unit. By treating bicycle 3 with acetic acid and sodium acetate the anthracycline system 4 is formed stereoselectivcly with the methyl and acetoxy groups in equatorial positions42. 

Cyclopropylchlorocarbene (8) should have 0-73 as calculated from Eq. 4. However, as determined experimentally from the measured relative reactivities, w xy 0.41. [34] Close analysis, including ab initio calculations, reveal that in order to add to alkenes, steric demands require the cyclopropyl unit of carbene 8 to twist away from the bisected conformation shown in 8 (where stabilizing electronic interactions of cyclopropyl a bonds and the vacant... 

The route to the second key intermediate 26, an achiral thiol bearing a cyclopropyl unit, requires nine steps from diethylmalonate (Scheme 11.9). 

Metal-mediated [5 -I- 2] cyclizations were built based on the hypothesis that the cyclopropyl moiety of cyclopropyl substituted enynes might participate in the rearrangement process as a three atom unit. This approach afforded indeed an efficient process for the formation of 7-membered rings starting from enynes with a cyclopropyl moiety on the olefin terminus [51]. A possible mechanistic model involves coordination of the enyne and oxidative coupling (a), followed by cleavage of the cyclopropyl unit (b) and reductive elimination (Fig. 10.38). 

According to the postulated reaction pathway, after the initial addition of the olefin to the activated diene, the cyclopropyl unit undergoes a ring enlargement reaction. The gold stabilized allyl cation participates then in a Nazarov-type electrocyclization leading to the final angular tricyclic compound. 

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