Carbocydization reactions

The understanding of the reactivity made possible the development of the rhodium catalyzed [4-+2+-2] carbocydization reaction as outlined in Tab. 12.9. This study demon strated that nitrogen-, sulfur-, and oxygen-tethered enynes furnish the desired cycload... 

Intramolecular Sm(n)-mediated coupling of ketones with distal vinylepoxides results in the corresponding carbocydic derivatives with good to excellent diaster-eoselectivity [125], When the reaction was conducted with enantioenriched substrates the products were obtained with poor ees, indicating low it-facial discrimination in this reaction. 

On the other hand, the reaction of bisallenes with Bu3SnSnBu3 may generate cis-bisallene Pd(SnBu3)2 (C) and/or a chelated cj-allylpalladium complex D reversibly. The fast carbocydization of these intermediates would give rise to a vinylpalladium complex E, which then yields a ds-compound by reductive elimination and/or a cis-bicyclodiene through (T-bond metathesis as a kinetically controlled product (Scheme 16.66). 

The intermolecular carbocyclization with a strained olefin, such as norbornene 35, has been frequently used for proof-of-prindple in challenging metal-catalyzed transformations. The use of rhodium catalysts fadhtates the intermolecular reaction, albeit in modest yield and with poor regioselectivity (Scheme 11.10). Ethylene 39 can also be utilized to this end, but generally affords the carbocydization products in low yield, as... 

The predominant formation of five-membered carbocydes or heterocycles 122 (Scheme 50) via a sequential conjugate addition-carbene insertion pathway is generally observed in the reactions of the appropriate alkynyliodonium salts 119 (R = long alkyl chain or other group with C-H bond available at C5) with various relatively hard nucleophiles. Typical nucleophiles used to initiate these selective cyclizations are enolate, azide, sulfinate, tosylamide, thioamide and some other anions. 

The Ferrier (II) reaction is quite efficient to form six membered carbocycles, but is unsuitable to prepare cyclopentitols. Five membered enollactone 14 was converted to the cyclopentanone derivative 16 as a single epimer upon treatment by LiAlH(OtBu)3 (Scheme 4) [41]. Spectroscopic studies established some mechanistic details. Accordingly, the hydride of the reducing agent rapidly added to the carbonyl and formed with the metal a stable alu-minate complex. The carbocydization occurred by protonation followed by fragmentation and aldol type cyclization process. 

Let s now consider concerted skeletal rearrangements which allow the direct transformation of a sugar structure into a carbocyde. The Claisen rearrangement has been used for the direct conversion of sugar C-glycosides to eight membered carbocycles, respectively. The reaction has precedents in the transformation of 2-methylene-6-vinyl-tetrahydropyrans to cyclooctenone derivatives, a transformation that has been applied in the synthesis of several natural products [54],... 

A broad variety of special aldehydes and ketones are easily accessible by Heck-type reactions with allylic alcohols and their homologs [3]. The potential for the synthesis of carbocydic structures is illustrated by the macrocyde 11 [4] which obviously is the product of a fourfold Heck reaction (Scheme 3). A domino process consisting of a double Heck reaction followed by an intramolecular Aldol condensation leads to the annulated ring system 15 [5]. 

Quite recently, novel cyclization reactions involving CO to give carbocydic and heterocyclic compounds, which are characteristic for mthenium catalysts, have been developed. Ruthenium complexes provide new avenues for cydization reactions. In addition, CO is often used as a reducing agent, and reductive carbonylations of nitro compounds catalyzed by mthenium complexes are very attractive reactions that provide phosgene-free processes [3]. 

This catalytic sequence is known as Kharasch addition or atom transfer radical addition (ATRA) [4]. Various polyhalogenated compounds such as CCI4 and CCI3CO2R are used as the organic halides, and transition metal salts or complexes are used as the catalyst [3]. Intramolecular version of the Kharasch addition reaction (atom transfer radical cyclization, ATRC) has opened novel synthetic protocols to the synthesis of carbocyde or heterocyles catalyzed by transition metals [5-7], and this has become a very important field in free radical cydization in organic synthesis. Transition metal-catalyzed Kharasch reactions sometimes afford telomers or poly-... 

The acyloin coupling reaction is suitable for the synthesis of medium and large (> 12 membered) carbocydic systems, using high-dilution techniques... 

Conjugate reduction of a, 3-unsaturated carbonyl compounds and subsequent carbon-carbon bond formation provides a versatile domino process of carbocyde and heterocyle synthesis [20]. These reactions proceed by the formation of enolate 49 through conjugate reduction of a,P-unsaturated carbonyl compound 48 with a reducing agent (M-H) followed by trapping of the enolate with an electrophile 50 to give 51 (Scheme 9.11). 

In their continuing efforts to synthesize the carbocydic core of sesquiterterpenes such as ophiobolin A, Wicha and co-workers constructed a 5,8-flised carbocydic system. Reaction of 333 with 5 mol % of 4 in refluxing dichloromethane furnished the desired bicyclic system 334 in 95% overall yield. 

Purpose, This experiment demonstrates the use of the Diels-Alder reaction in the preparation of six-membered carbocydic rings. The cyclic producfs are obtained by reaction of a conjugated diene with an alkene. The illustration given here involves the treatment of 1,3-butadiene (generated in situ) with maleic anhydride to form the corresponding Diels-Alder product. These addition prodwcfs are often called adducts. 

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