3.4- dihydro-2//-pyran alkene

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

Inverse electron-demand Diels-Alder reaction of (E)-2-oxo-l-phenylsulfo-nyl-3-alkenes 81 with enolethers, catalyzed by a chiral titanium-based catalyst, afforded substituted dihydro pyranes (Equation 3.27) in excellent yields and with moderate to high levels of enantioselection [81]. The enantioselectivity is dependent on the bulkiness of the Ri group of the dienophile, and the best result was obtained when Ri was an isopropyl group. Better reaction yields and enantioselectivity [82, 83] were attained in the synthesis of substituted chiral pyranes by cycloaddition of heterodienes 82 with cyclic and acyclic enolethers, catalyzed by C2-symmetric chiral Cu(II) complexes 83 (Scheme 3.16). 

A simple approach for the formation of 2-substituted 3,4-dihydro-2H-pyrans, which are useful precursors for natural products such as optically active carbohydrates, is the catalytic enantioselective cycloaddition reaction of a,/ -unsaturated carbonyl compounds with electron-rich alkenes. This is an inverse electron-demand cycloaddition reaction which is controlled by a dominant interaction between the LUMO of the 1-oxa-1,3-butadiene and the HOMO of the alkene (Scheme 4.2, right). This is usually a concerted non-synchronous reaction with retention of the configuration of the die-nophile and results in normally high regioselectivity, which in the presence of Lewis acids is improved and, furthermore, also increases the reaction rate. 

Pyrano[2,3- ]-l,2,4-trioxines 260a-c arise in low yield from the reaction of the appropriate alkene, 3,4-dihydro-277-pyran 258, with singlet oxygen in the presence of methylene blue (MB), followed by condensation of the intermediate oxetane 259 with acetaldehyde or acetone as appropriate (Scheme 43) <1997H(46)451>. 

Hydrogenation of the triple bond in 195 to a cis double-bond, followed by acid-catalyzed cyclization of the alkene, leads to 6-substi-tuted 2-alkoxy-5,6-dihydro-2//-pyrans (196). This method was used by H. Newman86 for the preparation of 2-ethoxy-5,6-dihydro-6-methyl-2H-pyran (196, R1 = Et, R = Me). 

Six-membered oxaenoncs [e.g., 5,6-dihydro-2i/-pyran-2-one,109, 2,2-dimethyl-l,3-dioxin-4-one,110-112 2,2-dimethyl-2/f-furo[3,4- ]pyran-4,7(3//,5//)-dione113] also give cyclobutanes on irradiation in the presence of alkenes. The corresponding six-membered thiaenones usually deactivate via ZjE isomerization.114... 

Reactions of unsaturated esters with electron-rich alkenes have been reported to yield only cyclobutane derivatives. However, NMR examination of the products has indicated the formation of substituted 3,4-dihydro-2H-pyrans. The most informative feature of the spectra is the C-2 proton coupling constants of ca. 3 Hz with the two different protons at... 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

Not only alkenes but also other unsaturated substrates were similarly converted into v/oazidophenylselenyl derivatives, e.g. 3,4-dihydro-2//-pyran (74%), methyl methacrylate (75%), etc. Tri-O-methyl-D-glycal and tri-O-acetyl-D-glycal also gave the expected products the former two stereoisomers in a 4 6 ratio for cis and trans, and the latter another two in the reverse ratio. 

Phosphonoacrolein, (Et0)2P(0)C(CH0)=CH2, takes part in hDA reactions with alkenes and cyclic conjugated dienes, to give phosphono-substituted 3,4-dihydro-2W-pyrans and their annulated derivatives. The reaction with alkynes gives the 1 1 adduct, a 4//-pyran, initially but this undergoes a second cycloaddition leading to a tetrahydropyrano[3,2-6]pyran 3... 

Ru-catalysed enyne metathesis offers a short approach to chiral derivatives of 3-vinyl-5,6-dihydro-2//-pyrans. Some epimerisation can occur at the pyranyl C atom at elevated temperatures (Scheme 3) <02T5627>. The bispropargyloxynorbomene derivative 6 undergoes a cascade of metathesis reactions in the presence of alkenes and Grubbs catalyst incorporating an enyne-RCM that leads to fused cyclic dienes. A dienophile can be added to the reaction mixture, resulting in Diels-Alder reactions and the formation of functionalised polycyclic products <02TL1561>. 

Sequential one-pot RCM-cyclisation of dienes and alkene isomerisation have been achieved through conversion of the Ru-alkylidene RCM catalyst into a Ru-hydride catalyst, providing an efficient route to 3,4-dihydro-27/-pyrans from acyclic dienes <02JA13390>. 

Tandem RCM and cross-metathesis reactions of allyl hexa-l,5-dien-3-yl ether with alkenes leads to 2-alkylidene 3,6-dihydro-27f-pyrans (Scheme 6) <07TL1417>. 

Chalcones can be prepared by a Heck reaction between an aryl iodide and an aryl vinyl ketone. Demethylation allows spontaneous cyclisation to the flavanone <03TL9107>. An arene-alkene photocyclisation is observed during the irradiation of dihydro-2-methyl-2-vinylnaphtho[l,2-6]pyran-4-one and benzotricyclo[5.3.1.0 ]undecenes 24 result <03TL2011>. 

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