Reduction of pyrans

Stepwise catalytic reduction of pyran-2-ones to the dihydro (300) and tetrahydro (301) pyran-2-ones is achieved by suitable choice of catalyst and reaction conditions (Scheme 17). Anhydrous copper(II) sulfate simultaneously dehydrates the alcohol (78JHC1153). 

Crimmins reported synthesis of Leighton s macrolide intermediate (Scheme 69). The key steps are a reductive cleavage of bicyclic acetal to form 2,6-fra s-tetrahydropyran 325, that is essentially reduction of pyran oxo-nium cation, and a diastereoselective 1,5-anfi-aldol reaction of the boron enolate to establish a stereogenic center at C7 by generating a C7-C8 bond, that is in contrast to the 1,5-anff-aldol forming ClO-Cll bond (Carreira... 

A 2,4)6-trisubstituted 2H or 4/f) pyran (38, R = R = Ph) was reported to result in low yield by catalytic reduction of 2,4,6-triphenyl-pyrylium salts by oxidation or by treatment with concentrated sulfuric acid it regenerated the triphenylpyrylium cation. There was no subsequent confirmation of this reaction. The reduction of pyrylium salts with sodium borohydride affords 1,5-diones by way of 4H-pyrans and 2,4-dien-l-ones by way of 2H-pyrans. ... 

Propanol with magnesium in reduction of chlorobenzene, 47, 104 Propionyl fluoride, 46, 6 n Propylamine, 46, 85 n Propylhydrazine, 46, 85 C ( Propyl) N phenylmtrone, genera tion from phenylhydroxylamme and n butyraldehyde, 46, 97 Purification of tetrahydrofuran (Warning), 46,105 4H Pyran 4-one, 2 6 dimethyl 3,5 diphenyl, 47, 54... 

The cyclization of y -hydroxy ketones is useful for the formation of pyrans,306,403 both directly and via rearrangement, as illustrated in Eq. 231.153 As with their acyclic counterparts, these cyclizations also occur with the silyl ethers of the hydroxy ketones where Et3SiH/BiBr3 is used with the TBS and TES ethers.342,404 A methyl thiomethyl ether is also capable of undergoing the reductive cyclization 405 In like manner, 1,4-diols and e-hydroxy ketones provide oxepanes, with I ds Si H or PhMe2SiH/TMSOTf being especially effective (Eqs. 232 and 233).336,406 The trimethylsilyl ether of the alcohol also provides the oxepane.306... 

Scheme 6 Cathodic reduction of 2-chloro-2-phenylacetyl chloride to pyran-2-one and pyran-4-one derivatives [82].

The oxygen and sulfur heterocycles, including pyrylium and thiopyrylium salts, are readily reduced catalytically. Moreover, it is usually possible to control the degree of reduction both pyran-2- and -4-ones can readily be reduced to dihydro derivatives, in the former case the 5,6-dihydro compound. Examples of catalytic reduction of the oxo substituents to either the corresponding alcohol or to the methylene group are known, but these reactions normally occur only under pressure. 

The diisobutylaluminium hydride (DIBAL-H) reduction of perhydrofuro[2,3- ]py derivatives, 58, is a novel entry to highly functionalized cyclopentanes containing high enantiomeric purity. The reduction targets the acetal group of the furo[2,3-/ ]pyran framework <2001CAR63>. 

The reduction of pyrylium salts to monomeric 2H- or 4//-pyrans has been accomplished by complex hydrides, hydrogen-transfer agents, electrochem-ically, and by catalytic hydrogenation. 

The reduction of pentaphenylpyrylium tetrafluoroborate with lithium borohydride in THF at -70°C gave, however, 60% of 2/f-pyran 154 together with 10% of the AH isomer and traces of dienone 156.211... 

On the other hand, analogous reductions of 158a,b with corresponding 4//-pyrans 151a and 151c at 80°C for 12 hours have led to dienones 152a,c, apparently via unstable 2H isomers 159a,b.209... 

Pentasubstituted tricyclic 4//-thiopyran 48 ( = 2, R = Me) was also prepared by the reduction of the corresponding thiopyrylium salt 227b with LiAlH4.269 The reduction with zinc proceeds via a dimeric 4//-thiopyran intermediate.272 Ion 229 (R = Ph, n = 2) was reduced with zinc in hydrochloric acid to a mixture of 2//-thiopyran 230 (R = Ph, n = 2), 4//-thio-pyran 232, and the corresponding perhydrothiopyran.100... 

Thienyimethanol (360a) underwent allylic rearrangement to 4-hydroxy-2//-pyran (2) with oxalic acid at 20°C for 6 days.341 2-Thienylmethylamine (360b) with nitrous acid (Demyanov reaction) gave a mixture of 360a and 2.3 An 86% yield of a mixture of 2,6-di-te/ -bu tyl-2//- and 4//-thiopyrans 362 and 363 was obtained when 2,5-dihydrothiophene ketone 361 (readily accessible by Birch reduction of 2-pivaIoyl-5-ter/-butylthiophene) was reduced with zinc and sodium hydroxide in the presence of trimethylsilyl chloride.286... 

The reduction of bis-2,6-hydroxycarbonyl-4//-pyran (32) (R = H) to the corresponding dihydropyrandicarboxylic acid was accomplished with sodium amalgam.379... 

The formation of fulvene (450) by sodium borohydride reduction of 2-formyl-4//-pyran (371) in water347 may be explained by the mechanism shown in Scheme 21. 

The keto group in condensed 4//-pyran derivative 264 was successfully reduced by reduction of the tosyl hydrazide with sodium borohydride.290... 

Optically active alcohol 6 (94% e.e.) can be obtained by LiAIH4 reduction of trans-methyl 2ft, 6ft-2-methoxy-5,6-dihydro-2//-pyran-6-carboxy late (see Scheme 9, 2 R = Me) [25],... 

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