Diphenylmethylene Acetals

Diphenylmethylene acetals are used to protect 1.2 diols including catechols. They can be cleaved by a variety of methods including acid hydrolysis, dissolving metal reduction and hydrogenolysis. The steric bulk of the phenyl rings has been used to control the stereochemistry of reactions at proximate centres.  

Modulated acid lability of protecting groups is extremely important in ribonu-cleoside synthesis. A comparison of the relative rates of hydrolysis of a some diarylmethylene protecting groups for the 2 3 -cu-diol system of uridine shows that aryl substitution achieves much greater add lability than the conventional isopropylidene group. Comparison of the half-times for the hydrolysis of the di-(p-anisyl)methylene (B), xanthen-9-ylidene (C) and 2,7-dimethylxanthen-9-yli-dene (D) acetals in trifluoroacetic acid/water/methanol (1 2 7 v ) at 30 revealed that the 2J-dimethylxanthen-9-ylidene system is 20 times more labile than the isopropylidene acetal. 

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In extensions of the original work, Liptdk and coworkers [28] have shown that reductive opening of dioxolane diphenylmethylene acetals hy LiAlH4-AlCl3 regioselec-tivcly produces the axial dipbenylmethyl ethers with an adjacent equatorial hydroxyl group (Scheme 13). 

A. BotMs, 1. Hajkb, M. Kajtdr-Heaedy, and A. Liprik, Hydrogenolyris of dro ol nr-type diphenylmethylene acetals by AlCIHj to axial diphenylmethyl ethers, J. Carbokydr. Chem. 12 191 (1993) and references therein. 

In an alternative route employing fewer overall steps, Borbas and Lipt k119 synthesized the trisaccharide hapten as a 4-aminophenyl glycoside protected as the (V-trifluoroacetyl derivative 13 for potential conjugation to protein.108 Ethyl 1-thio-a-L-rhamnopyranoside as the 2,3-O-diphenylmethylene acetal 23 was the precursor for both rhamnosyl residues. Condensation of 23 with glycosyl bromide 24 with silver triflate as promoter, followed by O-deacetylation and O-benzyliden-... 

Methylene acetals and diphenylmethylene acetals (see section 3,2.5) are the runts of the acetal family litter. They are not used often but they have properties that in special circumstances can be useful. The prime virtues of methylene acetals are their hydrolytic stability and negligible mass they can survive rather harsh reaction conditions unscathed. Likewise, the prime detraction of methylene acetals is their stability they require rather brutal conditions to remove them hence, they have limited utility. 

Coriarin A (Scheme 3.97) displays tumour remissive activity owing to host-mediated immunostimulation rather than direct cytotoxirity. The final step of a synthesis of Coriarin A entailed simultaneous hydrogenolysis of seventeen benzyl ethers and four diphenylmethylene acetal groups to give the final target in 80% yield.86... 

Protection of a catechol as its diphenylmethylene acetal also featured in a synthesis of the Galanthamine alkaloids.182 Here, the deprotection reaction was accomplished using trifluoroacetic acid — conditions that also effected protonolysis of the aryl silane [Scheme 3.98]. 

Stereoselective cleavage of diphenylmethylene acetals. Formation of axial diphenylmethyl ethers of pyranosides is noted. Preferential coordination to the more available equatorial oxygen atom prior to the hydride delivery may explain the results. 

There are only two common methods for synthesising diphenylmethylene acetals. The first involves an acid catalysed acetal exchange reaction using benzo-phenone dimethyl acetal [Scheme 3.100) -and the second involves reaction of the diol with diphenyldichloromethane in the presence of pyridine. The protection depicted in Scheme 3.101 was achieved by simply heating the reagents in the absence of base. ... 

A kinetic study of the reductive opening of the diphenylmethylene acetal in methyl 2,3-0-diphenylmethylene-o -L-rhamnopyranoside has been compared to earlier quantum calculations. ... 

Methylene, 287 Pivaldehyde Acetal, 289 Acetonide, 289 Cyclohexylidene. 289 Diphenylmethylene, 289 Ethyl Orthoformate, 290 Diisopropylsilylene Derivative, 290... 

Methylene, 424 Pivaldehyde Acetal, 426 2-BOC-ethylidene, 426 2-Moc-ethylidene, 426 Acetonide, 426 Cyclohexylidene, 427 Diphenylmethylene, 427 Ethyl Orthoformate, 428 Diisopropylsilylene Derivative, 428... 

