A- tetrahydropyranes

A tandem combination initiated by a Mukaiyama reaction generates an oxonium ion that cyclizes to give a tetrahydropyran rings.48... 

Fall D, Duval RA, Gleye C, Laurens A, Hocquemiller R. Chamuvarinin, an aceto-genin bearing a tetrahydropyran ring from the roots of Uvaria chamae. J Nat Prod 2004 67 1041-1043. 

The latest example of a Pdn-catalyzed Wacker/Heck methodology was published by Rawal and coworkers. During the total synthesis of mycalamide A, an inter-molecular Wacker oxidation with methanol acting as nucleophile and a subsequent ring closure via Heck reaction led to a tetrahydropyran moiety in a 5.7 1 diastereom-eric mixture [184]. 

Fig. 3.22 Oxidation of a phenyl group by RuO in the presence of a tetrahydropyran ring [246]...

Details of the Janda-Chen synthesis were as follows. A tetrahydropyran (THP) linker was attached to the NCPS support enabling attachment of alcohols via THP ether formation.13 The THP-NCPS resin 1 is derivatized with / -(+)-4-hydroxy-2-cyclopentanone 2, giving the THP ether-based resin 3, followed by coupling of the C13 20 fragment by enone-cuprate addition. The cuprate required was generated from the corresponding E-vinyl stannane 4. The resulting enolate was trapped as the silyl end ether... 

Hi) By formation of seven- from six-membered rings The expansion of a di- or a tetra-hydropyran ring fused to a three-membered ring has been used as a synthetic approach to oxepins. Thus the synthesis of oxepin (193) was attempted by thermal dehydrochlorination of a tetrahydropyran (equation 57) obtained from a dichlorocarbene addition reaction (65CI(L)184). Unfortunately the equilibrium appeared to favor the keto tautomer to the apparent exclusion of the enolic oxepin form (193). 

Another series, pyralomycins 2a-2c (99-101), have a tetrahydropyran ring in place of the cyclohexene ring. They inhibit the growth of Micrococcus luteus at 0.2-25 pg/ml [83]. 

It has also been suggested (18, 19) that a distinction should be made between the two lone pairs of a given ether-oxygen atom, one being more 2p-like and a better donor than the other lone pair which is more sp -like and a poorer donor. As a consequence of this, the 2p-like lone pair of the oxygen atom in a tetrahydropyran ring, for instance, cannot be perfectly anti peri planar to an axial alkoxy group (12, 20). I prefer to consider that... 

It was felt that this potential conformational problem could be circumvented by joining the two ends of the alkyl substituents, at C-9 and C-10 in arteether, to form a tetrahydropyran ring. Pentacyclic ether 154 was synthesized, but found to be less active than arteether. 

Cueto, M., Darias, J., Rovirosa, J., and San-Martin, A., Tetrahydropyran monoterpenes from Plocamium cartilagineum and Pantoneura plocamioides, J. Nat. Prod., 61, 1466, 1998. 

Substituents in cyclohexanes prefer an equatorial orientation. Nonetheless, an electronegative substituent X situated a to the oxygen in a tetrahydropyran will prefer an axial position. This is the anomeric effect. 

The enantioselective synthesis of the C(18)-C(25) segment of lasanolide A 324 can be achieved via an oxonia-Cope-Prins cascade cyclization of a-acetoxy ether 325. The in situ reduction of the oxocarbenium ion intermediate 326 with Bu3SnH prevents the formation of a tetrahydropyran 4-one side product (Scheme 81) <20050L1589>. 

A [Ni(cylam)](Cl04)2-catalyzed electrolytic reduction of alkenyl bromides 342 allows for an environmentally friendly entry to 4,5- A-tetrahydropyrans 343. Either (E)- or (Z)-alkenes can be used as substrates, but diastereoselectivity is not observed if an isomeric mixture of alkenes is subjected to the cyclization conditions (Equation 146, Table 15) <1996JOC677>. 

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