Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Diazoketones Stevens rearrangement

The CJS insertion reaction was suppressed completely upon catalytic decomposition of diazoketones 361, where the sulfur substituent was alkyl, acyl or thioacyl. It is presumed that sulfonium ylides occur as intermediates which give cepham (or cephem) derivatives in all cases270,343) rather than products of a Stevens rearrangement. [Pg.216]

While [2,3]-onium ylide rearrangements are proceeding most often by concerted pathways (review [388]), [l,2]-onium ylide rearrangements (Stevens rearrangements) often involve radical intermediates (recent reviews [389, 390]). A study aimed at the comparison of Rh(II) and Cu(II)-catalyzed oxonium ylide reactions of oxygenated diazoketones using 3 mol% of the former and 15 mol% of the latter showed that the copper-catalyzed process provided better yields and selectivities for [1,2]-rearrangement products (see Part 2, Sect. 6, Fig. 107) [391, 392]. [Pg.406]

West and Naidu found that the diazoketone 358, prepared by alkylating the benzyl ester of L-proline with 5-bromo-l-diazopentan-2-one, cyclized to give a transient spirobicyclic ammonium ylide 359 when heated with coppeifll) acetylacetonate in toluene (Scheme 44) (355,356). This unstable ylide underwent a diastereoselective [1,2]-Stevens rearrangement to give the quinolizidinone 360 and its bridgehead epimer in a ratio of 95 5. However, some racemization (possibly through an achiral diradical intermediate) must have occurred, since 360 had an ee of only 75%. Reduction of the ester and defimctionalization of thioketal 361 with the unusual combination of sodium and hydrazine in hot ethylene glycol completed a synthesis of the unnatural (- )-enantiomer of epilupinine (ent-331). [Pg.155]


See other pages where Diazoketones Stevens rearrangement is mentioned: [Pg.306]    [Pg.330]    [Pg.522]   
See also in sourсe #XX -- [ Pg.520 ]




SEARCH



Diazoketone rearrangement

Diazoketones

Diazoketones, rearrangement

Steven rearrangement

© 2024 chempedia.info