4-Iodo tetrahydropyrans

Phenols add intramolecularly to Michael acceptors. " Under acidic conditions, a one-pot sequence starts with initial electrophilic acetylation of the activated aromatic ring and is followed by cyclization." With an appropriate leaving group in the /f-position (OMe. or other amines such as in the unsaturated carbonyl compound (e.g., 4) is formed. Other approaches to pyroncs include the self-condensation of protected //-hydroxy acrylates,intramolecular aldol reactions followed by condensation,thermal cycli-zations of unsaturated ()-chloro esters,and an iodo-cyclization-elimination sequence w th Michael acceptors.Oxymercuration of an unsaturated alcohol is an alternative cyclization approach to tetrahydropyrans. 

Tetrahydropyran 4.5-Diacetoxy-2-isothiocyanat-3-iodo-6-methyl-E21e, 5303 (En + KSCN/ I2)... 

Tetrahydropyran 6-Azido-3,4-bis-[methoxy-methoxy]-5-iodo-2-[(methoxy-methoxy)-methyl]-E21e, 5201 (En + IN3)... 

Tetrahydropyran 6-(Acetoxy-methyl)-4,5-diacetoxy-3-iodo-2-isothiocyanat- E21e, 5303 (En + I2/KSCN)... 

Isolable allyl samarium species are formed in tetrahydropyran at —15 °C by the reaction of Sml2 with (ft)-l-iodo-2-dodecene (Scheme 300).1085... 

Our group performed a similar allylation study with a-iodo esters bearing a tetrahydrofuran (THF) or a tetrahydropyran (THP) ring adjacent to the radical... 

Perez-Balado, C. and Marko, I.E., 1-iodo-l-selenoalkenes as versatile alkene 1,1-dianion equivalents. Novel connective approach toward the tetrahydropyran subunit of polycavemoside A, Tetrahedron, 62, 2331, 2006. 

A variety of alkenes undergo azidoiodination with sodium azide, potassium iodide, and Oxone on wet alumina to give azido-iodo compounds regioselectively in high yield (eq 79). These compounds are useful precursors to vinyl azides, amines, and aziridines and are typically synthesized with more expensive and exotic reagents. Similar methods have been used in the iodolac-tonization and iodoetherification of unsaturated carboxylic acids and alcohols to make five- and six-membered lactones, tetrahy-drofurans, and tetrahydropyrans (eq 80). ... 

Metal-activated alkene additions can be classified as stoichiometric or catalytic processes. Stoichiometric processes for THP synthesis typically involve the use of mercury(II) salts and to a lesser extent iodo and seleno reagents. The progress of intramolecular oxymercuration is determined by the stabiUty of the cationic intermediates. Product stereochemistry is under substrate control and usually leads to the thermodynamically more stable THP product. Catalytic variations generally involve palladium complexes [44], but other transition metals are becoming more common (e.g., Pt [45], Ag [46], Sn [47], Ce [48]). The oxidation state of Pd determines the catalyst reactivity. Palladium(O) complexes are nucleophilic and participate in tetrahydropyran synthesis through jt-allyl cation intermediates, whereas Pd(II) complexes possess electrophilic character and progress through a reversible t-complex. 

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