Ethers dienyl

The addition proceeds in three discrete steps and the intermediates can be isolated. Simple alkenes are less reactive than alkynes and do not undergo the addition to aHylic boranes, but electron-rich alkyl vinyl ethers react at moderate temperatures to give 1,4-dienes or dienyl alcohols (440). 

Iron pentacarbonyl and l-methoxy-l,4-cyclohexadiene react as shown by Birch and oo-workera, but in dibutyl ether this solvent has been found superior. The tricarbonyl(methoxy-l,3-cyclohexadiene)iron isomers undergo hydride abstraction with triphenylmethyl tetrafluoro-borate to form the dienyl salt mixture of which the 1-methoxy isomer is hydrolyzed by water to the cyclohexadienone complex. The 2-methoxy isomer can be recovered by precipitation as the hexafluoro-phosphate salt. By this method the 3-methyl-substituted dienone complex has also been prepared from l-methoxy-3-methylbenzene. The use of the conjugated 1-methoxy-1,3-cyclohexadiene in Part B led to no increase in yield or rate and resulted chiefly in another product of higher molecular weight. An alternative procedure for the dienone is to react tricarbonyl(l,4-dimethoxycyclohexadiene)iron with sulfuric acid. ... 

Synthesis of functionalized aryloxy 1,3-butadienes and their transformation to dienyl ethers via Diels-Alder cycloaddition reactions [136]... 

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

Sensitive dienol ether functionality in the diene carboxylate was shown to be compatible with the conditions of the aqueous Diels-Alder reaction (Eq. 12.26).84 The dienes in the Diels-Alder reactions can also bear other water-solubilizing groups such as the sodium salt of phosphoric acid and dienyl ammonium chloride (Eq. 12.27).85 The hydrophilic acid functionality can also be located at the dienophile.86... 

A similar desymmetrization of the dienyl ethers was accomplished using cationic rhodium(l)/BIPNOR complex. In this reaction, an oxygen-containing co-solvent was necessary, and the best result was obtained with a 3 1 mixture of toluene and 1,2-dimethoxyethane (glyme) (DME) (Equation (21)). 

It has been noted that when the aldol condensation is conducted on simple aliphatic aldehydes, such as octanal, in the presence of an alkylating agent and a quaternary ammonium salt the 2-position is alkylated and the alk-3-enal is isolated (Scheme 6.14) [16]. In the absence of the quaternary ammonium salt, the normal aldol adduct is formed with no alkylation. It is noteworthy that in the presence of epichlorhydrin the dienyl ether is formed. 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

Bislactim ethers of the general type (171) react with butyl lithium or LDA in THF or glyme at - 78°C to give the lithium derivative (188) by the abstraction of a proton from position 3. This incorporates a diazapenta-dienyl anion, which was originally represented as an ion pair (83T2085). 

Synthesis of 1,3-butadienyl ethers is different from that employed when R is a simple alkyl group. David and coworkers350 351 elaborated two approaches to dienyl ethers. In the first method, a sugar derivative (650) having an isolated hydroxyl group3510 reacts under basic catalysis with 2,7-dimethyl-3,5-octadiyne-2,7-diol (651) to yield two en-ynyl ethers (652 and 653). Partial hydrogenation of 652 and 653 gives trims- and cis-l,3-butadienyl ethers (654 and 655) see Scheme 12. 

Cycloaddition of Butyl Glyoxylate to ttans-Dienyl Ether 4... 

Allyltributyltin, 10 Vinyltrimethylsilane, 343 Homoallylic ethers Allyltrimethylsilane, 11 Crotyltrimethylsilane, 86 Trityl perchlorate, 339 Homopropargylic alcohols Allenylboronic acid, 36 D imethoxy [ 1 -trimethylsilyl-1,2-buta-dienyljborane, 218 Lithium acetylide, 44 Triisopropoxy[l-(trimethylsilyl)-l,2-buta-dienyl]titanium, 218 Hydroperoxides Oxygen, singlet, 228 Hydroxy acids a-Hydroxy acids... 

Dibromoborane-Dimethyl sulfide, 92 Dichloro(dimethylamino)borane, 120 Diisobutylaluminum hydride-Boron trifluoride etherate, 116 Diisopinocampheylborane, 117, 182 Diisopinocampheylmethoxyborane, 86 Dimesitylmethylborane, 8 Dimethoxy[l-trimethylsilyl-l,2-buta-dienyl]borane, 218 (R,R)- and (S,S)-frans-2,5-Dimethyl-borolanes, 119... 

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

In those cases where a hydroxyethyl substituent is attached to the cyclohexadiene ting, an oxidative cyclization procedure has been developed to effect conversion to cyclohexadienyliron complexes.14 Thus, treatment of complex (52) with manganese dioxide gives the cyclic ether (53) which can be converted to the dienyl complex (54 Scheme 7) by treatment with tetrafluoroboric acid in acetic anhydride. 

The first method, resolution, is unattractive unless both enantiomers are useful in synthesis. In some cases, such as the resolution of dienecarboxylic acid derivatives mentioned earlier (via the phenylethyl-ammonium salt), the resolution is efficient and provides optically pure materials in good yield.39 60,63 In certain cases, the dienyliron complex can be treated with a chiral nucleophile to give a mixture of dia-stereomers which are separated and then reconverted to enantiomerically pure dienyl complex.64 An example of this method is the resolution of complex (27 Scheme 33), via the menthyl ethers (195) and... 

The reaction of a,/3-unsaturated benzodioxepin 116 with fert-butyllithium in THF at — 78 °C produced a lithium alkoxide, which was trapped in situ with 2,2,2-trifluoroacrylate to give acryloyl (lZ,3T)-dienyl ether 117 with high diastereoselectivity (Scheme 28) <20050BC1308>. The procedure has found application in the synthesis of carbo-sugars via Diels-Alder reaction. 

Hydroxypyridine may be allylated by Pd(0)-catalyzed nucleophilic attack of oxygen onto allene <2001T7965>. Reaction with 2 equiv of allene in THF in the presence of Pd(OAc)2 and tris-(2-furyl)phosphine gives 1,3-dienyl ether 105 in 34% yield (Equation 71). This process occurs in similar yield when performed on 3-hydroxyisoquinoline. 

In this section, the preparation and reactivity of ce-lithiated dienyl and trienyl ethers as well as dienyl thioethers as acyl anion equivalents will be considered749-751. 

Unsymmetric ethers cannot be selectively obtained (additional formation of symmetric ethers) except if one of the alcohol is tertiary, hence leads rapidly to a carbocation which reacts with the other alcohol. Furthermore, secondary reactions and especially polymerisation can be observed when the alcohol has very reactive substituents such as dienyl groups. 

RCM has been used to effect the cycloisomerisation of allyl pent-2,3-dienyl ethers to dihydropyrans <06TL3971> and in the synthesis of the spiro[indolino-3,2 -pyran] 5 <06JOC2346>. 

---