Oxidopyrylium ylides cycloadditions

Synthesis - Cycloadditions - Novel intramolecular oxidopyrylium ylide cycloadditions have been reported (e.g. (86) (87)88 and (88) (89) 8. ... 

Thermolysis of the pyranyl acetate (228) gave rise to the isomeric perhydroazulenes (229a-b), presumably via intramolecular cycloaddition of an intermediate oxidopyrylium ylide.127 A similar cyclization occurred for the pyran-4-one (230), leading to (231a), which was isolated as the acetate (231b).128 No evidence was presented for an oxidopyrylium intermediate and the timing of carbonyl-enol tauto-merization is unknown. Thus, another mechanism may apply. 

Dihydrofurans are valuable synthetic compounds. Nevertheless, little is known about intramolecular cycloaddition of carbonyl ylides to alkynes. In one example, alkynyl pyran-4-one (233) was cyclized to dihydrofuran (234 Scheme 69).128 Possibly this reaction proceeds via an oxidopyrylium ylide intermediate as shown. 

The intramolecular thermal [5+2] cycloaddition of 3-alkoxy-4-pyrones with sulfur- (e.g., 416) or silicon- (e.g., 419) tethered alkenes has been shown to occur with complete regio- and stereochemical control to give adducts 417 and 421, respectively. The adducts can be converted by reduction and oxidation, respectively, to the bicyclic products 418 and 421 (Scheme 69) <1993JOC5585>. It should be noted that this thermal [5+2] cycloaddition has not been realized in a bimolecular mode <1977JOC3976>. This methodology serves as an alternative to the reaction of electron-deficient alkenes with pyrone-derived 4-methoxy-3-oxidopyrylium ylides <1992TL2115>. 

The adduct obtained from the stereo- and regiospecific [5+2] cycloaddition of the 3-oxidopyrylium ylide 27 to 3-methylenetetrahydrofuran-2-one is readily converted into the polycycle 28 which is a late intermediate in the synthesis of the polygalolides <07OL873>. 

Li and co-workers showed that indole 203,upon treatment with MeOTf followed by CsF, undergoes a dearomative [5+2]-cycloaddition reaction through oxidopyrylium ylide 204 to give oxacyclohepta[fc]indole 205. The reactions are diastereoselective, give exclusively endo selectivity and proceed under mild conditions (14AG(I)11051). 

Recently, the first example of organocatalyzed asymmetric [5+2] dipolar cycloaddition of oxidopyrylium ylides was achieved by the same research group, leading to structurally diverse 8-oxabicyclo[3.2.1]octane derivatives 306 in good yield and stereoselectivity (Scheme 2.83). The combination of a chiral primary aminothiourea 140 m and a second achiral thiourea 305 offered an optimal dual-catalysis system to promote the formation of oxidopyrylium ylides from acetoxypyranones 304 and subsequent intramolecular cycloaddition [115]. 

As you have surely noted, several examples of 1,3-dipolar cycloaddition reactions have been discussed throughout the course of the Classics series due to the amazing library of products that result from their employment. Among the 1,3-dipoles typically employed in these events, of which a representative collection is shown in Scheme 5, oxidopyrylium species (whose resonance form is a carbonyl ylide) constitute a relatively unexplored tool... 

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