Dimethyl-3-(diphenylmethylene)benzodiazepine was prepared by condensation of o-phenylenediamine with 3-(diphenylmethylene) pentane-2,4-dione in ethanol-acetic acid.9 3-Methylenepentane-2,4-diones monosubstituted in the methylene group do not condense under these... 

Wolfrom and Tanghe. On partial hydrogenation, 1,2-0-benzylidene-a-D-glucofuranose was obtained. A 3,5.52,281,282 g 5,6- 0-benzylidene acetal of the mono-isopropylidene acetal (25) have been isolated. The five-membered benzylidene ring is formed in higher proportion at higher temperatures. The preparation of a 5,6-0-(diphenylmethylene) derivative of the monoacetal (25) was effected somewhat unusually by interaction with dichlorodiphenylmethane in pyridine at 0°. 

A-(Diphenylmethylene)glycine t-butyl ester (t-butyl glycinate-benzophenone Schiff base) (171) is a reactive prochiral nucleophile and a-allyl-a-amino acids can be prepared by allylation and hydrolysis of the allylated product. Asymmetric allylation of 171 with cinnamyl acetate (41) afforded 172 regioselec-tively with high % ee when the reaction was carried out in presence of achiral phosphite P(OPh)3, and a ehiral phase-transfer catalyst of alkaloid [0-allyl-(9-anthracenylmethyOcinchonidinium iodide] [65,66]. 

Carbanion generated from iV-(diphenylmethylene)glycine t-butyl ester was enantioselectively allylated by allylic acetate in reaction catalyzed by achiral Pd complexes and chiral Pt catalyst (112) (eq. 219). 

Zinc acetate catalyst produces essentially 100% o-methylol phenol (8) in the first step. The second step gives an approximately equal quantity of 2,2 - (5, 45%) and 2,4%-diphenylmethylene (6,45%) bridges, indicating little chelate-directing influence. In addition, a small quantity (10%) of methylene ether units (9) (dibenzyl ether) is observed at moderate reaction temperature. 

A wide range of adhesive types and chemistries are used to bond wood elements to one another (Table 2), but relatively few adhesive types are utilized to form the composites themselves. The vast majority of pressed-wood products use synthetic thermosetting adhesives. In North America the most important wood adhesives are the amino resins (qv), eg, urea-formaldehyde (UF) and melamine-formaldehyde (MF), which account for 60% by volume of adhesives used in wood composite products, followed by the phenolic resins (qv) eg, phenol-formaldehyde (PF) and resorcinol-formaldehyde (RF), which account for 32% of wood composite adhesives (12,13). The remaining 9% consists of cross-linked vinyl (X-PVAc) compounds, thermoplastic poly(vinyl acetates) (PVA), soy-modified casein, and polymeric diphenylmethylene diisocyanate (pMDI). Some products may use various combinations of these adhesives to balance cost with performance. 

Cycloaddition. W-(Diphenylmethylene)-4-methylben-zenesulfonamide (2) has been used as a dienophile in a hetero [3 + 2] cycloaddition. Dipolar trimethylenemethane (TMM, 21), generated by means of thermolysis of methylenecyclopropane (22), undergoes [3 + 2] cycloaddition with the reagent to regio-selectively afford the acetal of the a-methylene-y-lactam product 23. This product is quite acid-sensitive and may he isolated as the corresponding a-alkyhdene-y-amino ester. The cycloadduct serves as a synthetic precursor to y-amino acid derivatives (eq 13). 

C36H46N2, 1,l4-Bis-(2, 6, 6 -trimethylcyclohex-l -enyl)-3,12-dimethyl-tetradeca-1, 3,5,7,9,11,13-heptaene-6,9-nitrile, 37B, 80 Cft 2H3 8O2, 1-Diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene ethyl acetate solvate, 44B, 134... 

The diphenylmethylene ether, introduced by Robinson, is on the other hand a much more useful and selective protecting group. It is removed by hydrogenolysis or by treatment with hot acetic acid and has been used to prepare a number of partially benzylated and methylated polyphenols. A typical example is the synthesis of methyl m-digallate (61) by condensation of (56) and (59) to give the intermediate (60) hydrogenolysis then gives (61) [91]. 

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