Cyclopropanation ring opening stereochemistry

The threo stereoisomer was the major product obtained by the synthesis in Scheme 13.14. This stereochemistry was established by the conjugate addition in Step A, where a significant (4-6 1) diastereoselectivity was observed. The C(4)-C(7) stereochemical relationship was retained through the remainder of the synthesis. The other special features of this synthesis are in Steps B and C. The mercuric acetate-mediated cyclopropane ring opening was facilitated by the alkoxy substituent.19 The reduction by NaBH4 accomplished both demercuration and reduction of the aldehyde group. 

Can Hyperconjugation in a 1,3-Diradical Control the Stereochemistry of Cyclopropane Ring Opening and Make a Singlet the Electronic Ground State of the Diradical ... 

Mechano-Stereochemistry of Cyclopropane Ring-Opening Reactions... 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

In agreement with the results of experiments on pyrolysis of l,l-bis(trimethyl-silyl)cyclopropane, additional (2/2)CASPT2/6-31G calculations predict that a rapid 1,2-silyl shift will occur in 12a, forming 13a. However, if cis- and trans-l,l-bis(trimethylsilyl)-2,3-dimethylcyclopropanes (11b and 11c) were pyrolyzed, then, as shown in Figure 22.10, the stereochemistry of ring opening could, presumably, still be inferred from the stereochemistry of the double bonds in the expected rearrangement products (13b and 13c). 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

Ring-opened cyclopropane radical cations have also been postulated to account for the stereochemistry of the aminium radical cation catalyzed rearrangement of l-aryl-2-vinylcyclopropanes [225]. These systems, of course, contain substituents that may veil the true nature of the cyclopropane radical cations by delocalizing spin and charge. In addition, we note that the experimental findings allow some latitude in their interpretations. 

Irradiation of a mixture of diphenyl disulfide and diphenyl ditelluride in the presence of vinyl cyclopropane 141 with visible light gives the ring-opened thiatelluration product 142 as an E/Z mixture.208 Similar reaction with alkynes gives the thiatelluration product 143. The product stereochemistry depends on the nature of the alkyne.208 A mixture of diphenyl diselenide and diphenyl ditelluride reacts with alkynes in a similar way (Scheme 78).208 Similar reactions with allenes are less satisfactory.209... 

The ring-opening reactions of cyclopropane and its derivatives with electrophiles (Eq. (1)) are especially interesting from a mechanistic point of view because they involve electrophiUc cleavage of a carbon-carbon single bond. They thus present problems of stereochemistry and the effect... 

Cyclopropanes can undergo attack by electrophiles with either inversion or retention of configuration at the carbon to which the electrophile becomes attached. The stereochemistry at this center, plus the fact that the nucleophile enters with inversion in nearly all cases, can best be accounted for by an intermediate, corner-substituted cyclopropane for many ring-opening reactions. In some cases a direct, single step process may compete with this mechanism. The possibility of an edge-substituted cyclopropane as a reaction intermediate under special circumstances cannot be ruled out at present. 

---