Bicyclic cyclopentenone skeleton

In early 1994, Padwa et al. (119) synthesized both mono- and bicyclic core skeletons of the illudins and ptaqualosins. By utilizing a 1,1-disubstituted cyclopropane as the core of the dipolar cycloaddition, Padwa was able to add a number of dipolarophiles including cyclopentenone and cyclohexenone to produce cycloadduct 233 amenable to subsequent transformations to form the illudins and ptaqualosins. The cycloaddition forms the bicyclic constrained ether in... 

One of the characteristic reactions of the Ti reagent is the intramolecular nucleophilic acylation of the titanacycle with esters, and 2,7- and 2,8-enyne esters afford interesting mono- and bicyclic skeletons [125], Titanacycle 297, formed from a, /i-unsaturated ester 296, is converted to the alkenyltitanium 300 by protonation, and the cyclopentenone 301 is formed by intramolecular acylation [126], As 297 is a tautomeric form of the Ti enolate, it reacts with electrophiles such as aldehydes to give 298, which cyclizes to 299. The a,/i-alkynic ester 302 generates the titanacycle 303 and is converted to the bicyclo[3.1.0]hexane system 305 via 304 [126]. The titanacycle... 

Al-Methylmorpholine iV-oxide, preferably as monohydrate, as well as MesNO not only induces cyclopentenone formation under very mild conditions (0-20 °C) and in frequently excellent yields, but also removes some earlier limitations. The reductive processes (eqs 58 and 59) do not occur when these promoters or DMSO are used under oxygen. Hence two efficient syntheses of (—)-kainic acid have employed cyclizations of allylpropar-gylamine derivatives as the key step in one of these (eq 62), as well as in a synthesis of the dendrobine skeleton, use of a chiral precursor was shown to result in enantiospecific bicyclization. ... 

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