Cydopropanations asymmetric

Hodgson et al. showed that a series of bis- and tris-homoallylic terminal epoxides underwent intramolecular cydopropanation to give a range of bicydic alcohols. A short asymmetric synthesis of sabina ketone based on this chemistry was demonstrated (Scheme 5.20). A practical advantage with this process is that the volatile epoxides can be replaced with readily available chlorohydrins, an extra... 

The Rh2(DOSP)4 catalysts (6b) of Davies have proven to be remarkably effective for highly enantioselective cydopropanation reactions of aryl- and vinyl-diazoacetates [2]. The discovery that enantiocontrol could be enhanced when reactions were performed in pentane [35] added advantages that could be attributed to the solvent-directed orientation of chiral attachments of the ligand carboxylates [59]. In addition to the synthesis of (+)-sertraline (1) [6], the uses of this methodology have been extended to the construction of cyclopropane amino acids (Eq. 3) [35], the synthesis of tricyclic systems such as 22 (Eq. 4) [60], and, as an example of tandem cyclopropanation-Cope rearrangement, an efficient asymmetric synthesis of epi-tremulane 23 (Eq. 5) [61]. 

Nitromethane, cyanomethyl phenyl sulfbne 78a, and benzyl cyanoacetate 78b underwent the asymmetric cydopropanation with 2-bromo-2-cydo-... 

For stoichiometric asymmetric cydopropanations using ethyl diazoacetate-derived sulfur ylides and a unified model for cyclic substrates, see V.K. Aggarwal, E. Grange, Chem. Eur. J. 2006, 32, 568-575. 

In 1994, asymmetric cydopropanation (ACP) with ruthenium catalysts was first reported by Nishiyama and coworkers [ 19,20] by adoption of their chiral bis(oxazolinyl)pyridine (Pybox) ligands. The reaction profiles of Ru Pybox catalysts reveal extremely high trans selectivity with high enantioselectivity (or di-astereoselectivity) of cyclopropane products at the relatively low reaction temperatures (around 20-50 °C) so far reported for ruthenium catalysts. After 1997,... 

SHAFIPLESS Asymmetne epoxidation 343 SHARPLESS Asymmetric d tydroxylalion 344 SHERAOSKY Rearrangement 345 SHESTAKOV Hydrazine synthesis 346 SIEGRIST Stilbene synthesis 347 SIMMONS - SMITH Cydopropane synthesis 348... 

Table 6.3. Asymmetric cydopropanations. The cat. column refers to the catalysts in Figure 6.9. For the structure of 1-menthyl, see Scheme 6.34.

Schulz E (2005) Use of NdV-Coordinating Ligands in Catalytic Asymmetric C-C Bond Formations Example of Cydopropanation, Diels-Alder Reaction, Nudeophilic Allylic Substitution. 15 93-148... 

A Cu-Box complex supported on monolith (51) was developed for the enantio-selective cydopropanation of ethyl diazoacetate. The flow reactions using (51) provided an increase in enantioselectivities of about 20% relative to those for the homogenous batch process (Scheme 7.38) [142]. Pyridine-oxazolidine based monoliths (52) and (53), whose central metals were Ru and Cu, respectively, were also developed [143,144]. Mesoporous silica was utilized as a support for the Cu-Box complex for asymmetric cydopropanation in a flow reador. Aza(bisoxazoline) was easily immobilized on siliceous mesocellular foam MCF) microparticles, which are... 

In the same area, Xiao et al. have developed asymmetric cydopropanations of (J,Y-unsaturated a-ketoesters with stabilized sulfur ylides catalyzed by C2-symmetric ureas [132]. Although the stereoselectivities had to be further improved (de 2% and ee <80%), this novel methodology opened up access to other highly functionalized 1,2,3-trisubstituted cyclopropanes. In 2007, Cao et al. reported a novel... 

Imogai, H., Bernardmelli, G., Granicher, C., Moran, M., Rossier, J.C., and Muller, P. (1998) ds-Disubstituted cydopropanes via asymmetric catalytic cydopropenation synthesis of cydopropyl-dehydroamino acids and of dictyopterene C. Helv. Chim. Acta, 81,1754-1764. 

Table 20.3 Catalytic asymmetric cydopropanation via carbene generation of yiides.

Another chiral auxiliary-based approach was documented by Davies (Equation 5) [8, 34]. These studies involved diazo compounds that incorporate vinyl and C=0 substitution and revealed that these stable carbene precursors participate in a variety of useful transformations, including diastereoselective cydopropanations. Importantly, Davies found that the cydopropanations with 33, in contrast with those of their simpler diazoacetate counterparts, are highly diastereoselective. The reaction with styrene and chiral diazo compound 33 is representative treatment of styrene with 33 in the presence of a simple Rh catalyst resulted in the formation of product 34 with high asymmetric induction dr 98.5 1.5) and in 84% yield. In this example, the panto-lactone auxiliaiy is a convenient, readily available chiral alcohol. 

---