1.2- Diols from aromatic aldehydes

The most general procedures are based on low-valent titanium. Good yields of diols are obtained from aromatic aldehydes and ketones by adding catechol to the TiCl3-Mg reagent prior to coupling.250... 

The apphcation of these types of ionic hquids as reagents and solvents for the chemoselective, regioselective, and stereoselective syntheses of 1,2- or 1,3-bro-moesters from aromatic aldehydes and 1,2- or 1,3-diols at room temperature has been studied (Fig. 12.63). Good to excellent yields and moderate enantiomeric excesses were obtained under these reaction conditions. 

Reactions of aldehydes and their derivatives have been the subject of several reports. Benzene-1,2-dicarboxaldehyde condenses" with 2,3-dihydro-naphthazarone to give the p-quinone (75), and a similar reaction occurs between diketones such as PhCOCH2CH2COPh and naphthalene-1,4-diol. 4-Nitrobenzaldehyde undergoes disproportionation in the presence of cyanide ions and methanol, resulting in methyl 4-nitrobenzoate as the main product. The acid-catalysed rearrangement of arylhydrazones (76) derived from aromatic aldehydes and diaryl ketones leads to amino-biphenyls (77). 

Smaller aldehydes form cyclic acetal-type oligomers readily in aqueous conditions.60 Diols and polyols also form cyclic acetals with various aldehydes readily in water, which has been applied in the extraction of polyhydroxy compounds from dilute aqueous solutions.61 E in water was found to be an efficient catalyst for chemoselective protection of aliphatic and aromatic aldehydes with HSCH2CH2OH to give 1,3-oxathiolane acetals under mild conditions (Eq. 5.7).62... 

The cobalt carbonyl complex is also an effective catalyst for the siloxy-methylation of aromatic aldehydes.110 Arylethane-l,2-diol disilylethers are obtained in good yields, resulting from incorporation of one molecule of CO and two molecules of HSiR3. Good selectivity for the siloxymethylation product is observed at 0°C in hexane. At 15°C, faster reaction rates are observed, but the selectivity for the CO-incorporated product is lower. In contrast, aliphatic aldehydes react under these conditions (1 atm CO, 0°C) to give only a small amount of CO-incorporated product, with a major product resulting from hydrosilylation. 

An alternative strategy for preparation of monoacylated 1,2- and 1,3-diols is oxidative deavage of cyclic acetals prepared from a diol and an aliphatic or aromatic aldehyde (Scheme 10.7). For this purpose the required acetal does not need to be isolated, but can be generated in situ [25]. Acetals prepared from strongly electrophilic aldehydes, for example nitrobenzaldehydes, will, however, usually be difficult to oxidize (and to hydrolyze). 

Aldol condensation of the zinc enolate of resin-bound alkyl ester 29 with aromatic aldehyde or ketone forms a P-hydroxy ester, which upon treatment with DIBAL-H leads to simultaneous reduction and cleavage of the ester moiety from the resin to give a soluble 1,3-diol 31 [31], Parallel synthesis utilizing three ester and nine carbonyl building blocks afforded a library of 27 analogs which was screened for antioxidative efficiency using a ferric thiocyanate assay. 

Narasaka reported that, although the reactions proceeded in a stoichiometric manner, the mixture of TiCl2(Oi-Pr)2 with chiral diol (9a, see Fig. 1) derived from tartaric acid promoted the addition of TMSCN to aromatic aldehydes in the presence of 4 A molecular sieves to yield the corresponding cyanohydrins with an ee of up to 96% [40]. 

The dione (Scheme 22) reacts photochemically with aromatic aldehydes in processes similar to the reactions between quinones and aldehydes. Irradiation of the diketone (343) leads to the formation of the remarkable diol (344). The structure of this product was verified by T-ray analysis. The reaction, brought about by irradiation through Pyrex in various solvents, proceeds via the intermediate keto alcohol (345), a compound isolated at shorter reaction times. There is some doubt in the minds of the authors as to whether the reaction arises by hydrogen abstraction from the y- or the s-positioh. They propose that if the former occurs then the rearrangement by hydrogen migration to yield (I) is kinetically favoured. 

