Cyclization cyclopropane synthesis

An asymmetric cyclopropane synthesis has been achieved by the addition-cyclization reaction of a chiral a,/ -unsaturated sulfoxide with a Grignard reagent (equation 32)49. 

Danheiser, R. L., Morin, J. M., Yu, M. and Basak, A. 1981. Cationic cyclizations initiated by electrocychc cleavage of cyclopropanes. Synthesis of lactones, tetrahydropyrans, and tetrahydrofurans. Tetrahedron Lett. 22 4205-4208. 

Nandi, S., Ray, J. K. (2009). Palladium-catalyzed cyclization/cyclopropanation reaction for the synthesis of fused N-containing heterocycles. Tetrahedron Letters, 50,6993-6997. 

The synthesis of different substituted finans by cyclization of 4-pentynones using potassium tert-butoxide in DMF was reported <96TL3387>. Dihydrofuran 32 can be prepared by a destannylative acylation of l-[(2-methoxyethoxy)methoxy]-2-(phenylsulfonyl)-2-(tributylstannyl)-cyclopropane. Treatment of 32 with BFj-EtjO yields 3-acyUurans via an intramolecular Prins-type reaction of the resulting oxonium ion intermediate <96TL4585>. 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

Cationic cyclization. A key step in the synthesis of the diterpenes cafestol5 and atractyligenin4 involves a novel cation cyclization of bicyclic cyclopropanes to the tetracyclic systems of the diterpenes (equations I and II). Thus treatment of 1 with a slight excess of triflic anhydride and 2,6-lutidine effects cyclization to the rather unstable pentacycle 2 with the kaurene system. The related conversion of 3 to 4 can be effected with triflic anhydride and 2,6-di-r-butyl-4-methylpyridine in 1-nitropropane. 

A very convenient asymmetric synthesis of cyclopropane or epoxide systems developed by Johnson (184) is based on the use of chiral sulfur ylides as the agents that induce optical activity. Generally, this method consists of the asymmetric addition of a chiral sulfur ylide to the C=C or C=0 bond and subsequent cyclization of the addition product to form a chiral cyclopropane or epoxide system together with chiral sulfinamide. A wide range of chiral... 

Cycloproparenes may be prepared by formation of one of the lateral cyclopropane o-bonds either via biradical closing, or via 1/3/elimination. The first reported synthesis of a benzocyclopropene derivative (see Section 1)" is an application of the former of these approaches. Upon irradiation, 3//-pyrazoles 70 loose Nj, and the intermediate biradical 71 cyclizes to 72. There is evidence that the intermediate biradical is in the triplet state, but an alternative interpretation in favor of an excited singlet state has also been presented. A variety of 1,1-disub-stituted benzocyclopropenes has been synthesized by the 3ff-indazole route, which is however limited. Cycloproparenes lacking substituents at Cl are not accessible in this way, because the required indazoles occur in the IH tautomeric form 73. 

Tetralone synthesis. Murphy and Wattanasin7 have devised a simple tctralone synthesis involving cyclization of aryl aroyl cyclopropanes. A ehaleonc (2) is cyclo-propanated by 1 to give a 1 1 mixture of 3 and 4, which can be separated. This step is not necessary since 4 is readily cpimerized to 3 under the conditions for... 

---