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Rings, bicyclic, stereoselectivity

Allylic cations46, oxonium ions47,48,31 and thionium ions47c are known to be the most effective initiators for bicyclization. Stereoselective Irons ring fusion is obtained and the yield varies between moderate and high. If termination is realized by proton elimination then mixtures of regioisomers can be expected. [Pg.125]

An example of a completely stereoselective cyclization of an allylsilane, leading to a seven-mem-bered ring in excellent yield, is known158. Treatment of the hydroxy compound with methanesulfonyl chloride and triethylamine in acetonitrile gives a single isomer of the bicyclic compound via cyclization of the (T )-iminium ion. [Pg.824]

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. [Pg.965]

A key step in the synthesis in Scheme 13.11 was a cycloaddition between an electron-rich ynamine and the electron-poor enone. The cyclobutane ring was then opened in a process that corresponds to retrosynthetic step 10-IIa 10-IIIa in Scheme 13.10. The crucial step for stereochemical control occurs in Step B. The stereoselectivity of this step results from preferential protonation of the enamine from the less hindered side of the bicyclic intermediate. [Pg.1179]

There are several reports dealing with the use of tetrahydropyrrolo[l,4]oxazinones derived from natural proline or prolinol as chiral auxiliaries for the synthesis of enantiomerically pure compounds. The preparation of the heterocycle is described in Scheme 33 (Section 11.11.7.4). The presence of a rigid bicyclic skeleton allows stereoselective introduction of different substituents. The final ring opening of the system (generally by hydrolysis) provides enantiomerically pure compounds with the possibility of recycling the starting chiral auxiliary. [Pg.507]

Birch reduction-alkylation of 5 with 2-bromoethyl acetate was carried out with complete facial selectivity to give 57. This tetrafunctional intermediate was converted to the bicyclic iodolactone 58 ( > 99% ee) from which the radical cyclization substrate 59 was prepared. The key radical cyclization occurred with complete regio- and facial-selectivity and subsequent stereoselective reduction of the resulting tertiary radical gave 60 with the required trans BC ring fusion.The allylic alcohol rmit of (+)-lycorine was obtained by a photochemical radical decarboxylation, 62 63. [Pg.6]

A series of bicyclo[3.3.0]octanols are accessible by electroreductive tandem cyclization of linear allyl pentenyl ketones 189, as shown by Kariv-Miller et al. [189]. The electrolyses are carried out with an Hg-pool cathode and a Pt-flag anode. As electrolyte, tetrabutylammonium tetrafluororborate is used. The reaction is stereoselective, yielding only two isomers 192 and 193. In a competing reaction, a small amount of the monocyclic alcohol is formed. Since all the monocycles have the 1-allyl and the 2-methyl group in trans geometry it is assumed that this terminates the reaction. The formation of a bicyclic product requires that the first cyclization provides the cis radical anion which leads to cis-ring juncture [190] (Scheme 37). [Pg.104]

The reaction with monomethyl malonate in acetic acid, which does not occur at 0-10°C, proceeds smoothly when sonication is applied (Allegretti et al. 1993). From cyclohexene, only the cis ring fusion in bicyclic lactone is observed the product is formed at 80% yield for 15 min at 10°C. The overall transformation, in brief, is shown in Scheme 6.16. The stereoselectivity of the sonochemical process probably reflects the enhanced reaction rate, which does not allow equilibration processes to take place. [Pg.331]


See other pages where Rings, bicyclic, stereoselectivity is mentioned: [Pg.246]    [Pg.672]    [Pg.110]    [Pg.314]    [Pg.771]    [Pg.784]    [Pg.310]    [Pg.112]    [Pg.216]    [Pg.135]    [Pg.6]    [Pg.176]    [Pg.183]    [Pg.29]    [Pg.496]    [Pg.1197]    [Pg.115]    [Pg.92]    [Pg.29]    [Pg.82]    [Pg.98]    [Pg.99]    [Pg.103]    [Pg.147]    [Pg.343]    [Pg.423]    [Pg.59]    [Pg.79]    [Pg.516]    [Pg.561]    [Pg.26]    [Pg.37]    [Pg.372]    [Pg.112]    [Pg.83]    [Pg.7]    [Pg.128]    [Pg.1220]    [Pg.312]    [Pg.599]   
See also in sourсe #XX -- [ Pg.49 , Pg.839 ]




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Rings, bicyclic, stereoselectivity conformation

Rings, bicyclic, stereoselectivity fragmentation

Rings, bicyclic, stereoselectivity reactions

Rings, bicyclic, stereoselectivity stereoselective reactions

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