Vinyl benzyl ether

Commercial chromatography silica gel promotes effectual Diels-Alder cycloaddition of optically active pyrone lactate ester (5) with benzyl vinyl ether (6), affording the endo adduct 7 in an approximately 4 1 mixture of diastereoisomers [16] (Equation 4.1). 

Posner G. H., Anjeh T. E. N., Carry J. C., French A. N. A New and Efficient Asymmetric Synthesis of an A-Ring Precursor to Physiologically Active 1-a-Hydroxyvitamin D3 Steroids Proc. - NOBCChE 1994 21 383-389 Keywords inverse electron-demand Diels-Alder cycloadditions, (S)-lactate and Lewis acids (-)-Pr(hfc)3 with benzyl vinyl ether... 

Azadienes of this sort were studied simultaneously by Mariano et al., who reacted mixtures of (1 ,3 ) and (1E, 3Z)-l-phenyl-2-aza-l,3-pentadiene 275 with several electron-rich alkenes, e.g., enamines and enol ethers (85JOC5678) (Scheme 61). They found the (l ,3 )-stereoisomer to be reactive in this process affording stereoselectively endo 276 or exo 277 piperidine cycloadducts in 5-39% yield, after reductive work-up with sodium borohydride. The stereochemistry of the resulting adducts is in agreement with an endo transition state in the case of dienophiles lacking a cis alkyl substituent at the /8-carbon (n-butyl vinyl ether, benzyl vinyl ether, and 1-morpholino cyclopentene), whereas an exo transition state was involved when dihydropyrane or c/s-propenyl benzyl ether were used. Finally, the authors reported that cyclohexene and dimethyl acetylenedi-carboxylate failed to react with these unactivated 2-azadienes. 

In fact, the copolymers of methacrylic acid with maleic anhydride (14) and the copolymers of vinyl alcohol with maleic anhydride (127) obtained respectively from optically active (l-methyl-benzyl)-methacrylate or (l-methyl-benzyl)-vinyl-ether and maleic anhydride, were optically active, but their rotatory power was rather small. 

A similar unsuccessful attempt to perform a Claisen rearrangement on benzyl vinyl ether 7 resulted in a highly successful 1,3-benzyl shift to afford ethyl 3.3-dilluoro-2-oxo-4-phenyl-butanoate (8) in 82% yield.10... 

Because the transition state of electrophilic addition is rather open or only very weakly bridged, the stereoselectivity is poor, resulting in primarily atactic polymers. Polystyrene and most poly(vinyl ether)s prepared cationically at ambient temperature are atactic with similar proportions of meso and racemic dyads [243,244]. However, meso addition is slightly preferred with vinyl ethers, and varies from 60 to 70% for most monomers, including isobutyl, neopentyl, n-butyl, and ethoxyethyl vinyl ethers [245]. It is higher with benzyl vinyl ether (89%). This tendency to... 

The enantiopure 1-oxa-1,3-diene 278 is prepared by acylation of benzyl vinyl ether with oxalyl chloride. This generates acyl chloride 276, which acylates the lithium salt of 2-oxazolidinone 277. In the presence of Me2AlCl, diene 278 adds to (Z)-l-acetoxy-2-ethoxyethene giving mostly adduct 279, whereas, when using Me3SiOTf as promoter of the hetero Diels-Alder addition, diastereomer 280 is the major adduct (Scheme 13.79). Adducts 279 and 280 have been converted into ethyl P-d-mannopyranoside and ethyl-P-L-mannopyranoside, respectively [149]. 

The Diels-Alder reaction of enantiomerically pure 2-pyrone (5)-lactate 351 (Scheme 67) with benzyl vinyl ether at -30°C in the presence of a... 

Generally, on PhI(OAc)2-promoted oxidation of o-methoxyphenols in MeOH containing a large excess of electron-rich dienophiles, the resulting o-quinone monoketals may undergo an intermolecular Diels-Alder reaction with the dienophiles to afford the corresponding dimers. 4-Methoxycarbonyl-2-methoxyphenol (421) was submitted to PhI(OAc)2-promoted oxidation in MeOH containing benzyl vinyl ether (BVE) or dihydrofuran (DHF)... 

Notably, the cycloaddition of 3,5-dibromo-2-pyrone is more stereoselective than monobromo-2-pyrone counterparts 4 and 5. For instance, cycloadditions with acrylonitrile and benzyl vinyl ether provided the cycloadducts 44 and 51 in endolexo ratios of 76 24 and 100 0 (entries 4 and 11), respectively, while 5-bromo-2-pyrone 5 afforded the corresponding cycloadducts in ratios of 54 46 and 67 33, respectively. 

The resultant 3-alkynyl-5-bromo-2-pyrones 83 are potent ambident dienes, undergoing cycloadditions with both electron-rich and electron-poor dienophiles. As exemplied in Scheme 24, 3-[(trimethylsilyl)-ethynyl]-5-bromo-2-pyrone 83a underwent efficient cycloaddition with benzyl vinyl ether, as well as with methyl acrylate, to afford the corresponding cycloadducts 84 and 85, respectively. 

The ambident enophilic character of the resultant 3-aryl-5-bromo-2-pyrone 81 was demonstrated by the easy cycloadditions of 3-phenyl-5-bromo-2-pyrone 94 (Scheme 28) with both methyl acrylate (MA) and benzyl vinyl ether (BVE), affording the corresponding cycloadducts 95 and 96 in good yields with varying endo/exo selectivity. 

A series of 5-substituted-3-bromo-2-pyrones can be thus readily synthesized as summarized in Figure 3. The resultant 2-pyrones are expected to be potent ambident dienes as exemplified by the cycloadditions of 5-phenyl-3-bromo-2-pyrone with methyl acrylate and benzyl vinyl ether (Scheme 34). 

Silica gel has been reported to accelerate Diels-Alder reactions of 3-methoxy-carbonyl-2-pyrone and a related chiral dienophile with butyl vinyl ether or benzyl vinyl ether (Scheme 4) [31]. Related dienophiles, 4,6-disubstituted 2-pyrones, were investigated in reactions with naphthoquinone or A-phenylmaleimide catalyzed by silica or other heterogeneous catalysts in the absence of solvent [32], An example is included in Scheme 4. 

An approach to the synthesis of dihydroxyvitamine D3 and analogs involves the Diels-Alder reaction of the simple unsubstituted 2-pyrone with benzyl vinyl ether (Scheme 10.7). It is well known that the reaction progresses with extreme difficulty under normal conditions. Temperature is precluded as the potential cycloadduct easily decarboxylates. The biactivation mode induces sufficient reactivity as demonstrated by Posner et al. [35]. In the presence of the chiral ytterbium catalyst... 

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