Methoxy allene

The array of dienophiles amenable to these hetero Diels-Alder reactions is not limited to enol ethers and enamines since allylsilanes and simple alkenes have also been successfully employed [370, 371]. More recently, it has been shown that methoxy allenes such as 4-41 undergo formation of 6H-l,2-oxazines 4-43 upon cycloaddition to nitrosoalkenes such as 4-34 and subsequent tauto-merisation of the intermediate exo-methylene compound 4-42 (Fig. 4-9) [372, 373]. In these studies, 4-43 proved to be a versatile synthetical intermediate allowing oxidative demethylation or reductive removal of the methoxy group as well as nucleophilic substitutions after the generation of an azapyrylium ion [372 - 374]. Furthermore, ring contraction reactions of these oxazines leading to pyrroles [373] and y-lactames [375] are known. 

H-l,2-Oxazine 6-Methoxy-5-methylen-3-trifluormethyl-5,6-dihydro- E9a, 99 (1-CF3 — 1-NO —ethen + Methoxy-allen) 6H-l,2-Oxazine 6-Methoxy-5-methyl-3-trifluormethyl- E9a, 99 (Methylen - En —CH3)... 

H-l,2-Oxazin 3-Ethoxycarbonyl-6-methoxy-5-methylen-5,6-dihy-dro- E9a, 99 (1-COOR-1-NO-ethen + Methoxy-allen) 4H-l,4-Oxazin 2,6-Dimethyl-3-ethoxycarbonyl-5-oxo-5,6-dihydro- VI/4, 549... 

Dimethylhydrazine. N,N-Dimethyl-thiocaibamoyl chloride. (Z)-2-Ethoxy-vinyllithium. Lithium di-(E)-propenyl-cuprate. Oxygen, singlet. Methoxy-allene. Methoxyallidenetriphenyl-phosphorane. 3-Methoxy-l-phenyl-thio-l-piopene. Pyridinium chloio-chromate. 

Base-induced cylization of methoxyallene carbinols, which are available from lithiated methoxy-allenes and carbonyl compounds, yields 2,5-dihydro-3-ethoxyfurans (Equation (92)) <69RTC609,... 

By Cycloadditions involving Thiocarbonyl Compounds. The photocatalysed cycloaddition of thiocarbonyl compounds to olefins to form thietans has been reviewed. The photo-addition of thiobenzophenone to cis- and /m j-dichloroethylene and to cw-dicyanoethylene apparently proceeds with retention of configuration in the olefin moiety. Allenes participate in such reactions, thiobenzophenone with tetramethylallene > and methoxy-allene, for example, giving the thietan derivatives (44) and (45). Thiobenzophenone in its triplet excited state apparently attacks the central... 

The steroeselective synthesis of (2s, 3s) norstatine derivatives is carried out through aldehydes ozonolysis in the presence of lithium methoxy-allene [82],... 

A solution of 0.22 mol of butyllithium in 150 ml of hexane was cooled below -40°C and 140 ml of dry THF were added. Subsequently 0.20 mol of 1-dimethyl amino--4-methoxy-2-butyne (see Chapter V, Exp. 14) were added in 10 min with cooling between -35 and -45°C. After an additional 15 min 100 ml of an aqueous solution of 25 g of ammonium chloride were added with vigorous stirring. After separation of the layers four extractions with diethyl ether were carried out. The solutions were dried over potassium carbonate and then concentrated in a water-pump vacuum. Distillation of the residue gave a mixture of 8-10% of starting compound and 90-92% of the allenic ether, b.p. 50°C/12 mmHg, n 1.4648, in 82% yield (note 1). 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Durch Reduktion der aus einem Propargylalkohol, Dimethylamino-methanol und Me-thyljodid erhaltenen quartaren Ammonium-Salze mit Natrium-bis-[2-methoxy-athoxy]-dihydrido-aluminat werden ebenfalls a-Hydroxy-allene erhalten z.B.1... 

Methoxy-olefins were obtained when the allenic phosphonium salt... 

Deprotonation of 3-methoxy-3-methylallene with BuLi followed by metal exchange with Ti(OiPr)4 affords a chiral allenyltitanium reagent [31], Addition of this reagent to enantioenriched (S)-2-benzyloxypropanal afforded a mixture of four diastereomeric products in which the anti,syn and anti,anti adducts predominated (Eq. 9.26) [31], The former was shown to derive from the matched pairing of the (S)-aldehyde with the (P)-enantiomer of the allenic titanium reagent. The latter is the major diastereomer of the mismatched (S)/(M) pairing. 

