Cyclization with pyridinium chlorochromate

A novel one-pot tandem oxidation-cyclization-oxidation process was successfully applied in the transformation of unsaturated alcohols 250 [Eq. (5.315)].860 The intermediate aldehyde formed by oxidation with pyridinium chlorochromate (PCC) undergoes a carbonyl-ene cyclization followed by an additional oxidation to form 3-substituted piperidinones. 

Alternatively, the mesylate 77 was treated with sodium iodide in refluxing 2-butanone to produce the olefin 90 (Scheme 3). " Oxymercuration-demercuration reaction of 90 using mercnric acetate-sodinm borohydride afforded a 1 1 mixture of 91 and 92. Compound 92 was mesylated, rednced and cyclized in boiling ethanol, in the presence of sodium acetate, to give 93. This was converted by standard steps into 94, and subsequent hydrolysis of 94 followed by oxidation with pyridinium chlorochromate afforded the lactone 95 (28% overall yield from 92), which was nsed in the synthesis of retronecine (22). " ... 

Reduced Isocoumarins.- Kigelin (143 R=H) is a natural di-hydroisocoumarin its methyl ether (143 R=Me) has been synthesized in two steps from another natural compound called elemicin (allylbenzene). Cyclization with mercury(II) acetate gave the 1-isochromanol which was oxidized to (143 R=Me) with pyridinium chlorochromate.153 Isomeric with dihydroisocoumarins are the isochroman-3-ones one such compound has been synthesized by the action of hot mineral acid on 2-methoxymethylbenzyl cyanides such... 

Triol 984 is converted to acetonide 985 and the free hydroxyl group is sequentially oxidized to an aldehyde (986) with pyridinium chlorochromate and then to an acid under Jones conditions. Acidic workup furnishes the hydroxy butyrolactone 987. Treatment of THP-protected butyrolactone 988 with methyl Grignard reagent affords diol 989. Removal of the THP group, tosylation of the primary alcohol, and cyclization under basic conditions provides epoxide 990. Opening the oxirane ring with 2-(l,3-butadienyl)magnesium chloride in the presence of... 

Other interesting methods can generate furans. One method treats an allylic diol such as 125 with pyridinium chlorochromate (PCC see Chapter 17, Section 17.2.3), in an oxidative-cyclization process, to give 3-ethylfuran, 126. Thiophene derivatives can also be prepared from 1,4-dicarbonyl compounds. A Paal-Knorr thiophene synthesis reacts 2,4-hexanedione (115) with phosphorus pentasulfide (P4S10) to give 2,5-dimethylthiophene, 127. 

Desymmetrization of a meso-intermediate also held the key to enantio-selectivity in the approach to (—)-swainsonine (378) by Katsuki and his colleagues (Scheme 70). In this case, the (R,R)-(salen)manganese complex 524 mediated the enantioselective oxidation of the meso-pyrrolidine 525 with iodosylbenzene to the (2S)-hydroxypyrrolidine 526, which was further oxidized with pyridinium chlorochromate to yield lactam (—)-527 in an overall yield of 56% and an ee of 71%. Reaction with 4-chlorobutylmagne-sium bromide opened the lactam ring to give the chloroketone (+)-528, which underwent spontaneous cyclization to the cyclic imine 529 upon... 

Oxidative cyclization of selected 5,6-dihydroxyalkenes to ci.v-2,5-disubstituted tetrahydrofurans has been realized with chromium(VI) reagents. In fact, when Collins reagent or pyridinium chlorochromate are employed at 20 °C for 3 min, the cyclization affords the cis-2,5-products in moderate yield. The resulting (tetrahydrofuranyl)diols are diastereomerically pure by H- and i5C-NMR spectroscopy. Moreover, a comparison of the reaction mixture with authentic 2,5-trans samples indicates a stereoselectivity of 99.5%47. 

Oxidation of primary alcohols carrying unsaturation at the 5 or 6 position by means of the pyridinium chlorochromate provides the cyclized products. The same products are obtained by oxidation of analogously substituted aldehydes, with this reagent. ... 

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