Dihydropyranyl acetates

Alkylation of dihydropyranylacetates. The Pd(0)-catalyzed alkylation of allylic acetates by stabilized carbanions originally reported by Trost and Verhoeven (8,475 9, 451 -457)4 has been extended to alkylation of dihydropyranyl acetates as a route to natural C-glycopyranosides.5 Again the displacement is regioselective and results in net retention of configuration in reactions with most carbanions. However, alkylation with phenylzinc chloride or vinylzinc chloride occurs with inversion. 

Cyclic acetals 7 and 9 with one exocyclic oxygen atom react with bis(dimethylaluminum)selenide in the presence of cyclopentadiene at 100 JC giving the cycloadducts 8 and 10, each having a hydroxy group at the terminal position108. Similarly, the reaction of dihydropyranyl methyl acetal 11 affords the Diels-Alder adduct 12 with a side chain having a terminal formyl group108. 

The free OH can be modified by further chemical manipulation, and reaction with benzoyl chloride (BzCl) and pyridine (Py) gives 146 (the 0-benzyl group is in blue in the illustration). Treatment of 146 with 10% aqueous acetic acid cleaves the orthoester and gives a 1 1 mixture of hydroxy acetates 147 and 148, where the acetate units are marked in cyan. If 147 is isolated and treated with dihydropyran (see Chapter 26, Section 26.4.2) and an acid catalyst, the 2 -hydroxyl is converted to a dihydropyranyl ether (149) where the P3rranyl ether unit is marked in green. It is therefore possible to protect and deprotect the 5 -, 3 -, and 2 -hydroxyl groups selectively, although the abihty to differentiate the 2 - and 3 -hydroxyls is more difficrdt to achieve. 

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