1.3- Dioxolan-2-ones groups

The presence or absence of the dioxolane protecting group in dienes dictates whether they participate in normal or inverse-electron-demand Diels-Alder reactions.257 The intramolecular inverse-electron-demand Diels-Alder cycloaddition of 1,2,4-triazines tethered with imidazoles produce tetrahydro-l,5-naphthyridines following the loss of N2 and CH3CN.258 The inverse-electron-demand Diels-Alder reaction of 4,6-dinitrobenzofuroxan (137) with ethyl vinyl ether yields two diastereoisomeric dihydrooxazine /V-oxide adducts (138) and (139) together with a bis(dihydrooxazine A -oxide) product (140) in die presence of excess ethyl vinyl ether (Scheme 52).259 The inverse-electron-demand Diels-Alder reaction of 2,4,6-tris(ethoxycarbonyl)-l,3,5-triazine with 5-aminopyrazoles provides a one-step synthesis of pyrazolo[3,4-djpyrimidines.260 The intermolecular inverse-electron-demand Diels-Alder reactions of trialkyl l,2,4-triazine-4,5,6-tricarboxylates with protected 2-aminoimidazole produced li/-imidazo[4,5-c]pyridines and die rearranged 3//-pyrido[3,2-[Pg.460]

The 1,3-dioxolane group is probably the most widely used carbonyl protective group. For the protection of carbonyls containing other acid-sensitive functionality, one should use acids of low acidity or pyridinium salts. In general, a molecule containing two similar ketones can be selectively protected at the less hindered carbonyl, assuming that neither or both of the carbonyls are conjugated to an al-kene. ... 

Just as certain pyranose sugars can give rise to bis-acetal or bis-ketal derivatives which constitute linearly fused 5 6 6 systems (cf. Section 12.17.2.1.7), another set of bis-acetals and bis-ketals - in many cases derived from the same sugars - correspond to angularly fused 5 6 6 systems. These, like their linearly fused analogues, serve to protect, selectively, four hydroxyl groups of the parent sugars, and cyclic carbonates (l,3-dioxolan-2-ones) may fulfill similar functions. 

There are many studies of the mechanism and kinetics of the polymerisation of cyclic oxygen compounds, but only relatively few of these are concerned with cyclic formals. In the present paper I will review this field, and I hope to show that formals have some special characteristics which distinguish their polymerisations from those of other cyclic oxygen compounds. Since it is not possible to deal with all aspects of this group of reactions in one lecture, I will concentrate attention on questions of chemistry and mechanism, and I will not deal with other aspects, such as the thermodynamics and kinetics of these polymerisations. Most of the published work has been done with 1,3-dioxolan (I) and there are only very few papers on any other cyclic formals, although the patent literature on the homo- and copolymerisation of (I) and other cyclic formals is quite extensive. 

It is not difficult to show by conductance measurements that when the polymerisation of dioxolan becomes of first order, roughly at the first half-life, all the available acid has reacted with monomer or oligomer to produce active centres [10]. Thus on the Mainz theory one would expect to find a number of OH groups of the same order of magnitude as the number of perchloric acid molecules introduced this is evidently not so, as long as the water concentration in the system is significantly less than that of the acid [10]. For 1,3-dioxepan the protonation is very much faster than for DXL and is complete long before the first half-life of the polymerisation. 

A more recent synthesis of 197 [365] is shown in Fig. 9. Enders introduced the stereogenic centre of (S)-lactic acid into the crucial position 10 in 197. The vinylsulfone B, readily available from lactic acid, was transformed into the planar chiral phenylsulfonyl-substituted (q3-allyl)tetracarbonyliron(+l) tetra-fluoroborate C showing (IR,2S,3 )-configuration. Addition of allyltrimethyl silane yielded the vinyl sulfone D which was hydrogenated to E. Alkylation with the dioxolane-derivative of l-bromoheptan-6-one (readily available from 6-bro-mohexanoic acid) afforded F. Finally, reductive removal of the sulfonyl group and deprotection of the carbonyl group furnished 197. A similar approach was used for the synthesis of 198 [366]. 

Partial fluorination of 4-arylthio-l,3-dioxolan-2-ones occurs preferentially at the carbon atom adjacent to the thio group [67]. However, a remarkable solvent effect is encountered. In the more polar solvent, dimethoxyethane substitution occurs, while in the less polar dichloromethane a larger portion of the desulfurization with cleavage of the phenylthio group takes place. This is attributed to the fact that the intermediate radical cation is more stable in the polar solvent and undergoes deprotonation, while in the less polar solvent, the less stabilized radical cation dissociates into a dioxolane cation and a phenylthio radical. 

In the catalytic system shown in Scheme 9, a hydrogen bond between one hydroxy function of the diol catalyst and the carbonyl group of the substrate is regarded as the driving force of catalysis. Here, the spatial orientation of the bulky a-1-naphthyl substituents of the TADDOL (a,a,a, a -tetraaryl-l,3-dioxolan-4,5-dimethanol) scaffold generates the chiral environment controlling the enantioselectivity of the reaction. 

Telomers 422 (n = 2) and other 4-halogeno-l,3-dioxolan-2-ones were shown288 to react readily with ammonia or primary aliphatic amines, with formation of 4-hydroxy-2-oxazolidones (460). The latter, for which the trans arrangement of the hydrogen atoms of the oxazoli-done ring was deduced from H-n.m.r. data, readily underwent replacement of the hydroxyl group by a phenyl group on reaction with... 

Chiang et al., 1983) but not with 2-methoxy-l,3-dioxolan [80] itself (Ahmad et al., 1979). This method and the method of using acetoxy as leaving group therefore nicely complement one another as 2-hydroxy-1,3-dioxolan which would be the intermediate obtained from [80] can be generated from the... 

The value of kHO- for the breakdown of 2-hydroxy-1,3-dioxolan is more than 10 times greater than for the breakdown of dimethyl and diethyl hemi-orthoformate (Table 12). This contrasts with what is found with the hydronium-ion catalysed breakdown when acyclic hemiorthoesters react faster than cyclic ones, and may be due either to a greater ease of ionization of the hydroxyl group when attached to the dioxolan ring or to the leaving group in the breakdown of the 2-hydroxy-1,3-dioxolan being p-hydroxy-alkoxide rather than an alkoxide as in the breakdown of dimethyl and diethyl hemiorthoformate. 

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