5,6-dihydropyran-2-ones

Cyelohydration of alkoxylvinylaeetylenie aleohols 180 (prepared from methoxy-butenyne by the Favorsky reaetion) leads to dihydropyran-4-ones 181-183 under eonditions of the Kueherov reaetion (75MI1 88MI1 93MI2). 

Complete diastereoselection is observed in the HDA reaction of Danishefsky s diene with o-substituted benzaldehyde chromium tricarbonyl complexes. Decomplexation is facile and good yields of 2-aryl-2,3-dihydropyran-4-ones result <96SL258>. Cis-2,3-disubstituted pyranones are accessible from the Lewis-acid catalysed HDA reaction between (triisopropylsilyloxy) dienes and aldehydes and dehydrogenation of the resulting dihydropyrans <96JOC7600>. 

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Additional evidence for a ring current in pyran-4-one is provided by the observation that the ring protons are deshielded by 1 p.p.m. compared to those in 2,3-dihydropyran-4-one. 

Irradiation of reduced pyran-4-ones frequently gives dimeric products which sometimes revert to the monomer. 2,6-Dimethyl-2,3-dihydropyran-4-one (559) in water gives a 96% yield of a mixture of three dimers (560) in a reaction which is reversible (73CJC1267). 

The cyclization of the alkynic enol ether (556) leads to a 5,6-dihydropyran-4-one (63JOC687). 

Dihydropyran-4-ones are formed with good enantiomeric excess by a chiral Lewis acid catalysed reaction of aldehydes with Danishefsky s diene and cyclisation of the initial aldol product. The overal process equates to a hetero-Diels-Alder cycloaddition (95JOC5998). Lactams also react with the electron rich diene under the influence of a Lewis acid, yielding 7-aza-l-oxaspiroalkenones (95JOC7724). 

Carbomethoxypivaloylketene 733 can undergo a hetero-Diels-Alder reaction with ethoxyalkyne to yield methyl 2-fer/-butyl-6-ethoxyM-oxoM//-pyran-3-carboxylate (Equation 292). Likewise, carbomethoxypivaloylketene 733 undergoes a hetero-Diels-Alder reaction with ethoxyethene to afford the intermediate dihydropyran-4-one, which upon elimination of ethanol affords methyl 2- v/-butylM-oxo-4//-pyran-3-carboxylate (Scheme 185) <2001T6757>. 

A stereospecific synthesis of dihydropyran-4-ones 937 can been achieved by a mercury(ll)-catalyzed rearrangement of l-alkynyl-2,3-epoxy alcohols 936. The configuration of the epoxide is transferred unchanged to the product (Equation 367, Table 44) <1998JOC3798>. 

Mercury(ll)-mediated removal of both dithianes from alcohol 938 followed by in situ cyclization of the unmasked diketone provides the dihydropyran-4-one 939 during a synthetic approach to thyrsiferol (Equation 368) <2005JOC5249>. 

Trifluoracetic acid-promoted cyclization/dehydration is used to construct a dihydropyran-4-one ring during the total synthesis of (—)-maurenone, establishing the relative stereochemistry of the natural product <2006JOC117>. A double aldol reaction forms two dihydropyran-4-one rings during the total syntheses of (—)- and (+)-membrenone C <20030L1729>. 

Ozonolysis of the double bond in 1,4-cyclohexadiene 943 followed by reductive work-up and dehydration provides the dihydropyran-4-one 944, an intermediate during a formal synthesis of leucascandrolide (Equation 369) <2002CC2066>. 

The use of hDA methodology as a means of constructing the dihydropyran-4-one ring system continues to attract much interest. A review concerning the enantioselective hDA reaction covers the synthesis of dihydropyran-4-ones using this cycloaddition approach <2000AGE3558>. 

The use of the zinc complex of the BINOL ligand 946 in the hDA reaction between Danishefsky s diene 947 and aldehydes proceeds in excellent yield and enantioselectivity to afford dihydropyran-4-ones 948 (Equation 370, Table 45) <2002OL4349>. An asymmetric diethyl zinc addition can occur in tandem with the cycloaddition reaction between Danishefsky s diene 947 and isophthalaldehyde using a related catalyst 949 (Equation 371) <20030L1091, 2005T9465>. 

Hydrogenolysis of the racemic isoxazolyl alcohol 955 followed by an acid-mediated cyclization of the resulting diketone provides the dihydropyran-4-one 956, an intermediate during a synthesis of C-aryl glycosides (Equation 373) <20020L977>. 

Hydrazone anion 957 induced ring opening of the enantiopure 4-substituted oxetan-2-ones 958 followed by cycliza-tion/dehydroamination of the resulting P-ketohydrazones 959 affords dihydropyran-4-ones 960 in good to excellent yield and enantioselectivity (Scheme 256, Table 46) <20020L1823>. 

Treatment of the chiral aijl-unsaturatcd sulfoxide 961 with TMSI results in formation of the unstable dihydropyran-4-one 962 (Equation 374) <2002T10145>. 

Dihydropyran-4-ones can be prepared by addition of various nucleophiles to 3-ethoxy-5,6-dihydropyran-2-ones prepared from the hDA reaction between Brassard s diene and aldehydes <20050L2421>. 

A copper(l)-catalyzed 1,4-addition of vinylmagnesium bromide to dihydropyran-4-one 1053 proceeds in good yield and with high diastereoselectivity to afford the tetrahydropyran-4-one 1054, a key intermediate during synthetic studies towards the pederin family of antitumour agents (Equation 411) <2000J(P1)2357>. 

Pd-catalysed oxidative heterocyclisation features in a synthesis of substituted dihydropyran-4-ones from P-hydroxy-ynones. The process involves domino Wacker - Heck coupling of the ynones with ethyl acrylate and, although yields are only moderate, the stereochemistry of the ynone is retained in the product (Scheme 24) <06JOC8390>. 

A tandem aldol - intramolecular conjugate addition involving enones and aldehydes provides a route to 2,3-dihydropyran-4-ones which does away with the need to preform the activated dienes needed for a HDA reaction (Scheme 25) <06TL1537>. 

The oxo-HDA reaction of 2-carbonyl derivatives of pyridine 1-oxide with electron-rich dienes catalysed by bisoxazoline Cu(II) complexes proceeds by a Mukaiyama-aldol route and affords dihydropyran-4-ones 33 in good yield and excellent enantioselectivity <07JOC240>. 

The location of the silyl protecting group in the coupling product from the reaction between a silyl dithiane and two different chiral epoxides is controlled by the order of the addition of the epoxides. Subsequent cyclisation of the derived 1,3-diketones provides an efficient route to a variety of 2,6-disubstituted dihydropyran-4-ones 39 <07JOC4280>. 

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