1.2- Dihydropyrans, ring opening

Tetraacetylethene yields dihydropyrans in hDA reactions with ethyl vinyl ether and cyclopentadiene, but 1,2-dimethylenecyclohexane affords both the hDA and normal Diels-Alder adducts, the latter presumably arising via ring opening of the former adduct to a zwitterionic intermediate (Scheme 2) <99EJ03343>. 

The metal is chelated by an ene-1,2-dithiolate (dithiolene) of the dihydropyran ring which is fused to a partially reduced pteridine 178. The syntheses of these molybdopterin-related proligands were described as model compounds for verification of the stability in vitro <2001CC 123>. The proligand 179 is a relatively stable compound however, after several weeks, the pyran ring opened and oxidation to the diol derivatives 180 was observed (Scheme 35). 

Hot mineral acids open dihydropyran rings and recyclization to a benzene ring is feasible when a suitable substituent is present, as for example in 6-ethyl-3-vinyl-2,3-dihydro-4H-pyran (605) (74T1015). 

The reaction of a,/3-unsaturated carbonyl compounds with enamines also leads to dihydropyrans, although it is not always possible to isolate these since they react further to give either ring-opened by-products or bicyclic derivatives arising from a Stork annelation. There has been considerable discussion on the mechanism of this reaction, although the initial nucleophilic attack of the enamine on the /3-carbon of the diene is not in doubt (63JA207). It is possible that a zwitterionic species is involved, either as an intermediate or merely in equilibrium with the dihydropyran (67JCS(C)226). 

The a-oriented alkynyl sugar 166, formed originally, is transformed to the /1-isomer 169 by forming the Co complex 167. Due to the stable propargylic cation of the Co complex, facile opening and closing of the dihydropyran ring in 167 yield the thermodynamically stable //-isomer 168 [43]. 

Treatment of cycloprop [c]pyrans 520 with acid leads to chromans 521 with a 5,8-substitution pattern, which is difficult to obtain by other synthetic methods (Scheme 114). This novel reaction is believed to proceed via a retro-hDA opening of the dihydropyran ring to afford 522 followed by an acid-catalyzed aldol-type cyclization and dehydration (Scheme 114) <20040L3191>. 

Radical induced ring opening of the cyclopropyl ketone 827 affords the radical intermediate 828, 6- W >-trig cyclization of which furnishes 3,4-dihydropyran-2-one 829 in good yield (Scheme 233) <1998TL6971, 20050BC316>. 

Treatment of the diazofuran 905 with dirhodium tetraoctanoate leads to decomposition affording 5,6-dihydropyran-2-one 907. The reaction proceeds via ring opening of a zwitterionic cyclopropane intermediate 906 (Scheme 247) <1997TL5623, 2000JOC4261>. 

Magnesium in methanol effects the reductive ring opening of the cyclic peroxide 911, recyclization of which affords 4,6,6-trimethyl-5,6-dihydropyran-2-one in good yield (Equation 362) <2004JOC2851>. 

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>. 

At 150 °C, (+)-mahanimbine (5) undergoes racemization, presumably by opening and re-closure of the dihydropyran ring. At higher temperatures (200 °C), curryanine (cyclomahanimbine) (6) and curryangine (7) are formed 5 the structure... 

The 4-benzyl 1,4-dihydropyran 520 rearranges on irriadiation, or thermally to the 2-benzyl isomer 521 which yields 522 via electrocyclic ring opening. Irradiation of 277-chromenes 523 gives an intensely red photoproduct 524. 

Platinum-catalyzed cyclization of a 2,3-dihydrofuran to the tethered alkyne provided the fused tricyclic compound 238, as shown in Scheme 71. Acid-promoted benzannulation of 238 then produced the dihydrobenzofuran, presumably via a retro-hetero-Diels-Alder opening of the dihydropyran ring <20040L3191>. 

Cyclobutenes possessing an angular O-functionality, obtained from a Lewis acid-mediated [2+2] cycloaddition of cyclic silyl enol ethers to ethyl propynoate and subsequent reduction and butenylation, undergo a ring-opening metathesis that produces a substituted dihydropyran that forms part of a c -diene. After desilylation, an oxy-Cope rearrangement leads to the fused tetrahydropyran 4 <03JA14901>. 

The addition of alkoxycarbonyl nitrenes, generated photochemically from azides, to substituted dihydropyrans and tri-O-acetyl-D-glycal57"59 in alcoholic solution gave the products of alcoholysis 30 and 31-33 of the intermediate aziridines by a one-pot procedure59. The attack of the nitrene takes place mainly on the less hindered face of the double bond, but complete control of the diastereoselectivity was not accomplished even the ring opening of the aziridine can follow both anti and syn stereochemistry. 

The polymerization of 2,3-dihydropyran has been reported. However, this compound is a vinyl ether. The polymerization occurs mainly through the double bond and not by ring opening [3]. Kamio et al. [117] found that the polymer obtained by polymerizing 2,3-dihydropyran with... 

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