Oxidopyrylium ylides

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>. 

The [5 + 2]-intramolecular annulation of 3-oxidopyrylium ylide 461 prepared from the corresponding acetate 460b by using DBU affords 465 (90TL6769). Transformation of 465 by way of several reactions gives the bicycloundecanone nucleus 466 of the secodolastane terpenes 467. 

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

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]. 

---