Spiro-3,4-dihydropyrans

Ma and coworkers reported a similar reaction for the synthesis of spiro-3,4-dihydropyranes [44]. The reaction between cyclic p-oxo aldehydes and aromatic p,Y-unsaturated a-ketoesters catalyzed by the cinchona alkaloid derivatives afforded the corresponding spirocyclic compounds in good yields and moderate to good enantioselectivities. 

Ma and coworkers reported the reaction of p-oxo aldehydes 79a with p,7-unsaturated ketoesters 91 catalyzed by bifunctional amino-squaramides (XXni) [53]. The reaction afforded diverse spiro-3,4-dihydropyrans 92, after acylation, in moderate to good yields and good stereoselectivities, as shown in Scheme 10.32. 

After successfully achieving the asymmetric synthesis of spiro-3,4-dihydropyrans via chiral tertiary amine-catalyzed domino Michael-hemiacetalization reaction of... 

Formation.—Phenyl-, 2-naphthyl-, 2-thienyl-, and 2-furanyl-thiets are prepared from aryl methyl ketones, dimethylamine, and methanesulphonyl chloride. X-K y crystal structures establish that the compounds arising from the cycloaddition of alkynes to thioxanthones are thiets, and not spiro-dihydropyrans, as originally proposed. Additional work shows that alkynyl sulphides also give thiets with thioxanthone, and that the adduct (104) is in rapid equilibrium with the open form (105) in solution, although they can be isolated separately. Thiet (106) was isolated after thermolysis of a Dewar thiophen—azide adduct. ... 

RCM has been used to effect the cycloisomerisation of allyl pent-2,3-dienyl ethers to dihydropyrans <06TL3971> and in the synthesis of the spiro[indolino-3,2 -pyran] 5 <06JOC2346>. 

Scheme 12.64 Au-catalyzed synthesis of spiro or fused dihydrofurans and dihydropyrans.

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