Synthesis of natural products

It is primarily in its crucial and elegant use as a key step of numerous syntheses of natural products that the McMurry reaction has proved so successful. Indeed, intramolecular carbonyl coupling has proved to be one of the best methods for 

The McMurry reaction has also proved useful in the synthesis of a number of cembrenoid diterpenes, including crassin acetate methyl ether 119 [157], sar- 

The estrone methyl ether 130 [169], kempene-2 131 [170], and isokhusimone 132 [171] were easily isolated following McMurry reactions of their tricarbonyl precursors as their intermolecular dimerization is much slower than the intramolecular cyclization. 

FIGURE 11.2 Representative heterocycles synthesized by MW-assisted MCRs. 

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The synthesis of natural products containing the quinonoid stmcture has led to intensive and extensive study of the classic diene synthesis (77). The Diels-Alder cycloaddition of quinonoid dienophiles has been reported for a wide range of dienes (78—80). Reaction of (2) with cyclopentadiene yields (79) [1200-89-1] and (80) [5439-22-5]. The analogous 1,3-cyclohexadiene adducts have been the subject of C-nmr and x-ray studies, which indicate the endo—anti—endo stereostmcture (81). 

The preparation of isoxazolidine derivatives was first reported by Bodforss in 1918 (18CB192). The major synthesis of isoxazolidines involves the cycloaddition of nitrones with alkenes, and isoxazolidines have also enjoyed an increasing use as key intermediates in the synthesis of natural products and other heterocycles (79ACR396, 1892CB1498, 1892CB3291, 1882CB2105). 

Isoquinoline Alkaloids" Academic Press New York, 1972 (e) Kametani, T. In "The Total Synthesis of Natural Products" ApSimon, J., Ed. Wiley-... 

S. Hanessian, "Total Synthesis of Natural Products The Chiron Approach" (Pergamon Press, Oxford, 1983). 

J. ApSimon, Ed., The Total Synthesis of Natural Products, Vols. 1-6 (J. Wiley, New York, 1973-1984). 

Asymmetric Birch reduction and reduction-alkylation in synthesis of natural products 99CC1263. 

Synthesis of natural products containing bis-spiroactal fragment 99JHC1373. 

The Baeyer-Villiger oxidation is a synthetically very useful reaction it is for example often used in the synthesis of natural products. The Corey lactone 11 is a key intermediate in the total synthesis of the physiologically active prostaglandins. It can be prepared from the lactone 10, which in turn is obtained from the bicyclic ketone 9 by reaction with m-chloroperbenzoic acid (MCPBA) " ... 

The stereochemical outcome of the reaction is determined by the geometry of the transition state for the Claisen rearrangement a chairlike conformation is preferred,and it proceeds strictly by an intramolecular pathway. It is therefore possible to predict the stereochemical course of the reaction, and thus the configuration of the stereogenic centers to be generated. This potential can be used for the planning of stereoselective syntheses e.g the synthesis of natural products. 

Numerous examples of intramolecular Diels-Alder reactions have been repor-ted especially from application in the synthesis of natural products, where stereoselectivity is of particular importance e.g. syntheses of steroids. " ... 

With the use of chiral reagents a differentiation of enantiotopic faces is possible, leading to an enantioselective reaction. The stereoselective version of the Michael addition reaction can be a useful tool in organic synthesis, for instance in the synthesis of natural products. 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

A. number of rutro compounds used m ndturdl product synthesis have been prepared by the n of alkyl halides Some recent examples are summarized m Table 2 4 fi-Nitro carbonyl compounds are important for synthesis of natural products The reaction of alkyl vinyl ketones with sodium nitrite and acetic acid in THF gives thecorrespondmgfi-nitro carbonyl compounds in42-82% ""rhis method is better for the preparation of fi-nitro carbonyl compounds than the nitration of the corresponclmg halides... 

Barron, L. D., and Vrbancich, J. Natural Vibrational Raman Optical Activity. 123,151-182 (1984) Bestmann, H. J., Vostrowsky, O. Selected Topics of the Wittig Reaction in the Synthesis of Natural Products. 109, 85-163 (1983). 

For summaries of Sheehan s penicillin synthesis and related work, see (a) Fleming, I. Selected Organic Syntheses A Guidebook for Organic Chemists, John Wiley Sons New York, 1973, p. 80 (b) Johnson, F. In The Total Synthesis of Natural Products, Vol. 1, ApSimon, J., Ed., Wiley-Interscience New York, 1973, p. 331 (c) Holden, K.G. In Chemistry and Biology of f-Lactam Antibiotics, Morin, R.B. Gorman, M., Eds., Academic Press New York, 1982, Ch. 2, p. 99. 

For excellent discussions of the use of optically active starting materials in synthesis, see (a) Hanes-sian, S. The Total Synthesis of Natural Products. The Chiron Approach, Pergamon Press New York, 1983 (b) Scott, J.W. In Asymmetric Synthesis, Morrison, J.D. Scott, J. W., Eds., Academic Press San Diego, 1984, Vol. 4, p. 1. 

Jackson, A.H Smith, K.M. in Total Synthesis of Natural Products, Vol. 1 Ap-Simon, J., Ed. Wiley-Interscience New York, 1973 p143. 

Aziridine-2-carboxylates are playing important roles in the synthesis of natural products and pharmaceutically useful molecules. In this section, applications of chiral nonracemic aziridine-2-carboxylates in the synthesis of natural products are discussed. 

Abstract Since its discovery the chromium-mediated benzannulation reaction has been developed into a unique and useful tool in organic synthesis. In this review, topical aspects of this reaction concerning its mechanism and the chemo-, regio- and stereoselectivity are summerised and discussed in detail. Special attention is paid to the asymmetric benzannulation reaction and, finally, the importance of this reaction as a key step in the total synthesis of natural products is outlined. 

Enyne metathesis starting either from acetylenic boronates and homoallylic alcohols [104a,c] or from propargyl alcohols and allylboronates [104b] has recently been described. The resulting boronated dienes can be converted to allenes or cycloaddition products. The cross metathesis of vinylcyclopropyl-boronates directed toward the total synthesis of natural products has very recently been investigated by Pietruszka et al. [104d]. 

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