Meso-Hydrobenzoin

High enantiomeric excess in organocatalytic desymmetrization of meso-diols using chiral phosphines as nucleophilic catalysts was achieved for the first time by Vedejs et al. (Scheme 13.21) [36a], In this approach selectivity factors up to 5.5 were achieved when the C2-symmetric phospholane 42a was employed (application of chiral phosphines in the kinetic resolution of racemic secondary alcohols is discussed in Section 12.1). A later study compared the performance of the phos-pholanes 42b-d with that of the phosphabicyclooctanes 43a-c in the desymmetrization of meso-hydrobenzoin (Scheme 13.21) [36b], Improved enantioselectivity was observed for phospholanes 42b-d (86% for 42c) but reactions were usually slow. Currently the bicyclic compound 43a seems to be the best compromise between catalyst accessibility, reactivity, and enantioselectivity - the monobenzoate of hydrobenzoin has been obtained with a yield of 97% and up to 94% ee [36b]. 

Vedejs, E. Daugulis, O. Tuttle, N. Desymme-trization of meso-hydrobenzoin using chiral, nucleophilic phosphine catalysts./. Org. 

Acyloxylation of aryl olefins probably involves radical cations as intermediates. Acetoxylation of frans-stilbene in anhydrous acetic acid/sodium acetate yields mainly meso-diacetate, while in moist acetic acid mainly threo-2-acetoxy-l,2-di-phenylethanol is formed 100 Anodic oxidation of trans- and ds-stilbene in ace-tonitrile/benzoic acid produces with both olefins the same mixtures of meso-hydrobenzoin diacetate (62) and f/ireo-2-benzoyloxy-l,2-dip]ienylethanol (63) l01 Product formation is best rationalized by a ECiqE-sequence leading to theienerge-tically most favorable acyloxonium ion (64) (Eq. (125) ) ... 

Fu s planar chiral ferrocenyl DMAP catalyst 4b and also Vedejs PBO catalysts, particularly p-phenyl-PBO catalyst 2a, are also effective for this type of ASD reaction, as illustrated for the cases of unusual meso-diol 37 [49] and meso-hydrobenzoin [165], respectively (Scheme 8.19). 

In the reduction of an optically active ketone, the reaction results in the creation of an additional asymmetric center, and two diastereoiso-mers are possible. In many cases it has been found that both isomers are formed, often in comparable amounts. For example, reduction of camphor (XV) gives a mixture of bomeol and isobomeol (XVI). Numerous other examples may be noted fai the tables at the end of this chapter. However, the reduction of benzil (XVII) or benzoin (XVIII) is reported to give 90% of meso-hydrobenzoin (XIX). ... 

The isomeric composition of some of the phenylcyclopropanes obtained using 1,2-diphenyloxirane (la) as phenylcarbene presursor, deviates significantly from those observed under other experimental conditions. However, reinvestigation showed that 1,2-diphenyloxirane-derived phenylcarbene exhibits the same stereoselectivity as phenylcarbene derived from other sources, i. e. a preference for formation of the thermodynamically less stable phenylcyclopropanes. This is apparent from the almost identical isomer distribution observed when cw-2,3-dimethyl-1-phenylcyclopropane (2) was synthesized from (Z)-but-2-ene using la, phenyldiazomethane (lb), mejo-hydrobenzoin sulfite (Ic), ii/-hydrobenzoin sulfite (Id), meso-hydrobenzoin carbonate (le), and //-hydrobenzoin carbonate (11). - ... 

Schuster, C. (2004) Meso-hydrobenzoin derivatives as new chiral auxiliaries and... 

Broker,). (2004) Aryl substituted meso-hydrobenzoins as new chiral auxiliaries and enantiomerically pure linkers for stereoselective synthesis on solid support. Institute of Applied Synthetic Chemistry-Vienna University of Technology. PhD Thesis. 

The first stage of the process consists of refluxing the glycol with 1,1 -thiocarbonyldi-imidazole for 30 min 48 the resulting cyclic thiocarbonate is converted into the olefin by 70-80 hours refluxing with an excess of trimethyl phosphite. In this way, for example, cis-stilbene was obtained in 92% yield from meso-hydrobenzoin, and trans-cyclodecene in 81 % yield from trans-1,2-cyclodecanediol. 

Interestingly, thermal decomposition of 4,5-diphenyl-l,3,2-dioxathiolane 2-oxide depends on the stereochemistry of the starting diols. For example, the cyclic sulfite derived from meso-hydrobenzoin (66a) decomposed on heating to give a good yield of desoxybenzoin (67), while the trans isomer 66b (derived from if/-hydrobenzoin) furnished diphenylacetaldehyde (68) quantitatively [Eqs. (16) and (17)]. 

Consult with your instructor before performing this experiment, in which you will determine the stereoselectivity of the reduction of benzoin with sodium borohydride. Follow either the Miniscale or Microscale Procedure described for reducing 9-fluorenone to reduce benzoin with sodium borohydride. but use ethanol rather than methanol as the solvent. After slowly adding 3 M HCI to decompose the excess borohydride. add enough water to adjust the solvent composition to 50% v v) ethanol and water. You may recrystallize the crude product from 50% (v v) ethanol and water. Obtain the melting point and the IR. H. and NMR spectra of the purified product for characterization. Compare these data with those for racemic and meso-hydrobenzoin to determine the identity of the product and the stereochemistry of the reduction. If authentic samples of racemic and meso-hydrobenzoin are available, determine mixed melting points to support your assignment. 

Further, itwas shown [89] that electrolysis of benzoic acid in acetonitrile with triethylamine and cis- and trans-stilbene gives only meso-hydrobenzoin dibenzoate irrespective of the stilbene configuration. The authors attempted an explanation of this unexpected result on the assumption that stepwise oxidation and inversion of an intermediate unstable product occur. An important part is played here by adsorption and oxidation of stilbene itself at the electrode, although no evidence was obtained for its possible isomerization under the experimental conditions. 

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