Diol monoester formation

The mechanism of the aldol-Tishchenko reaction has been probed by determination of kinetics and isotope effects for formation of diol-monoester on reaction between the lithium enolate of p-(phenylsulfonyl)isobutyrophenone (LiSIBP) and two molecules of benzaldehyde. ". The results are consistent with the formation of an initial lithium aldolate (25) followed by reaction with a second aldehyde to form an acetal (26), and finally a rate-limiting intramolecular hydride transfer (Tishchenko... 

In 1998, Hasanayn and Streitwieser reported the kinetics and isotope effects of the Aldol-Tishchenko reaction . They studied the reaction between lithium enolates of isobu-tyrophenone and two molecule of beuzaldehyde, which results iu the formation of a 1,3-diol monoester after protonation (Figure 28). They analyzed several aspects of this mechanism experimentally. Ab initio molecular orbital calculatious ou models are used to study the equilibrium and transition state structures. The spectroscopic properties of the lithium enolate of p-(phenylsulfonyl) isobutyrophenone (LiSIBP) have allowed kinetic study of the reaction. The computed equilibrium and transition state structures for the compounds in the sequence of reactions in Figure 28 are given along with the computed reaction barriers and energy in Figure 29 and Table 6. 

FIGURE 28. Reaction sequence for formation of a diol monoester from the reaction between a hthium enolate and two aldehyde molecules. Reprinted with permission from Reference 29. Copyright 1998 American Chemical Society... 

The mechanism of the Serini reaction has been studied further by Ghera, and the pathway shown in Scheme 59 proposed for this general reaction of 1,2-trisubstituted r/c-glycol monoesters. A related process is seen in the formation of olefins by zinc-induced fragmentation of 1,3-diol monoesters a... 

The excellent level of anti selectivity in the formation of the 1,3-diol monoester can be explained by transition state A shown in Scheme 9.1. 

A chiral recognition was observed in aminolysis of 3-acyl-4(R)-methoxycarbonyl-l,3-thiazolidine-2-thione, a derivative of (R)-cysteine, by racemic amines to give an optically active amide [(S)-excess] and amine [(R)-excess]264). In the reaction of cyclic meso-1,3-diols with chiral N-protected phenylalanyl chlorides, Yamada et al.26S) observed the preferential formation of one of the two possible diastereomeric monoesters, which has been used for the synthesis of optically active steroids 266) and prostaglandins 267). 

The formation of diol-periodate esters is supported by physical evidence. The addition of ethane-1,2-diol to periodate solutions causes an initial rapid change in the uv absorption spectrum, followed by a slower change as the oxidation proceeds and lOJ is formed. Similar results are observed for other 1,2-diols except for highly substituted diols such as pinacol (Buist et al ). Buist and Bunton have shown that the cyclic periodate esters formed in alkaline solution from 1,2-diols and periodate can be detected by nmr. The initial fall in pH which occurs in the oxidations of ethane-1,2-diol and lightly substituted diols is also attributed to ester formation (Malaprade, Buist and Bunton ). Cyclic triesters, similar to the cyclic diesters formed from 1,2-diols, are formed from cyclic compounds containing the cis-l,2,3-triol system (Barker and Shaw , Dijkstra and from 1,2-0-isopropylidene-a-D-glucofuranose. In the latter case the presence of the triester has been demonstrated by nmr (Berlin and van Rudloff ). Monoesters of periodic acid have not been detected in any system, but they are postulated as intermediates in the formation of cyclic diesters from 1,2-diols (section 1.3.5). 

Values of the rate coefficients and k have been determined for ethane-l,2-diol andmethyl-substituted ethane-l,2-diols (Buisteta/. ). The general effect of methyl substitution is to decrease k . For meso- and ( )-butane-2,3-diols the values of kf are practically identical, although the equilibrium constant for forming the diester is much larger for the latter. This result suggests that formation of a monoester as an intermediate is rate determining, viz. 

Boronic acid is reported to form esters with D-sorbitol in alkaline solution. Van Bekkum et al. suggest the formation of a monoester involving the eryf/rro-configured diol while a mono-and a diester may be formed over a f/rreo-contigured site [40]. Tridentate D-sorbitol is found to occupy three of four coordination sites of the boron center in a ferrocene derivative with its hydroxyl functions at C2, C3, and C5 [46]. 

Reaction of hypobromous or hypochlorous acids with cholest-4-en-3j8-ol acetate or benzoate (160) gives the 5a-halogeno-3) ,4 -diol 4-monoester, (162) via the acyloxonium ion (161). The 3-monoesters (163) are available by opening of the 4/ ,5)9-epoxide (164). The facility of acyl migration between the two oxygen substituents is revealed by the formation of the 4) ,19-ether (165) when either monoester (162) or (163) was treated with lead tetra-acetate cf. p. 386). ... 

Figure 20 Possible mechanisms for sulfate ester hydrolysis catalyzed by members of the AS family. Formation of the sulfoenzyme (a) nucleophilic attack of the hydrated FGly oxygen on the substrate (b) nucleophilic attack of the sulfate oxygen of the substrate on the aldehyde FGly yields a diester sulfoenzyme, which hydrolyzes into the sulfoenzyme monoester. Enzyme desulfurylation (c) elimination of inorganic sulfate leaves an aldehyde moiety, which is hydrated to give a gerr7-diol (d) nucleophilic attack of a nucleophile, such as water, yields the hydrated gerr7-diol FGly.

The formation of 1,2-diol products from alkenes can be achieved using Prevost s reagent - a solution of iodine in carbon tetrachloride together with an equivalent of silver(l) acetate or silver(I) benzoate. " Under anhydrous conditions, this oxidant yields directly the diacyl derivative of the anh-diol (Prdvost conditions), while in the presence of water the monoester of the syn-diol is obtained (Woodward conditions). Thus, treatment of a cw-alkene with iodine and silver benzoate in boiling carbon tetrachloride under anhydrous conditions gives the tra 5-dibenzoate (5.92). With iodine and silver(I) acetate in moist acetic acid, however, the monoacetate of the cw-1,2-dihydroxy compound is formed. 

The symmetric diols are selectively monoacetylated, probably due to the higher rate of monoacetylation compared to the rate of diester formation from the monoester. Thus in the reaction... 

Acid hydrolysis produced the monoester 410a, which underwent cleavage to the diol 410b with potassium carbonate. For attack at the P-side, one has to start with bromohydrin formation (NBS/H2O), which introduces the bromo atom into the less hindered Jt-face [139]. 

---