Cycloalkenes synthesis

C104 (L-glycoside), 95f 101-102 Chaetiacandin, 101-102,104 Concanavalin-A, 146-148 Conduritols, 2-4 Cycloalkene synthesis. See Ascidiatrienolide A Herbarumins Pinolidoxin... 

The details of trans-medium-ring cycloalkene synthesis by silver-ion-assisted opening of igiem-dihalogenobicyclo[n,l,0]alkanes have been presented. Solvolysis of chlorofluorocyclopropanes (352) is also assisted by silver ion, and it provides a useful route to fluoroallyl alcohols (353). ... 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

Klunder A. J. H., Zhu J., Zwanenburg B. The Concept of Transient Chirality in the Stereoselective Synthesis of Fnnctionalized Cycloalkenes Applying the Retro-Diels-Alder Methodology Chem. Rev. (Washington, D. C.) 1999 99 1163-1190 Keywords stereochemistry, stereoselective synthesis, retro-Diels-Alder methodology... 

Two illustrations that show the power of this reaction for the preparation of strained cycloalkenes are the contractions of 102 to the propellane 103 , an application that has been reviewed , and of 104 to the bicyclo[2.1.1]hexene 105 . The utility of the Ramberg-Backlund rearrangement in the preparation of various natural products such as steroids , terpenoids and pheromones has been demonstrated. In addition to the synthetic applications mentioned in the previous subsection, several selected examples taken from the recent literature are given in equations 66-69. These examples further demonstrate the potential of this method for alkene synthesis in general. 

The synthesis of cyclic nitroalkenes via nitration of cycloalkenes is summarized in Table 2.2. Acyclic nitroalkenes are more readily prepared via the Henry reaction than by nitration of alkenes (see Section 3.2.1). 

Another important feature of this reaction is the synthesis of highly strained E-cycloalkenes. Thus E,Z-cycloocta-1,3-diene has been obtained from the Z,Z-compound in sensitized irradiation 309) (3.4), while E-cyclo-octene was obtained from the Z-isomer by direct irradiation (3.5) 310). The synthesis of a trans doubly bridged olefin has also been reported (3.6)311>. 

Further important industrial applications of olefin metathesis include the synthesis of 3,3-dimethyl-l-butene ( neohexene , intermediate for the production of musk perfume) from ethene and 2,4,4-trimethyl-2-pentene, the manufacture of a,co-dienes from ethene and cycloalkenes (reversed RCM), and the ROMP of cyclooctene and norbomene to Vestenamer and Norsorex , respectively. 

The inverse-electron-demand Diels-Alder reaction of 3,6-dichloro[l,2,4,5]tetrazine with alkenes and alkynes provides the synthesis of highly functionalized pyridazines. ° Also, the 4 + 2-cycloaddition reactions of the parent [l,2,4,5]tetrazine with donor-substituted alkynes, alkenes, donor-substituted and unsubstituted cycloalkenes, ketene acetals, and aminals have been investigated. ... 

Z-151, and hydroboration/protonation affords -cycloalkene. In 2002, Trost reported an alternative procedure for the synthesis of the -olefin from an alkyne by hydrosihlation... 

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