Contrary to some reports, electrophilic addition reactions may occur in other multiple-bond systems. In many of the reactions of aldehydes and ketones the first stage involves the addition of some entity across the carbon-oxygen bond, e.g., the formation of oximes, semicarbazones, hydrazones, hydrates (1,1-diols) and their ethers, and the aldol condensation. Most of these reactions entail a subsequent loss (elimination) of a small molecule e.g. water, ammonia, ethanol) and, while one must be careful to determine whether the rate-determining stage involves attack on the carbonyl compound or elimination from the adduct , there are some systems in which it is evident that electrophilic attack is involved in the slow stage of the reaction sequence. Examples of such reactions are the acid-catalysed formation of oximes of aliphatic - and aromatic carbonyl compounds, of furfural semi-carbazone , and of 1,1-diols from aldehydes or ketones . 

In the same study, a range of related amino acid derivatives were applied as catalysts, hut clearly failed to reach similar results compared to proline, both in terms of yield and enantioselectivity. It was shown already at that point that both the pyrrolidine ring as well as the carbo>ylate in the unique structure of proline are crucial for its activity. The scope of aldehydes was extended to several branched, aromatic aldehydes, with moderate yields and enantioselectivities. Nonbranched aldehydes were naturally excluded from the method due to aldehyde self-aldolisation and aldol condensation. List et al. were able to address this problem by modifying the conditions, temperature and solvent in particular. The revisited method applies to a range of aliphatic, nonbranched aldehydes, moderate yields but good enantioselectivities are obtainable. The approach was shortly after extended to the valuable, asymmetric synthesis of, 2-diols (Scheme 5.5). 

A wide range of (ra 5-fused furo[3,4-c]tetrahydropyrans were obtained from the InBr3-catalyzed Prins bicyclization of ( )-2-styrylbutane-l,4-diol with aliphatic and aromatic aldehydes in dichloromethane (14TL4110).The synthesis of a 5-fused furo[3,4-c]tetrahydropyran amides occurs through a... 

The use of chiral diols as co-catalyst in aldol reaction led to an improvanent of the achieved results [41]. Thus, when acetone (3a, 8.18 equiv.) was reacted with benzaldehyde (2 h) in DMSO at 0°C catalyzed by (5)-proline (30 mol%) the expected product 4 was obtained in 72% ee, while a 96% ee was achieved in the presence of (R)-BINOL (0.5 mol%). A hypothetical explanation from the authors for this effect is the possible template effect of the chiral diol which may activate and ordered the aldehyde and enamine nucleophile. The same reason was claimed for the beneficial effect achieved by addition of a 10 mol% of (3,5-bistrifluoromethylphenyl)thiourea in the aldol reaction between cyclohexanone (3b) and several aromatic aldehydes catalyzed by proline (1,10 mol%) in hexane a 25°C [42], In this case, reaction times, yields as well as diastereo- and enantioselectivities were improved (75-98%, 76-88% de, 98-99% ee), with these results being also attributed to the enhancement of the proline solubility by the formation of a host-guest proline-thiourea complex. 

Optically active tetrahydropyran derivatives can be obtained by domino cross aldol/acetal cyclization reaction of aromatic aldehydes with glutaraldehyde generated from the inexpensive tetrahydro-2/f-pyran-2,6-diol under equilibrium conditions, in yields ranging from 42% to 78% and good diastereo- (60-75% de) and enantioselectivities (93-99% ee) [75],... 

Various aromatic aldehydes were reacted with the active manganese prepared from using manganese chloride. Table 8.24 reports that simple treatment of aryl aldehydes with active manganese gives the corresponding 1,2-diols in moderate to good isolated yields under very mild conditions, rt in THF. The results are shown in Table 8.24. 

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