Hg(OAc)2 easily reacts with allene to yield methoxymercuration products, i.e. vinylmercury compound 41, with the trans isomer being the major product. The chirality of allene was transferred into the final products, indicating the intermediacy of a cr-bridged mercurinium ion [23-26], The stereoselectivity of this reaction was determined by the relative stability of intermediates 40 and 42 and steric hindrance for the incoming methoxy group. 

The protonation of 3-thio methyl-l-methoxy-l,2-allenes 90 affords 3-methylthio-enals 91 [43]. Hydrolysis can, on the other hand, be directed to the thioether moiety with HgCl2-MeOH leading to l,l-dimethoxy-3-ketones 92 [43],... 

However, with allylzinc bromide the reaction afforded the vinylic metal intermediate 111 with the allyl group connected to the carbon linked to a methoxy group, which may afford a new allenic product via /3-elimination. 

Lithio-l-methoxyallene 183 ° , readily accessible by deprotonation of methoxyal-lene with n-butyllithium in diethyl ether, turned out to be a versatile C-3 building blocL It adds to aldehydes and ketones giving hydroxyaUcylated allenes 184, which undergo a ring-closure reaction under basic conditions. Thus, 3-methoxy-2,5-dihydrofurans 185 are obtained. Subsequent acid hydrolysis leads to the formation of dihydro-3(2//)-... 

Hibino et al. reported the total synthesis of carbazomycin G (269) by the regioselective addition of methyllithium onto 3-methoxy-2-methylcarbazole-l,4-quinone (941) (653). The required immediate precursor of carbazomycin G, carbazole-l,4-quinone 941, was obtained from 3-(2-methoxyethenyl)-N-(phenylsul-fonyDindole (986). The benzannulation involves an allene-mediated electrocyclic reaction of a 67t-electron system generated from the 2-propargylindole 989, which was derived from the 3-vinylindole 986 in three steps. 

Zeng F, Lee H, Chidiac M, Allen C (2005) Synthesis and characterization of six-arm star poly (5-valerolactone)-Woci -methoxy poly(ethylene glycol) copolymers. Biomacromolecules 6 2140-2149... 

Whereas the lithium enolates of acetate and propionate esters of difluoroallylic alcohols are known to fragment rapidly, methoxy- and benzyloxy-acetates have been found151 to form chelated enolates which undergo a smooth [3,3]-sigmatropic rearrangement as their silyl ketene acetals to afford highly functionalized difluoro compounds (see Scheme 33). Allenic silyl ketene acetals (117) have been used152 to prepare 2-substituted methyl 3,4-dienoates (118). 

Reaction of l,8-dimethoxy-9-methylenethioxanthene with 1,8-dimethoxythiopyrylium cation followed by sequential deprotonation and oxidation affords the allene 362 which shows only one methoxy signal in the 111 NMR spectrum (Scheme 84) <2005PS(180)1339>. Methylation with Mc(0 1 BF4 gives the resonance stabilized mono-methylated cation 363 which reverts to the mono cation 364 over a period of about 4 hours and which is accessible directly from the allene 362 by treatment with HBF4 (Scheme 84) <2005PS(180)1339>. 

Unsuccessful attempts to prepare stable complexes of six-membered ring allenes that succeeded for their seven-membered counterparts (Section IV,C) include reaction of 299 with KOt-Bu in the presence of tris(triphenyl-phosphine)platinum(O) [Eq. (50)]118 and abstraction of methoxy from 301 (Scheme 37)119120 (L = CO) with trimethylsilyl triflate. The latter gave products suggestive of an intermediate complex of 1,2-cyclohexadiene, but it could not be detected or trapped. Based on fluxionality studies in the seven-membered counterpart (Section IV,C), equilibration of 302 with a... 

Alternatively, 1,1,3,3- and 1,2,3,3-tetrachloropropene 21 and 22 or 1,1,2,3,3 penta-chloropropane 23 underwent dehydrochlorination with potassium t-butoxide to give, probably through 1,3,3-trichloropropyne 24, the dichlorovinylidenecarbene 25 which was trapped by olefins to lead to the dichloroethenylidenecyclopropanes 26. Then, the highly reactive allene 26 added electrophiles as well as nucleophiles such as methoxides to give, for instance, l-methoxy(2-chloroethynyl)cyclopropane 27, Eq. (9) 22). 

J. C. A. Boeyens, C. C. Allen, and G. W. Perold, Rotamers and isomers in the fulgide series. Part 3. Structures of the bis-(4-methoxy-3-methylbenzylidene) succinic anhydrides, J. Chem. Soc., Perkin. 

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