Carbocyclic synthesis cyclopropane ring systems

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. 

Facile cycloalkenylations of carbonyl groups have been carried out with cyclopropylphosphonium fluoroborates . Complex carbocyclic systems, such as the sesquiterpene a-cedrene, can be effectively constructed by cationic cyclization Epoxide cleavage which follows upon dissolving metal reduction of proximal cyclopropane rings makes possible the ready synthesis of functionalized strained ring compounds inaccessible by other methods... 

The most prominent and most successful compound classes in this respect are arguably VCPs and MCPs. The ring-expanding annulations of these substrates provide an opportunity for the synthesis of various medium-sized carbocyclic ring systems. However, the number of examples of asymmetric reactions remains relatively low. Their application in asymmetric catalytic processes is the next challenge to render cyclopropane ring opening more useful and fruitful. 

Malonate-substituted cyclopropanes are often considered as synthetic building blocks due to the specific reactivity of their carbocyclic ring systems. For instance, total synthesis of ( )-bruguierol A was achieved in several steps that involved cyclopropanation of 2-(2-ally 1-4-methoxyphenyl)-2-methyl-1,3-dioxolane with dimethyl diazomalonate and [Rh2(esp)2] as catalyst (eq 44). The characteristic 2,3-benzofused 8-oxabicyclo-[3.2.1]octane core of bruguierol was constructed via a Sc(OTf)3-catalyzed intramolecular [3+2] cycloaddition of the cor-... 

Hudlicky T, Reed JW (1991) Rearrangements of vinyl cyclopropane and related systems. In Trost BM (ed) Comprehensive organic synthesis, vol 5. Pergamon Press, Oxford, p 899 Hudlicky T, Becker DA, Fan RL, Kozhushkov S (1997) Carbocyclic three-and four-member-ed ring compounds. In de Meijere A (ed) Houben-Weyl methods of organic chemistry, vol El7c. Thieme, Stuttgart, p 2538... 

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