Pyran preparation

Substituted unsaturated pyrans prepared by RCM using 19 as catalyst can be immediately submitted to zirconium-catalysed kinetic resolution of the racemic product at 70 °C. This provides a new route to medicinally important agents containing 6-membered cyclic ethers. A one-pot synthesis can give 63% conversion with >99% enantiomeric purity229. 

The detailed structure of the clathrate compound formed between 2,5,5-trimethylhex-3-yn-2-ol and 4-p hydroxyphenyl-2,2,4-trimethylthio> chroman has been determined by A -ray crystallography. tra/i5 -3,4-Dibromothiochroman and analogous dibromo-naphthothio-pyrans, prepared by addition of bromine to the appropriate A -thio-chromenes, may exist in sofa conformations. The dibromo-compounds undergo ring-contraction reactions with alkalis, yielding benzo[ ]thiophen derivatives. ... 

Intermediates. 3,4-Dihydro-2H-pyran [110-87-2] is prepared by a ring-expanding dehydration of tetrahydrofurfuryl alcohol. It is used as a protecting agent for hydroxyl compounds and as an intermediate. 2-Methylfuran is a chemical intermediate for 5-methylfurfural [620-02-0] (151) and... 

Hydroqulnollnes. Pyrans formed by reactions of a,P-unsaturated aldehydes with 1-ethoxycyclohexene and treated with hydroxjiamine are converted ia good yield to 5,6,7,8-tetrahydroquiQolines (117). These compounds can be dehydrogenated to the corresponding quiaolines. The parent reduced product has been prepared by heating O-aHylcyclohexanone oxime (118). 

The oxa-Pictet-Spengler reaction has been used with success to prepare dihydrofurano[2,3-c]pyrans and isochromans from l-(3-furyl)alkan-2-ols and 2-(3 ,4 -dihydroxy)phenylethanol, respectively. Furanyl alcohol 32 reacted with isobutyraldehyde 33 in the presence of p-toluenesulfonic acid to give the corresponding CI5-5,7-diisopropyl 4,5-dihydro-7H-furano[2,3-c]pyran 34 in good yield. ... 

Dihydropyrones 302 are used as intermediates in the syntheses of tetrahydro-pyrane. There are two approaches (i, ii) to their preparation [74JA7807 84AG991 85JA1246 86JA7060 86MI1 87AG(E)15]. 

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. 

Starting from l.l-dichloro-7b-ethoxy-2-methyl-1,1 a,2,7-tetrahydrobenzo[/)]cyclopropa[prepared from the corresponding benzothiopyran by addition of dichlorocarbene, the three 1-benzothiepins 6a-c are formed upon treatment with strong bases, i.e. sodium methoxide or ethoxide in dimethyl sulfoxide.73 The optimal yield of each 1-benzo-thiepin compound depends on the molar equivalents of base, as follows from different ring-opening mechanisms. 

The bicyclic system 5 can also be used to prepare annulated systems by cycloaddition-cycloreversion processes. With 2//-pyran-2-one (6) a mixture of cycloadducts is formed on... 

A second powerful route to functionalized allylboron compounds involves the reaction of an a-haloalkylboronatc and a vinyl organometallic reagent3 4-28-29, 50c-92 04. This method is especially useful for the preparation of allylboron compounds not accessible via the allylorganometal-lic route. Notable examples that fall into this category are ( )-4,4,5,5-tctramethyl-2-[4-(tetrahy-dro-2//-pyran-2-yloxy)-2-butenyl]-l,3,2-dioxaborolane (yield 41 %, 93% E) and (E)- or (Z)-2-(l,l-dimethyl-2-butenyl)-4,4,5,5-tetramethyl-1.3,2-dioxaborolane (yield 77-84%. 98% E or 93% Z). 

B. 2H-Pyran-2-one. A mixture of 9.81 g. (0.100 mole) of 5,6-dihydro-2/I-pyran-2-one, 200 mg. of benzoyl peroxide, 18.6 g. (0.105 mole) of A-bromosuccinimide [2,5-Pyrrolidinedione, 1-bromo-] (Note 7), and 800 ml. of carbon tetrachloride is prepared in a 2-1., three-necked, round-bottomed flask equipped with a reflux condenser and a mechanical stirrer. The resulting suspension is stirred and heated to reflux. After 1.5 hours at reflux, most of the solid is dissolved, and the solution gives a negative test with starch-iodide paper. The reaction mixture is then allowed to cool, dining which time succinimide [2,5-Pyrrolidine-dione] crystallizes out. The precipitate is removed by filtration, and the filtrate is concentrated under reduced pressure to leave crude 5-bromo-5,6 -dihy dro 211 -pyran-2 one [ 211 Py ran - 2-one, 5-bromo-5,6-dihydro] as an oil. 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

The present one-step procedure for preparation of 5,6-dihydro-2if-pyran-2-one is slightly modified from that described in the original paper.9 It is simpler and easier than the three-step method3 used in the past and represents the most convenient synthesis presently available. The present preparation of 2f/-pyran-2-one has several advantages compared to the alternatives mentioned above simplicity of apparatus and technique, mild reaction conditions, availability of reactants, and ease of product isolation. 

The preparation of 5-ACETYL-l,2,3,4,5-PENTAMETHYLCYCLO-PENTADIENE is of value in the synthesis of pentamethyleyclo-pentadiene and many pentamethylcyclopentadienyl metal carbonyl derivatives that are more soluble in organic solvents than those derived from cyclopentadiene. Simple preparations of 5,6-DIHYDRO-2-PYRAN-2-0NE and 2-//-PYRAN-2-ONE make these hitherto rather inaccessible intermediates available for cycloaddition and other reactions. The already broad scope of the Michael reaction has been widened further by including an efficient preparation of ETHYL (E)-3-NITROACRYLATE. Workers in the field of heterocyclic chemistry will find a simplified method for the preparation of 2,3,4,5-TETRA-HYDROPYRIDINE of help. 

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

Polymerization of 6,8-dioxabicyclo[3.2.1]octane, 2, has been most extensively studied among bicyclic acetals. This monomer is readily prepared from 3,4-dihydro-2H-pyran-2-carbaldehyde 1 by reduction with sodium borohydride followed by add-... 

More functionalized 5,6-dihydro-2H-pyran-derivatives 71 and 72 have been prepared [26] by cycloaddition of 1 -methoxy-3-trialkylsilyloxy-1,3-butadienes 69 with t-butylglyoxylate (70) (Scheme 5.6). Whereas thermal reactions did not occur in good yields because of the decomposition of the cycloadducts, application of pressure (10 kbar) allowed milder conditions to be used, which markedly improved the reaction yields. The use of high pressure also gives preferentially en Jo-adduct allowing a stereocontrolled synthesis of a variety of substituted 5,6-dihydro-2H-pyran-derivatives, which are difficult to prepare by other procedures. 

The same substrate (451) and 4-phenylhydrazono-3,4-dihydro-2//-l-benzo-pyran-2,3-dione (454) [prepared from the dione (452) with benzenediazo-nium chloride] gave 3-(o-hydroxy-a-phenyUiydrazonobenzyl)-2(l//)-quinox-alinone (455) (EtOH-AcOH, reflux, 90 min 80%). ... 

Over the years, many spiropyran structures have been prepared. The pyran component consists of benzopyran or naphthopyran and the heterocyclic part consists of indoline, benzothiazoline, benzoxazoline, benzoselen-azoline, phenanthridine, acridine, quinoline, benzopyran, naphthopyran, xanthene, benzodithiole, benzoxathiole, and saturated heterocyclic rings such as pyrolidine and thiazolidine. 

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