Synthesis of Bridged Bicyclic Ethers and Fused Oxetanes from Pyran-4-ones via Tandem Solvent Trapping and Norrish Type II Cyclization(1).

Polyalkyl pyran-4-ones 1a-c were irradiated in methanol or ethanol. Although the expected solvent trapping products 3 could be observed, extended irradiation times allowed exclusive formation of secondary photoproducts 4 and 5 in combined yields of 37-64%. These bicyclic compounds are believed to arise from gamma-hydrogen abstraction by the excited enone chromophore of 3, followed by closure of the resulting biradical through one of two possible pathways. Moderate stereoselectivity was observed in the radical coupling to produce 4, whereas the analogous closure to 5 was completely diastereoselective. Tautomerization of the enol precursors to 5 also occurred with complete selectivity for protonation from the exo face. Overall, this process converts simple, planar heterocycles and alkanols into complex products in a single transformation.

Cyclopentene Synthesis from 1,3-Dienes via Base-Induced Ring Contraction of 3,6-Dihydro-2H-thiopyrans: Studies on Diastereoselection and Mechanism.

An investigation of the scope and mechanism of a new synthesis of cyclopentenes from 3,6-dihydro-2H-thiopyrans is described. Alkyl halides substituted with an electron-withdrawing group in the alpha-position were reacted with sodium thiosulfate, yielding the corresponding Bunte salts, which could be transformed to reactive thiocarbonyl compounds by elimination of the elements of bisulfite with mild base treatment. In situ trapping by 1,3-dienes afforded in good yields a variety of 3,6-dihydro-2H-thiopyrans substituted with electron-withdrawing groups at the 2-position. Exposure of these cycloadducts to strong base at low temperature effected a novel ring contraction, affording 2-(methylthio)-3-cyclopentenes after quenching with methyl iodide. The level of diastereoselectivity exhibited during the generation of these cyclopentenes was found to be dependent on the nature of the electron-withdrawing group at the 2-position of the dihydrothiopyran as well as the substitution pattern originally present in the diene component. In some cases, reducing the temperature during the ring contraction resulted in the isolation of good yields of vinyl cyclopropanes of high isomeric purity. With one substrate, highly diastereoselective rearrangement of a vinyl cyclopropane to a cyclopentene was unambiguously demonstrated, suggesting that this might be a key feature of the overall ring contraction mechanism.